Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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OFFERED LOAD ESTIMATION AND APPLICATIONS FOR
USING SAME IN A COMMUNICATION NETWORK
Background
1. Field of the Invention
The invention relates generally to communication systems,
and more particularly to offered load estimation and applications
for using same in a communication network.
io
2. Discussion of Related Art
In today's information age, there is an increasing need for
high-speed communication networks that provide Internet access
and other on-line services for an ever-increasing number of
~s communications consumers. To that end, communications
networks and technologies are evolving to meet current and future
demands. Specifically, new networks are being deployed which
reach a larger number of end users, and protocols are being
developed to utilize the added bandwidth of these networks
20 efficiently.
One technology that has been widely employed and will
remain important in the foreseeable future is the shared medium
communication network. A shared medium communication
network is one in which a single communications channel (the
25 shared channel) is shared by a number of users such that
uncoordinated transmissions from different users may interfere
with one another. The shared medium communication network
typically includes a number of secondary stations that transmit on
the shared channel, and a single primary station situated at a
3o common receiving end of the shared channel for, among other
things, coordinating access by the secondary stations to the
shared channel. Since communication networks typically have a
limited number of communication channels, the shared medium
communication network allows many users to gain access to the
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network over a single communication channel, thereby allowing
the remaining communication channels to be used for other
purposes.
Many techniques are known which the primary station can
use for coordinating access by the secondary stations to the
shared channel. The ability of the primary station to meet
specified performance goals depends on a number of factors,
including the particular techniques) employed and the number of
secondary stations attempting to access the shared channel at
io any given time (often referred to as the "offered load").
Furthermore, the ability of the primary station to meet specified
performance goals often depends on the ability of the primary
station to adapt to changes in the offered load over time, and
more specifically on how quickly the primary station can adapt to
~5 such changes. Thus, the primary station must be able to estimate
the offered load of the network and react accordingly.
Brief Description of the Drawing
In the Drawing,
2o F1G. 1 is time line depicting a shared channel in accordance
with a preferred embodiment of the present invention, with the
shared channel divided into successive frames including a
request interval for providing contention access;
FIG. 2 is a three-dimensional graph depicting a planar
25 region ABC representing the set of possible contention outcomes
in accordance with a preferred embodiment of the present
invention;
FIG. 3A is a three-dimensional graph showing the locus of
expected outcomes within the planar region ABC in accordance
3o with a preferred embodiment of the present invention;
FIG. 3B is a two-dimensional graph showing the locus of
expected outcomes within the planar region ABC in accordance
with a preferred embodiment of the present invention;
2
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F IG. 4 is a two-dimensional graph showing the planar region
ABC divided into three regions based on the distance of points
from the locus of expected outcomes in accordance with a
preferred embodiment of the present invention;
s FIG. 5 is a three-dimensional graph showing the planar
region ABC intersected with three planes So, lo, and Co in
accordance with a preferred embodiment of the present invention;
FIG. 6 is a two-dimensional graph showing the three planes
So, lo, and Co intersecting at the point of maximum likelihood of
to SUCCESS outcomes within planar region ABC in accordance with
an embodiment of the present invention;
FIG. 7 is a two-dimensional graph showing the three planes
So, lo, and Co in accordance with a preferred embodiment of the
present invention;
15 FIG. 8 is a block diagram showing a shared medium
communication network in accordance with a preferred
embodiment of the present invention;
FIG. 9 is a state diagram showing three possible states for a
MAC User in accordance with a preferred embodiment of the
2o present invention;
FIG. 10 is a block diagram showing a primary station in
accordance with a preferred embodiment of the present invention;
and
FIG. 11 is a block diagram showing a secondary station in
25 accordance with a preferred embodiment of the present invention.
Detailed Description
As discussed above, the primary station must be able to
estimate the offered load of the network and react accordingly.
3o The present invention includes techniques for estimating offered
load based on a history of contention outcomes. The present
invention also includes applications for utilizing the estimated
offered load for determining a request interval size and for
3
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determining a contention access mode in a communication
network. The present invention is described herein with reference
to various embodiments.
s 1. Offered Load Estimation Model
In accordance with the present invention, the shared
channel is divided into discrete time slots, and is often referred to
as a "slotted channel." The slotted channel is organized into
successive frames, where each frame consists of a number of
io slots. The number of slots in each frame can be fixed or variable.
For convenience, Tk represents the number of slots in a frame k.
A portion of each frame (referred to as the "request interval") is
used for transmitting requests for contention access, and
particularly for placing reservations for bandwidth. The number of
is slots in each request interval can be fixed or variable. For
convenience, Mk represents the number of slots in the request
interval of the frame k (referred to as "request interval k").
Assuming that R slots are needed to transmit a request, the
request interval k therefore provides MkIR request transmission
20 opportunities in which requests can be transmitted. Although Mk
is typically selected such that MkIR is an integer, there is no
requirement that Mk be so selected, and the value MkIR is
heuristically treated as being a real number for the purpose of
discussion.
25 For each request transmission opportunity in a request
interval, such as request interval k, there will be either (1 ) no
request transmission; (2) a single request transmission; or (3)
multiple request transmissions. When a single request is
transmitted in response to a request transmission opportunity, it is
3o presumed for the sake of discussion that the request is
successful. When multiple requests are transmitted, it is
presumed that the requests collide and are therefore
4
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unsuccessful. For convenience, the three outcomes are referred
to as IDLE, SUCCESS, and COLLISION, respectively.
Offered load estimation was addressed in a paper written by
Frits C. Schoute and published in the IEEE transactions on
Communications, Vol. COM-31, N0.4, April 1983. The paper is
related to the present invention because it provides a solution to a
similar problem, yet, in a different environment and using a different
technique. Schoute attempts to estimate the offered load in a Slotted
Dynamic Frame Length ALOHA environment, where all data is
transmitted in contention and hence reservation is totally absent. In
summary, for each slot having a COLLISION outcome, Schoute
computes the expected number of contending users based on the
known maximum throughput of the system (1/e), and increments the
offered load estimation accordingly. Schoute's solution can be readily
extended to a contention-based reservation context in accordance
with the present invention, provided that the goal is to maximize the
contention throughput in the request interval.
However, the goal of the present invention is not to maximize
the contention throughput in the request interval. Rather, the goal of
2o the present invention is to estimate the offered load based on the
number of observed IDLE, SUCCESS, and COLLISION outcomes in
each request interval k. Therefore, the offered load estimation
techniques of the present invention differ substantially from the
offered load estimation technique of Schoute.
For the sake of simplicity, it is assumed that only certain
requests are eligible for transmission during the request interval k.
Specifically, only those requests that are available for
transmission prior to request interval k (including "new" requests
and requests made as part of a collision resolution scheme) are
3o eligible for transmission in request interval k. Therefore, any
requests that become available for transmission during request
interval k cannot be transmitted in request interval k, and must
5
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wait until request interval (k+1 ). A system that adheres to such a
rule is often referred to as a "gated" system.
Given that the system is gated, all requests that become
available for transmission during frame (k-1 ) will be transmitted
s during request interval k. For convenience, Nk_, represents the
total number of requests that become available for transmission
during frame (k-1 ) that are transmitted during request interval k.
The Nk_, requests can be conceptualized as becoming available
randomly over the Tk_, slots in frame (k-1 ), such that the average
to rate that requests become available during frame (k-1 ) is equal to:
Eq. 1 gk_, = Nk_~ITk_,
Thus, gk_, represents the average number of requests per slot
is over the frame (k-1 ), such that:
Eq. 2 Nk_, = gk_, x Tk_,
Because the Nk_, requests are transmitted in the Mk/R
2o request transmission opportunities in request interval k, it
therefore follows that the average number of requests transmitted
per request transmission opportunity during the request interval k
is equal to:
2s Eq. 3 Gk = Nk_,/(Mk/R)
_ (9k-, x Tk-, )/(Mk/R)
_ (9k-, x Tk-, x R)/Mk
The probability distribution of the number of requests
3o transmitted per request transmission opportunity can be
approximated by the binomial distribution:
Eq. 4 P[m] _
s
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where A is the number of requests that become available during
frame (k-1 ) (i.e., A = Nk_,), B is the number of request transmission
opportunities in frame k (i.e., B = Mk/R), and m is the random
variable representing the number of request transmitted in a
request transmission opportunity.
Therefore, the probability of SUCCESS, IDLE, and
COLLISION outcomes during request interval k can be
io approximated as:
1 1 A-1
Eq. 5 Pk[S] = A x~ x (1-~ )
Eq. 6 Pk~~~ _ ~~' S
is
Eq. 7 Pk[C] = 1 -A x$ x ~1-g~ -IL1 g
-J
Provided that B is large, the binomial distribution P[m] can
be approximated by the Poisson distribution:
Eq. 8 P[m] _ [(A/B)m x exp(-A/B)] I m!
By definition, Gk = AIB. Substituting G~ for A/B in Eq. 8
results in the Poisson distribution:
Eq. 9 P[m] _ [Gkm x exp(-Gk)] / m!
Therefore, the probability of SUCCESS, IDLE, and
COLLISION outcomes during request interval k can be
3o approximated as:
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Eq. 10 Pk[S] = Gk x exp(-Gk)
Eq. 11 Pk[I] = exp(-Gk)
Eq. 12 Pk[C] = 1 - Gk x exp(-Gk) - exp(-Gk)
s
Based on the above probabilities, the expected number of
SUCCESS, IDLE, and COLLISION outcomes during request
interval k is equal to:
Eq. 13 Ek(S) = Pk[S] x MkIR
Eq. 14 Ek(I) = Pk[I] x Mk/R
Eq. 15 Ek{C) = Pk[C] x Mk/R
With respect to request interval k, the values Mk and R are
~s known a priori, and the actual number of SUCCESS, IDLE, and
COLLISION outcomes during request interval k can be measured.
For convenience, the actual number of SUCCESS, IDLE, and
COLLISION outcomes measured during request interval k are
referred to as Sk, Ik, and Ck, respectively. Because Sk, Ik, and Ck
2o are probabilistically equal to Ek(S), Ek(I), and Ek(C), respectively,
any one of Eq. 13, Eq. 14, and Eq. 15 can be used to determine
the estimated offered load gk_, during the request interval {k-1 ).
Working from Eq. 14 and using the measured number of
IDLE outcomes Ik to determine the offered load results in the
2s following transformations:
Eq. 16 Ik = Ek(I)
= Pk(I) x Mk/R
_ (1/R) x Mk x exp(-Gk)
30 (R x Ik)IMk = exp(-Gk)
Substituting Gk from Eq. 3 and solving for gk_, gives the
estimated offered load during the request interval (k-1 ):
a
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Eq. 17 gk_, = [Mk/(R x Tk_,)l x In[Mk/(R x Ik)l
Thus, the estimated offered load gk_, can be calculated
s based on values that are either known a priori (i.e., Mk, R, and Tk_
,) or measurable (i.e., Ik).
In many situations, the estimated offered load determined
according to Eq. 17 may not be an accurate estimate of the actual
offered load. This is because the Poisson distribution according
to to Eq. 8 only approximates a binomial distribution if B is large.
Depending on the number of request transmission opportunities in
a single frame, the value B may or may not be large enough to
ensure that the estimated offered load gk_, is accurate. When the
number of request transmission opportunities in a single frame is
~s not large enough to provide a statistically significant number of
request transmission opportunities, the offered load estimation
model must be adapted.
A. Offered Load Estimation Using Sample Window
2o A first adaptation of the offered toad estimation model
computes the estimated offered load over a number of
consecutive frames. The number of consecutive frames must be
large enough to provide a statistically significant number of
request transmission opportunities, and yet not so large that the
25 offered load varies considerably within the number of frames. For
convenience, the number of frames n over which the estimated
offered load is calculated is referred to as the "sample window."
Assuming that 1; represents the number of IDLE outcomes in
sample window frame i, then the total number of IDLE outcomes
30 over the sample window is equal to:
s
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n
Eq. 18 I = ~li
i=1
As in Eq. 16 above, the number of IDLE outcomes in the
sample window frame i can be estimated by the corresponding
expected number of outcomes as follows:
Eq. 19 I; = E;(i)
= P;[I] x M;IR
= M;IR x exp(-G;)
io
such that:
Eq. 20 I = E M;IR x exp(-G;)
Using the transformations from Eq. 16 and Eq. 17 above, it
is possible to calculate the instantaneous offered load g; for each
sample window frame i based on values that are either known a
priori or measurable. By selecting an appropriate sample window,
it is expected that the instantaneous offered load g; does not vary
2o considerably over the sample window. Therefore, the
instantaneous offered load g; can be approximated by an offered
load g that is the same for each sample window frame i (i.e., g, _
9z=....=9~=9).
Substituting Gk from Eq. 3 into Eq. 20 and substituting g for
2s each g; results in the following:
Eq. 21 I =
Except for the estimated offered load g, all of the elements
30 of Eq. 21 are either known or measurable. An objective of the
present invention is to derive from Eq. 21 an estimator function for
g based on the known and measurable variables, such that g =
,o
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f(l). For now, it is assumed that there is such a function f(I), the
details of which are presented later.
In certain situations, it is desirable to regularly update the
estimated offered load in order to reflect the actual offered load of
the network as it changes over time. In such situations, it is
important that the estimated offered load adapt quickly to changes
in the actual offered load.
One way to update the estimated offered load is to consider
consecutive disjoint sample windows of size n, and to update the
io estimated offered load at the end of each sample window. This
approach is simple, and requires relatively infrequent updates.
However, it is also relatively stow to adapt to changes in the
actual offered load, and can therefore be quite inaccurate if the
actual offered load changes significantly between sample
~s windows.
A more accurate way to update the estimated offered load is
to use a sliding sample window and to update the estimated
offered load each frame. While this approach requires more
frequent updates, is adapts more quickly to changes in the offered
20 load. However, it can still be inaccurate if the actual offered load
changes significantly between frames.
In order to improve the performance of the sliding sample
window approach, a weighting scheme is employed which assigns
a higher weight to the x most recent frames in the sample window.
2s Thus, in a sample window having n frames, the x most recent
frames are assigned a weighting factor a and the (n-x) "older"
frames are assigned a weighting factor Vii, where a > Vii. The total
weight of the frames in the sample window is equal to:
3o Eq. 22 n' = ax + ~3(n - x)
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The weighting factor (i can be arbitrarily set to one (1 }, so
that the total weight of the frames in the sample window is equal
to:
s Eq.23 n' =ax+n-x
=n+(a-1)x
The weighting factor a is selected so that the weight
assigned to the x most recent frames is equal to a predetermined
to percentage X of the total weight n' as follows:
Eq. 24 ax / n' - X
With the assumption that g is constant within the sample
is window, it can be expected that the ratio Y = T;_,/M; (i.e., the ratio
of the frame size to the request interval size} will also be constant
for i = 1 to n. Applying this assumption to Eq. 21 results in:
Eq. 25 I =
For convenience, T represents the weighted average of the
number of slots per frame over the entire sample window as
follows:
2s Eq. 26 T = [To + ... . + T~_x_, + aT"_X + .... + aT~_,] I n'
For convenience, M represents the weighted average of the
number of slots per request interval over the entire sample
window as follows:
Eq.27 M=~M~+....+M"_X+aM~_X+,+....+aM~J/n'
'12
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Substituting M into Eq. 25 results in:
Eq. 28 I - exp(-g x Y x R) x n' x M/R
where Y = T;_11M;. Heuristically, the ratio T;_1/M; can be
approximated by the ratio TIM, such that Y = TIM. Substituting Y
= TIM into Eq. 28 results in:
io Eq. 29 l
The estimator function for g is obtained by taking the natural
logarithm on both sides of Eq. 29 and solving for g as follows:
Eq. 30 g' = f(I) _
where g' is the estimator function for g.
B. Offered Load Estimation Using Single Frame
2o A second adaptation of the offered load estimation model
computes the estimated offered load over a single frame.
Estimating offered load using a single frame is desirable due
particularly to its simplicity in not requiring the maintenance and
evaluation of historical data as required when estimating offered
25 load over a sample window. One problem with estimating offered
load over a single frame, though, is that the number of request
transmission opportunities in a single frame does not represent a
statistically significant sample, and therefore the observed
outcomes for the frame may or may not be indicative of the actual
30 offered load. However, it is known that certain outcomes are
more probable than other outcomes. For example, it is unlikely
(but possible) that there will be all SUCCESS outcomes with no
13
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IDLE or COLLISION outcomes, or an equal number of IDLE and
COLLISION outcomes with no SUCCESS outcomes. Thus, the
set of all possible outcomes can be divided into a set containing
those outcomes that are likely and therefore "trusted," and a set
s containing those outcomes that are unlikely and therefore
"untrusted." If an observed outcome falls within the set of
"trusted" outcomes, then it is used to update the estimated offered
load; otherwise, the observed outcome is ignored and is not used
to update the estimated offered load. The problem then is to
to define the set of "trusted" and "untrusted" outcomes.
Since there are Mk/R request transmission, opportunities in
frame k and each request transmission opportunity results in
either a SUCCESS, IDLE, or COLLISION outcome, then the sum
of the number of outcomes is equal to Mk/R as follows:
~s
Eq. 31 Ik + Sk + Ck = Mk/R
When mapped onto a three-dimensional graph in which Ik,
Sk, and Ck represent the three axes, Eq. 31 defines a planar
2o region ABC as shown in FIG. 2. Planar region ABC contains all of
the possible states of request interval k such that any observed
point Z(Ik,Sk,Ck) falls on the planar region ABC.
Within the planar region ABC, certain points are more likely
than other points as outcomes of request interval k. Assuming the
2s offered load estimation model above is accurate, the most likely
points within planar region ABC are those representing the
expected number of SUCCESS, IDLE, and COLLISION outcomes
according to Eq. 13, Eq. 14, and Eq. 15, respectively, shown as
curve L in FIG. 3A. Thus, the curve L describes a locus of
3o expected outcomes. For convenience, the planar region ABC and
the curve L are shown in two-dimensional view in FIG. 3B. It
should be noted that the maximum probability of SUCCESS is at
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the point P*, which maps to Sk = 0.368, Ik = 0.368, and Ck = 0.264
in a preferred embodiment described in detail below.
One important attribute of the planar region ABC is that the
probability of a particular point is inversely proportional to its
s distance from the curve L (i.e., the closer the point is to the curve
L, the higher the probability). Thus, the planar region ABC can be
divided into regions} having "trusted" points and regions) having
"entrusted" points based generally on the distance of each point
from the curve L.
io In one embodiment, the planar region ABC is divided solely
on the distance from the curve L. Fig. 4 shows a two-dimensional
view of the planar region ABC, with the planar region ABC divided
into three regions according to the distance from the curve L.
Those points falling within a predetermined distance from the
is curve L (i.e., region 2) are considered to be "trusted" points, while
ail other points (i.e., regions 1 and 3) are considered to be
"entrusted" points. While region 2 captures all points meeting at
least a predetermined minimum probability, it is not an easy
region to work with, since it is computationally complex to
2o determine whether a particular point falls within the region.
In another embodiment, the planar region ABC is divided
according to its intersection with three planes Sk = So, Ik = lo, and
Ck = Co, as shown in three-dimensional view in FIG. 5. The three
planes intersect at point P* if So = 0.368 x Mk/R, to = 0.368 x Mk/R,
2s and Co = 0.264 x Mk/R, as shown in two-dimensional view in FIG.
6. Region BB'P*E, corresponds to the state of obtaining many
IDLE outcomes and few COLLISION outcomes in the request
interval k, which is reasonably probable if the effective offered
load within the frame is low. Region CC'P*D corresponds to the
3o state of obtaining many COLLISION outcomes and few IDLE
outcomes in the request interval k, which is reasonably probable if
the effective offered load within the frame is high. Region EP*D
corresponds to the state of obtaining many COLLISION
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outcomes, many IDLE outcomes, and few SUCCESS outcomes in
the request interval k, which is improbable irrespective of the
effective offered load. Region AB'C' corresponds to the state of
obtaining many SUCCESS outcomes (i.e., with probability greater
s than 0.368) with few COLLISION and IDLE outcomes, which is
desirable but improbable if the offered load estimation model is
accurate.
Except for the point P*, which falls in all regions, all points
on the curve L fall within either region BB'P*E or CC'P*D. Thus,
io regions BB'P*E and CC'P*D are good candidates for containing
"trusted" points. However, there are also points within regions
AB'C' and EP*D that are close to the curve L and are therefore
likely to be "trusted" points. In order to capture those "trusted"
points, the three planes are redefined such that So = 0.4 x Mk/R, lfl
1s = 0.4 x Mk/R, and Co = 0.3 x Mk/R, as shown in two-dimensional
view in FIG. 7. As a result, plane So now falls above point P*, and
planes to and Co intersect well below point P* at point X. Points
that fall within either region AB'C' (i.e., Sk > So) or region EXD
(i.e., Ck > Co and Ik < lo) are considered to be "entrusted" points,
2o while points that fall within region BB'C'CDXE are considered to
be "trusted" points.
2. Some Applications Utilizing Estimated Offered Load
As discussed above, the problem of estimating offered load
25 in a communication network is a generic problem with many
applications. One important application utilizes the estimated
offered load to improve access performance in a shared medium
communication network. Specifically, the estimated offered load
is used for determining certain operating parameters such as the
3o number of request transmission opportunities per frame and
certain access mode parameters that affect how the network is
accessed (described in more detail below).
is
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FIG. 8 shows a shared medium communication network 100
in accordance with a preferred embodiment of the present
invention. The shared medium communication network 100
allows a number of end users 110, through 110N to access a
s remote external network 108 such as the Internet. The shared
medium communication network 100 acts as a conduit for
transporting information between the end users 110 and the
external network 108.
The shared medium communication network 100 includes a
1o primary station 102 that is coupled to the external network 108.
The primary station 102 is in communication with a plurality of
secondary stations 104, through 104N (collectively referred to as
"secondary stations 104" and individually as a "secondary station
104") by means of channels 106 and 107. Channel 106 carries
1s information in a "downstream" direction from the primary station
102 to the secondary stations 104, and is hereinafter referred to
as "downstream channel 106." Channel 107 carries information in
an "upstream" direction from the secondary stations 104 to the
primary station 102, and is hereinafter referred to as "upstream
2o channel 107." Each end user 110 interfaces to the shared
medium communication network 100 by means of a secondary
station 104.
In a preferred embodiment, the shared medium
communication network 100 is a data-over-cable (DOC)
25 communication system wherein the downstream channel 106 and
the upstream channel 107 are separate channels carried over a
shared physical medium. In the preferred embodiment, the
shared physical medium is a hybrid fiber-optic and coaxial cable
(HFC) network. The downstream channel 106 is one of a plurality
30 of downstream channels carried over the HFC network. The
upstream channel 107 is one of a plurality of upstream channels
carried over the HFC network. In other embodiments, the shared
physical medium may be coaxial cable, fiber-optic cable, twisted
,z
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pair wires, and so on, and may also include air, atmosphere, or
space for wireless and satellite communication. Also, the various
upstream and downstream channels may be the same physical
channel, for example, through time-division
s multiplexing/duplexing, or separate physical channels, for
example, through frequency-division multiplexing/duplexing.
In the shared medium communication network 100 of the
preferred embodiment, the downstream channels, including the
downstream channel 106, are typically situated in a frequency
band above approximately 50 MHz, although the particular
frequency band may vary from system to system, and is often
country-dependent. The downstream channels are classified as
broadcast channels, since any information transmitted by the
primary station 102 over a particular downstream channel, such
1s as the downstream channel 106, reaches al) of the secondary
stations 104. Any of the secondary stations 104 that are tuned to
receive on the particular downstream channel can receive the
information.
In the shared medium communication network 100 of a
2o preferred embodiment, the upstream channels, including the
upstream channel 107, are typically situated in a frequency band
between approximately 5 through 42 MHz, although the particular
frequency band may vary from system to system, and is often
country-dependent. The upstream channels are classified as
25 shared channels, since only one secondary station 104 can
successfully transmit on a particular upstream channel at any
given time, and therefore the upstream channels must be shared
among the plurality of secondary stations 104. If more than one
of the secondary stations 104 simultaneously transmit on a
3o particular upstream channel, such as the upstream channel 107,
there is a collision that corrupts the information from all of the
simultaneously transmitting secondary stations 104.
,8
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In order to allow multiple secondary stations 104 to share a
particular upstream channel, such as the upstream channel 107,
the primary station 102 and the secondary stations 104 participate
in a medium access control (MAC} protocol. The MAC protocol
s provides a set of rules and procedures for coordinating access by
the secondary stations 104 to the shared upstream channel 107.
Each secondary station 104 participates in the MAC protocol on
behalf of its end users. For convenience, each participant in the
MAC protocol is referred to as a "MAC User."
to MAC protocols fall into two basic categories: contention-free
and contention-based protocols.
In contention-free protocols, end users access a shared
channel in a controlled manner such that transmissions are
scheduled either statically, or adaptively so that collisions are
is completely avoided. With static scheduling, such as that of a Time
Division Multiple Access (TDMA) scheme, a predetermined
transmission pattern is repeated periodically. The users may access
channel resources only during the time intervals assigned to them
individually. Contention-free protocols with static scheduling for
2o resource allocation is inefficient for a cable network supporting a large
number of users where, typically, only a fraction of the users are
active at any time. With adaptive scheduling, the transmission
pattern may be modified in each cycle to accomodate dynamic traffic
demand, via reservations or token passing. A fraction of the multiple
25 access channel, or a separate channel, is used to support the
overhead due to reservation, or token passing. A reservation scheme
typically requires a centralized controller to manage the reservations.
A token passing scheme, on the other hand, is usually implemented
in a distributed manner. Contention-free protocols with adaptive
3o scheduling are sometimes referred to as demand assignment multiple
access.
In contention-based protocols, users contend with one another
to access channel resources. Collisions are not avoided by design,
,s
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but are either controlled by requiring retransmissions to be randomly
delayed, or resolved using a variety of other contention resolution
strategies. The broadcast capability of a network, such as an HFC
cable network, can often be taken advantage for simplified control in
s the MAC layer. One approach for delaying retransmissions is a
binary exponential backoff approach, wherein a backoff window limits
the range of random backoff, and an initial backoff window is doubled
in successive attempts for retransmission. As the binary exponential
backoff approach is known to lead to instability in heavy load, the
io maximum number of retransmissions for a request can be used to
truncate the otherwise indefinite backoff.
Most contention-based protocols resolve collisions by using
feedback information on the number of users involved in the
collisions. If the number of conflicting transmissions can be
~5 determined from the feedback, then channel throughput arbitrarily
close to one packet per packet transmission time is known to be
achievable in principle, but with intractable complexity. More often
than not, for the sake of simplicity, feedback information used is
ternary indicating zero, one, or more transmissions, or binary
2o indicating exactly one transmission or otherwise.
An example of a contention-based protocol is known as an
ALOHA multiple access protocol. Its original version, which operates
with continuous or unslotted time, is referred to as Unslotted ALOHA.
Another version, which operates with discrete or slotted time, is
2s referred to as Slotted ALOHA. The behavior and performance of
Unslotted and Slotted ALOHA have been studied widely, and their
maximum throughputs are well known to be 11(2e) and 11e,
respectively.
One type of MAC protocol suitable for HFC cable networks
3o utilizes a reservation system in which each MAC User that wants to
transmit data on the shared channel is required to make a
reservation. in contention-free protocols with adaptive scheduling,
users with pending transmissions must reserve transmission
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resources. The protocol for reservation is itself a multiple access
protocol.
In the shared medium communication network 100, many users
share the upstream channel 107 for transmissions to the primary
station 102. However, at any time, it is likely that only a fraction of
these users are actually busy. If a contention-free protocol with static
scheduling (e.g., TDMA) is used, resources allocated to idle users are
wasted. This inefficiency is particularly intolerable when the load in
the system is light. Contention-based protocols behave well under
light load, but have limited throughput when offered load is high due
to excessive collisions.
The throughput of a reservation-based system is limited by the
percentage of available bandwidth that is allocated to the reservation
control channel. One approach for reducing the demand on the
reservation channel is to allocate a small field in the data packets for
piggy-backing additional requests (i.e., including a request along with
the transmitted data).
During steady-state operation, the number of users waiting to
to make reservations is typically small, particularly when piggybacking is
permitted. It is therefore advantageous for the reservation protocol
be a contention-based protocol with contention resolution. Unlike in a
conventional contention-based protocol, the users typically do not
contend with data packets, but instead contend with special
~5 reservation packets that are considerably smaller than data packets.
In a multiple access system with contention-based reservation,
each MAC User that has data to transmit but has not already made a
reservation waits for contention opportunities provided by the primary
station 102. Each contention opportunity is provided by the primary
2o station to a selected group of MAC Users, and allows each of the
MAC Users in the specified group to contend for a reservation at a
specific time, provided the MAC User has data to send.
Following each contention opportunity, the primary station 102
monitors for contention by the MAC Users and determines the
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outcome of contention for each contention opportunity, specifically
whether no MAC User contended, exactly one MAC User contended,
or more than one MAC User contended. For convenience, the
contention outcomes are referred to as IDLE, SUCCESS, and
s COLLISION, respectively. The primary station 102 then sends
feedback information to the MAC Users indicating the outcome of
contention for each contention opportunity. The feedback information
allows each MAC User to determine, among other things, whether or
not its own contention attempt was successful, and hence whether its
to request for bandwidth reservation has been accepted.
The advantage of a reservation system over a simple TDMA
system is derived from the fact that request packets used for
reservation, either in contention mode or a time division mode, are
considerably smaller than most data packets. !n a contention-based
is reservation system, the bandwidth wasted due to IDLE or
COLLISION outcomes is relatively small compared to the bandwidth
used for actual data transmission. Suppose that the ratio of the size
of a request packet to that of a data packet is v « 1, and that a
simple Slotted ALOHA multiple access scheme is used in the logical
2o control channel for contention-based reservation. Then, it can be
verified that the maximum throughput, S, in data packets per time
unit, achievable in such a system is given by:
1 1
Eq. 32 S = +v r
2s
where Sr is the maximum throughput for Slotted ALOHA, which is
equal to 11e (see Bertsekas and Gallager, Data Networks, Section
4.5, Prentice-Hall, 1987).
A reservation-based MAC protocol can be represented at a high
30 level by a state diagram for each MAC User as shown in FIG. 9. The
MAC User starts in the INACTIVE state, and remains there so long as
it has no data to transmit or it is waiting for an opportunity to transmit
22
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a request. When the MAC User receives data to be transmitted, the
MAC User transitions into the ACTIVE state upon receiving a
contention-free opportunity to transmit a request, provided it is not
required to contend for upstream bandwidth, as in the case of unicast
polling. Otherwise, the MAC User transitions into the CONTENTION
state upon receiving and transmitting a request in a transmission
opportunity for contention. In the CONTENTION state, the MAC User
contends for access to the channel until it is able to make a
successful reservation for itself, or until its request is rejected due to
to system overload. Upon making a successful reservation in this state,
the MAC User transitions into the ACTIVE state. Here, the MAC User
receives opportunities to transmit its data, and remains in the ACTIVE
state so long as it has data to transmit. During system overload, a
requests pending contention resolution may be denied further
is retransmission opportunities after a predetermined number of
attempts. When this happens, the MAC User moves from the
CONTENTION state to the INACTIVE state. If new data arrives while
the MAC User is in the ACTIVE state, the MAC User may be
permitted to include a piggyback request in the data it transmits.
2o Upon transmitting all of its data, the MAC User transitions back into
the INACTIVE state.
For each request transmission opportunity provided by the
primary station 102, the primary station receives either (1 ) no
transmission, indicating that no MAC User transmitted a reservation
25 request; {2) a reservation request, indicating that a single MAC User
transmitted a reservation request and identifying that MAC User; or
(3) a collision, indicating that more than one MAC User transmitted a
reservation request. For convenience, the three feedback states are
referred to as IDLE, SUCCESS, and COLLISION, respectively.
3o The primary station 102 schedules future request transmission
opportunities and data transmission opportunities based on the result
of the contention-based reservation. If a successful reservation is
made (i.e., if the result of the contention is SUCCESS), then the
23
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primary station 102 allocates bandwidth to the MAC User based on
the QoS requirements of the corresponding end user so that the MAC
User can transmit user information contention-free over the shared
channel. On the other hand, if multiple MAC Users respond (i.e., if
s the result of the contention is COLLISION), then the primary station
102 attempts to aid in resolving the collision by providing additional
request transmission opportunities.
In a preferred embodiment, the MAC protocol includes a
protocol commonly referred to as Multimedia Cable Network
io System (MCNS), which is defined in the document entitled MCNS
Data-Over-Cable Service Interface Specifications Radio
Frequency Interface Specification SP-RFI-102-971008 Interim
Specification (hereinafter referred to as the "MCNS Protocol
Specification"), incorporated herein by reference in its entirety. In
is the MCNS Protocol Specification, the primary station 102 is
referred to as a Cable Modem Termination System (CMTS}, and
the secondary stations 104 are referred to as Cable Modems
(CMs). The CMTS is responsible for packet processing, resource
sharing, and management of the MCNS MAC and Physical layer
2o functions. Each CM operates as a slave to the CMTS. MAC
Protocol Data Units (PDUs) transmitted on the downstream
channel 106 by the CMTS may be addressed to an individual CM
via unicast, or to a selected group of CMs via multicast or
broadcast. In the upstream channel, a MAC PDU may be sent by
2s any CM to the CMTS. MCNS supports variable length MAC
PDUs.
The MCNS Protocol Specification utilizes a slotted upstream
channel, such that the upstream channel 107 is divided into
successive time slots. The MAC protocol supports a plurality of
3o slot types for carrying different types of information. Each time
slot is capable of transporting a unit of information (for example, a
data packet or a control packet). The MCNS Protocol
Specification further divides the upstream channel 107 into
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successive frames, where each frame includes a number of slots.
The CMTS allocates bandwidth to a group of CMs by transmitting
on the downstream channel 106 a control message containing a
bandwidth allocation information element known as a MAP. The
s MAP specifies the allocation of transmission opportunities within a
given transmission frame. Bandwidth is allocated, frame by
frame, in terms of transmission opportunities for contention-based
reservation requests (or simply requests) as well as for user data.
A successful transmission in a contention opportunity results in
the reservation of a future data transmission opportunity.
More specifically, the upstream channel 107 is modeled as a
stream of mini-slots, providing for TDMA at regulated time ticks. The
use of mini-slots implies strict timing synchronization between the
CMTS and all the CMs. Hence, the CMTS is responsible for
~s generating the time reference to identify these mini-slots and
periodically allow for ranging opportunities so that all CMs maintain
their synchronization. The access to the mini-slots by the CMs is
controlled by the CMTS. To accomplish that, the CMTS transmits on
the downstream channel a MAP describing the use of each upstream
2o mini-slot in a specified future time interval. This message, in a way,
"maps" in a future time interval each mini-slot to its use. Of course,
the MAP has to be sent by the CMTS earlier than the effective time
interval that it describes in order to allow enough time for the CMs to
transmit in the mapped mini-slots.
2s In the MCNS Protocol Specification, each frame is organized
into discrete intervals. At least three different interval types are
defined. A request interval includes a number of mini-slots that are
allocated for transmitting requests (or small data packets) in
contention mode. A maintenance interval includes a number of mini-
3o slots allocated for registration of CMs. A data grant interval includes
a number of mini-slots allocated for transmitting data packets. The
MAP includes a number of information elements (IEs) that define the
different intervals in the frame.
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FIG. 10 is a block diagram showing an exemplary primary
station 102 in accordance with a preferred embodiment of the
present invention. In the preferred embodiment, the primary
station 102 includes a number of functional modules implemented
s on individual cards that fit within a common chassis. In order to
enable communication within the shared medium communication
network 100, the primary station 102 requires at least a minimum
set of functional modules. Specifically, the minimum set of
functional modules comprises an Adapter Module 210, a MAC
to Module 220, a Transmitter Module 240, and a Receiver Module
230. In the preferred embodiment, the minimum set of functional
modules allows the primary station 102 to support a single
downstream channel and up to eight upstream channels. For the
sake of convenience and simplicity, the exemplary embodiments
is described below refer to the single upstream channel 107,
although it will be apparent to a skilled artisan that multiple
upstream channels are supportable in a similar manner.
The Adapter Module 210 controls the flow of data and
control messages between the primary station 102 and the
2o secondary stations 104. The Adapter Module 210 includes
Control Logic 218 that is coupled to a Memory 212. The Control
Logic 218 includes, among other things, logic for processing data
and control (e.g., request) messages received from the secondary
stations 104, and logic for generating data and control (e.g., MAP)
2s messages for transmission to the secondary stations 104. The
Memory 212 is divided into a Dedicated Memory 216 that is used
only by the Control Logic 218, and a Shared Memory 214 that is
shared by the Control Logic 218 and MAC Logic 224 (described
below) for exchanging data and control messages.
3o The Control Logic 218 and the MAC Logic 224 exchange
data and control messages using three ring structures (not
shown) in the Shared Memory 214. Data and control (e.g.,
request) messages received from the secondary station 104 are
26
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stored by the MAC Logic 224 in a Receive Queue in the Shared
Memory 224. Control (e.g., MAP) messages generated by the
Control Logic 218 are stored by the Control Logic 218 in a MAC
Transmit Queue in the Shared Memory 214. Data messages for
transmission to the secondary station 104 are stored by the
Control Logic 218 in a Data Transmit Queue in the Shared
Memory 214. The Control Logic 218 monitors the Receive Queue
to obtain data and control (e.g., request) messages. The MAC
Logic 224 monitors the MAC Transmit Queue to obtain control
~o (e.g., MAP) messages, and monitors the Data Transmit Queue to
obtain data messages.
The MAC Module 220 implements MAC functions within the
primary station 102. The MAC Module 220 includes MAC Logic
224 that is coupled to a Local Memory 222 and to the Shared
is Memory 214 by means of interface 250. The MAC Logic 224
monitors the MAC Transmit Queue and the Data Transmit Queue
in the Shared Memory 214. The MAC Logic 224 transmits any
queued data and control (e.g., MAP) messages to
Encoder/Modulator 241 of Transmitter Module 240 by means of
2o interface 253. The MAC Logic 224 also processes data and
control (e.g., request) messages received from the Receiver
Module 230 by means of interface 255. The MAC Logic 224
stores the received data and control messages in the Receive
Queue in the Shared Memory 214 by means of interface 250.
25 The Transmitter Module 240 provides an interface to the
downstream channel 106 for transmitting data and control (e.g.,
MAP) messages to the secondary stations 104. The Transmitter
Module 240 includes a Transmitter Front End 242 that is operably
coupled to the downstream channel 106 and an
3o Encoder/Modulator 241. The Encoder/Modulator 241 includes
logic for processing data and control (e.g., MAP) messages
received from the MAC Logic 224 by means of interface 253.
More specifically, the Encoder/Modulator 241 includes encoding
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logic for encoding the data and control (e.g., MAP) messages
according to a predetermined set of encoding parameters, and
modulating logic for modulating the encoded data and control
(e.g., MAP) messages according to a predetermined modulation
s mode. The Transmitter Front End 242 includes logic for
transmitting the modulated signals from the Encoder/Modulator
241 onto the downstream channel 106. More specifically, the
Transmitter Front End 242 includes tuning logic for tuning to a
downstream channel 106 center frequency, and filtering logic for
io filtering the transmitted modulated signals. Both the
Encoder/Modulator 241 and the Transmitter Front End 242
include adjustable parameters, including downstream channel
center frequency for the Transmitter Front End 242, and
modulation mode, modulation symbol rate, and encoding
~5 parameters for the EncoderlModulator 241.
The Receiver Module 230 provides an interface to the
upstream channel 107 for receiving, among other things, data and
control (e.g., request) messages from the secondary stations 104.
The Receiver Module 230 includes a Receiver Front End 232 that
2o is operably coupled to the upstream channel 107 and to a
DemodulatorlDecoder 231. The Receiver Front End 232 includes
logic for receiving modulated signals from the upstream channel
107. More specifically, the Receiver Front End 232 includes
tuning logic for tuning to an upstream channel 107 center
2s frequency, and filtering logic for filtering the received modulated
signals. The DemodulatorlDecoder 231 includes logic for
processing the filtered modulated signals received from the
Receiver Front End 232. More specifically, the
Demodulator/Decoder 231 includes demodulating logic for
3o demodulating the modulated signals according to a
predetermined modulation mode, and decoding logic for decoding
the demodulated signals according to a predetermined set of
decoding parameters to recover data and control (e.g., request)
is
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messages from the secondary station 104. Both the Receiver
Front End 232 and the Demodulator/Decoder 231 include
adjustable parameters, including upstream channel center
frequency for the Receiver Front End 232, and modulation mode,
s modulation symbol rate, modulation preamble sequence, and
decoding parameters for the Demodulator/Decoder 231.
In the preferred embodiment, the primary station 102
includes a configuration interface 254 through which the
adjustable parameters on both the Receiver Module 230 and the
to Transmitter Module 240 are configured. The configuration
interface 254 operabiy couples the MAC Logic 224 to the
Demodulator/Decoder 231, the Receiver Front End 232, the
Encoder/Modulator 241, and the Transmitter Front End 242. The
configuration interface 254 is preferably a Serial Peripheral
is interface (SPI} bus as is known in the art.
FIG. 11 is a block diagram showing an exemplary
secondary station 104 in accordance with a preferred embodiment
of the present invention. The secondary station 104 includes a
User Interface 310 for intertacing with the End User 110. Data
2o transmitted by the End User 110 is received by the User Interface
310 and stored in a Memory 308. The secondary station 104 also
includes a Control Message Processor 304 that is coupled to the
Memory 308. The Control Message Processor 304 participates
as a MAC User in the MAC protocol on behalf of the End User
25 110. Specifically, the Control Message Processor 304 transmits
data and control (e.g., request) messages to the primary station
102 by means of Transmitter 302, which is operably coupled to
transmit data and control (e.g., request) messages on the
upstream channel 107. The Control Message Processor 304 also
3o processes data and control (e.g., MAP) messages received from
the primary station 102 by means of Receiver 306, which is
operably coupled to receive data and control (e.g., MAP}
messages on the downstream channel 106.
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An important consideration that affects performance in the
MCNS MAC protocol is the number of mini-slots allocated to the
request interval in each frame. Assuming that the number of slots per
frame Tk is substantially constant, the number of mini-slots allocated
to the request interval affects the number of mini-slots allocated to the
other intervals, particularly the data interval. A large number of mini-
slots allocated to the request interval decreases the likelihood of
collisions, but also decreases the number of mini-slots allocated for
transmitting data and therefore decreases the data throughput of the
to system. Furthermore, a small number of mini-slots allocated to the
request interval can increase the likelihood of collisions and therefore
decrease the data throughput of the system by preventing successful
requests from reaching the CMTS. Preferably, the number of slots in
the request interval is selected to maximize the likelihood of
is SUCCESS outcomes. This typically involves increasing the number
of slots in the request interval if the offered load is high, and
decreasing the number of slots in the request interval if the offered
load is low. Thus, the offered load is a key consideration in selecting
the number of slots per request interval.
2o Another important consideration that affects performance in the
MCNS MAC protocol is the type of contention access used. In
accordance with the MCNS Protocol Specification, at least two types
of contention access is supported. In a first type of contention
access, the secondary stations 104 are only permitted to transmit
25 request messages during the request interval. In a second type of
contention access, the secondary stations 104 are permitted to
transmit either request messages or small data messages during the
request interval. The second type of contention access can improve
performance when there are few collisions, but can decrease
3o performance when there are many collisions. Therefore, the second
type of contention access would only be utilized when the actual
offered load is low, where the first type of contention access would be
used when the actual offered load is high. Thus, the offered load is a
CA 02333447 2000-11-24
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key consideration in selecting the type of contention access in the
MCNS MAC protocol.
In the MCNS MAC protocol, the offered load is not known a
priori. Therefore, the offered load must be estimated using either the
s sample window technique or the single frame technique described
herein, typically by the Control Logic 218 as an element its MAP
generating logic.
A. Application of the Sample Window Technique
to The estimator function g' from Eq. 30 was derived using a
number of variables. These include the variable n representing the
sample window size; the variable x representing the number of
weighted frames in the sample window; and the variable X
representing the percentage variable that is used in Eq. 24 to derive
is the weighting factor a.
In accordance with a preferred embodiment of the present
invention, the variable n is equal to 16 frames. The variable n to be
large enough that there is a statistically significant number of request
transmission opportunities in the sample window, and yet not so large
2o that the offered load varies significantly over the sample window.
With reference to Eq. 21, it was accepted heuristically that the
instantaneous offered load g; for each frame in a sample window
does not vary considerably over the sample window, and therefore
that the instantaneous offered load g; can be approximated by an
2s offered load g that is the same for each sample window frame i (i.e.,
g, = g2 = .... = g~ = g). Simulations of the MCNS MAC protocol have
shown that a frame seldom exceeds 5 milliseconds in duration, and
the offered load variation during a 100 millisecond time interval is
typically less than ten percent of its origins! value. Therefore, the
3o instantaneous offered load can be approximated by g provided that
the sample window size n is less than approximately 20 frames.
In accordance with a preferred embodiment of the present
invention, the variable x is equal to 3, and the variable X is equal to
31
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0.4 such that a is equal to 3. The selection of these parameter values
will become apparent by understanding why a sliding window
approach with weighting is used.
It should be recalled that a first possibility was to update the
estimated offered load at the end of each sample window. This
approach is very inaccurate, and can lead to oscillations of the
estimated offered load around the actual offered load. To understand
why, assume that at the end of some sample window j, the offered
load is underestimated. Since the estimated offered load is used to
~o select the request interval size for each frame in the next sample
window (j+1 ), the CMTS will set the request interval size for each
frame in the next sample window (j+1 ) to a value smaller than it
should be. This will lead to collisions in frame 1 of sample window
(j+1 ). As these collided requests will be retransmitted in the frame 2
~5 of sample window (j+1 ), the probability of collision is further increased
in frame 2. The process will go on until the end of the sample window
(j+1 ), eventually leading to an extremely high probability of collision at
the end of the window, and hence a very small number of IDLE
outcomes. As a result, the estimated offered load will increase
2o significantly, leading to an over-estimation of the offered load.
Similarly, this over-estimation yields request interval sizes in sample
window (j+2) larger than they should be, resulting in a large number
of IDLE outcomes at the end of sample window (j+2). The result is an
under-estimation of the offered load, causing the cycle to repeat and
25 therefore causing the estimated offered load to fluctuate around the
actual offered load.
It should also be recalled that a second possibility was to
use a sliding sample window and to update the estimated offered
load at the end of each frame without using weighting. While
3o simulation results have shown this approach to be significantly
better than updating the estimated offered load at the end of each
sample window, this approach is also susceptible to fluctuations
around the actual offered load. To understand why, assume that
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at the end of some frame k, the offered load is under-estimated.
Therefore, the request interval in the next frame (k+1 ) will be
smaller than it should be, resulting in a higher probability of
COLLISION outcomes in frame (k+1 ). However, because the
s estimated offered load is determined over a window of 16 frames,
the increased probability of COLLISION outcomes in frame (k+1 )
will have little impact on the estimated offered load, so that the
estimated offered load is likely to increase slightly but remain
under-estimated. This under-estimation will cause more collisions
io in frame (k+2) and subsequent frames, as the actual offered load
increases faster than the estimated offered load due to an
increased number of retransmissions. After several frames, the
sample window will start including more and more of those frames
having a large number of COLLISION outcomes, leading to an
i5 over-estimation of the offered load. Once the estimated offered
load is over-estimated, the request interval sizes will be set bigger
than they should be, resulting in many IDLE outcomes. Again,
because the estimated offered load is determined over a window
of 16 frames, the increased number of IDLE outcomes in the
2o subsequent frames will have little impact on the estimated offered
load, so that the estimated offered load is likely to decrease
slightly but remain over-estimated. After several frames, the
sample window will start including more and more of those frames
having a large number of IDLE outcomes, leading again to an
25 under-estimation of the offered load causing the cycle to repeat
and therefore causing the estimated offered load to fluctuate
around the actual offered load.
These examples demonstrate that fluctuations can occur
about the actual offered load when the response to changes in the
3o actual offered load is too slow. With reference to the sample
window approach, the slow response problem would not exist if
the sample window is small. However, reducing the size of the
sample window also reduces the number of request transmission
33
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opportunities in the sample window, which degrades the accuracy
of the estimation. Thus, the sample window must remain
relatively large, but the estimated offered load must adapt quickly.
The solution of course is to add a weighting factor that
emphasizes the most recent frames and yet still considers a large
number of frames. As discussed above, the x most recent frames
are given a weighting factor of a, while the older (n-x) frames are
given a weighting factor of ~i that is arbitrarily set to one. The
weighting factors must be selected appropriately to obtain fast
to response and an accurate estimated offered load. If the ratio a/~
is too small or if x is too large, then not enough weight is given to
the most recent frames, and therefore the estimated offered load
will adapt too slowly. On the other hand, if the ratio a/(3 is too
large, then too much weight is given to the most recent frames,
i5 and therefore the estimated offered load will be inaccurate.
Simulation results have shown that good performance is
obtained when x is equal to 3 and X is equal to 0.4, such that a is
equal to 3. This selection of x is consistent with the expected
operation of the MCNS MAC protocol. Because the starting
2o backoff window size does not exceed Mk/R and the ending backoff
window size does not exceed 2xMk/R, any retransmissions from
collisioons in frame (n-2) will all show, with a high probability, in
frames (n-1 ) and n. For the same reason, the effect of collisions
in frames prior to (n-2) is minimal in frame n. Therefore, frames
25 (n-2), (n-1 ), and n are most representative of the actual offered
load for frame (n+1 ), and therefore are weighed more heavily than
the prior frames.
B. Application of the Single Frame Technique
3o In accordance with a preferred embodiment of the present
invention, the Control Logic 218 determines the number of IDLE,
SUCCESS, and COLLISION outcomes during the request interval k,
3a
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referred to as Ik, Sk, and Ck, respectively. Based on the number of
IDLE, SUCCESS, and COLLISION outcomes during the request
interval k, the Control Logic 218 decides whether the outcomes
represents a "trusted° or "untrusted" point. Specifically, the outcomes
s are deemed to represent an "untrusted" point and are ignored if Sk >
So or if Ck > Co and Ik < la (where So = 0.4 x Mk/R, to = 0.4 x Mk/R, and
Co = 0.3 x Mk/R). Otherwise the outcomes are deemed to represent a
"trusted" point and are used to update the estimated offered load.
All logic described herein can be embodied using discrete
components, integrated circuitry, programmable logic used in
conjunction with a programmable logic device such as a Field
Programmable Gate Array (FPGA) or microprocessor, or any
other means including any combination thereof. Programmable
logic can be fixed temporarily or permanently in a tangible
is medium such as a read-only memory chip, a computer memory, a
disk, or other storage medium. Programmable logic can also be
fixed in a computer data signal embodied in a carrier wave,
allowing the programmable logic to be transmitted over an
interface such as a computer bus or communication network. All
2o such embodiments are intended to fall within the scope of the
present invention.
The present invention may be embodied in other specific
forms without departing from the essence or essential
characteristics. The described embodiments are to be considered
2s in all respects only as illustrative and not restrictive.
We claim:
