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A METHOD AND A DEVICE FOR DELAY REDUCTION IN A COI~~LJNICATION
NETWORK
TECHNICAL FIELD
The present invention relates to reduction of a total delay of
the traffic in a communication network carrying ATM traffic,
wherein a multitude' of users share a Gammon physical medium. The
reduction is particularly advantageous and useful in the use of
so called CBR traffic (Constant Bit Rate) which has a low
t0 tolerance for delay in the network.
PRIOR ART
In transmission of speech via the telephone network it is
important to limit the total delay from transmitter to receiver
(round trip delay). If the delay exceeds about 50 ms in for
example a telephone connection an ero canceller is required
since the delay then becomes annoying to the persons speaking
because about this point the persons start to speak at the same
time. When these 50 ms in different standards and specifications
were allocated to different local, transit and international,
stations the acce:>s network was constituted by a twin cable
having no delay. This results in that nowadays when transmission
systems, switching and ATM (Asynchronous Transfer Mode) are
introduced also in the access network there is no time from the
"delay budget" to allocate. This results in tough requirements
regarding a low total delay in the access network.
The reason for the delay in traditional local networks can also
be expressed by observing the fact that transport of large data
packages and use oi_' a store-and-forward technique in routers and
bridges takes time. First there is a delay when the user
accumulates data i:or filling a whole data package. The same
delay effect is repeated every time the package is received by a
bridge or a router which has to await the entire package before
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it can be transmitaed onwards to the next link. The delays vary
when the package waits in a queue at links, having a heavy load.
If all packages are given the same priority there is no
possibility for aytime critical package to go past the queue.
Regarding the queue problem, ATM has a priority function which
makes it possible :for time critical traffic to go past a queue.
Also, a terminal can demand that a certain bandwidth is
allocated for a particular channel, which guarantees a required
capacity. The system relies on that the traffic is classified as
either Constant Bit Rate (CBR) Variable Bit Rate (VBR) or
Available Bit Rate (ABR). A terminal which intends to transmit a
video or audio sequence via CBR or VBR must reserve the required
bandwidth when the connection is set up.
If all traffic has the same priority the problem will of course
remain. The problem will be particularly big when a narrow band
CBR signal (speech) packed in ATM cells is to be transmitted and
when the physical layer only offers a fixed low transmission
rate. This is often the case in the uplink direction (the
traffic from the users or/and equipment towards a common node,
for example a local exchange station o.r a so called "head end" )
when PON (Passive Optical Network) and COAX system based on TDM
(Time Division Multiplex) are used for transmitting ATM traffic.
If the delay also varies over time, this will further add to the
total delay at th.e receiver. The more the delay varies the
larger a CDV buffer (Call Delay Variation) will be required.
For example, when .a single bus is used by a multitude of nodes
for transmitting cells in the uplink direction (from user nodes
towards a common node) in a TDM based network, the physical
medium, in this case the bus, is a common resource which must be
used in the best possible manner. Since only one user at the
time can transmit on the bus the cell in the user nodes must
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wait for "their turn" before they can be transmitted over the
physical medium.
This problem arises within a number of application areas where a
multitude of users are to share a time multiplex common physical
medium. Closest at. hand is speech (telephone) and transmission
using a low constant bandwidth over an ATM connection. Another
area is cable television. When studying the state of the art it
turns out that mo~;t documents are found in the technical field
of cable television. However, no document shows how one can
minimize said delay time.
US 5,546,199 discloses a method for generation (synthesis) of a
carriers for the uplink direction in a cable television system
by using a reference frequency in the downlink direction. The
object is to provide a low-cost carrier having an exact
frequency. The carrier is then used in the common medium, in
this case a coaxial cable. However, the document does not
disclose how one can reduce the total delay in the system. Other
documents found, for example US 4,553,161 describes
synchronization of uplink data traffic.
SUi~ARY
The invention shall, using a device and a method in a
communication network where a multitude (n) users/nodes share a
common physical medium, reduce the total delay in transmission
of data package generated at a fixed rate.
The best or simplest would of course be if the connection had a
bandwidth which waa considerably larger than the need and that
there was a function for dynamic bandwidth allocation which
immediately generated transmission permission when a data
package/cell is ready in one of the nodes. As a difference to
this ideal case the bandwidth which is to be shared by several
nodes is usually very limited and fully used for payload.
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If all traffic has the same priority it will not help to have
dynamic allocation of transmission permission. In the case when
all nodes simultaneously have a cell ready to transmit the last
cell will have to wait a whole period T, where T is the time
between two cells from a particular connection (the period) when
all other nodes must have a transmission permission each during
that time.
According to the invention the problem is solved by a device and
a method where the generation of ATM cells is synchronized at
the users with the generation of. uplink time slots on the common
physical medium so that a cell is ready to be transmitted in
exactly the same moment when it receives a transmission
permission. The condition is that an ATM cell can be fitted in a
time slot. Hence, it is required that the generation of ATM
cells is synchronized at the user nodes with the allocated time
slots in the uplink channel. The synchronization can be
implemented in a number of ways but the important thing is that
the generated ATM cells arrive at the same rate as the time
slots on the common physical medium, so that the data packages
can be transmitted directly when they have been packed onto said
medium without delay. The random phase which is a result between
the time slots and the ATM generation in an initial state can be
corrected, for example, using a phase delay device and a phase
detector or simply by not using a cell and to start the next
cell in the corre<a phase. It is assumed that TDM is used and
that in the normal case each connection make use of a fixed
"time slots" for transmission of ATM cells. A frame can be
defined as n time slots, one for each connection. A connection
uses the "same" time slot in all frames. The period for the
frames is constant. It is also possible that the stream of time
slots is not defined in frames of the same length but only as a
constant flow of 'time slots. It is however important that the
period between two time slots intended for a particular
connection is const=ant .
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The advantages which are obtained using this method are clear.
The short delay for the packed ATM cells in the user nodes
contribute to reduce the total delay in the network.
5 It is assumed that the characteristics of the invention defined
in the appended claims are new. However the implementation,
function and other advantages are best understood using the
description and the accompanying drawings.
DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will be described below in
conjunction with the accompanying drawings in which:
figure 1 schematically and simplified shows the uplink traffic
in a PON/Coax system having a multitude of nodes and
a common physical medium according to the state of
the art"
figure 2 shows a block diagram in an environment where the
invention as described below is useful,
figure 3 shows a block diagram of how a user node is
implemented and a method of performing
synchronization according to the invention,
figure 4 shows a block diagram of how a user node is
implemented and an additional method of performing
synchronization according to the invention.
DESCRIPTION OF A PREFERRED EMBODIMENT
Figure 1 shows the uplink direction for a PON/Coax system having
n nodes. The delay in the uplink traffic from n nodes towards a
headend according to figure 1 mainly consists of two parts,
partly the packing into ATM cells and partly the delay time
before the cell can be transmitted.
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In order to obtain the correct background and comprehension it
can be suitable t:o use two examples. In the first example
dynamic bandwidth allocation is used and in this case some form
of MAC function (M:edium Access Control) for continuous control
and allocation of 'the available capacity between connections or
groups of connections is used. The object is to as soon as
possible transmit a cell when it is ready, i.e. to dynamically
control the bandwidth allocation. Here we use:
n = max number of connections
t = the time to transmit a cell, which depends on the
capacity of the connection.
T = the time between 2 cells from a connection (the period),
(64 kb/s voice connection results in T 6 ms).
A condition for the' capacity of the connection to be enough for
all n connections is thus that nxt T or equivalently that n
T/t.
At maximum use of the present bandwidth: n = T/t.
The waiting time varies and the maximum waiting time occurs when
all connections have a cell to transmit at the same time (= n
cells). The last cell then will have the maximum waiting time T.
This is however not likely, but the available bandwidth often
limits n to such low values that the probability for a waiting
time close to T is not negligible.
If we instead have a static bandwidth allocation, which we have
in the invention, consecutive transmission permissions are given
to each ATM connection at a fixed time interval T1 regardless if
there is a cell to transmit or not. The method results in a
polling at the frequency fl - 1./T1 per connection. A
sufficiently high capacity for the connection is obtained when
T1 T. In order to use the available capacity of the connection
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maximally, T1 is to be chosen as large as possible but not
larger than T. For 69 kb/s speech T 6 ms. In such a non-
synchronized polling method the waiting time has a uniform
distribution in they interval 0 - T1.
The conclusion is that the waiting time varies continuously for
the two described methods between 0 and 6 ms. The waiting time
can however be controlled as a difference from the packing
delay which is fixed and not controllable.
l0 When using the proposed method most of the delay is avoided
except for the part which is a result from the packing. The
total delay for these parts is therefore reduced to about 6 ms.
It should however be noted that any additional delay which can
result during transmission or in queues are not taken into
account.
The description bE~low mainly concerns transmission of 69 kb/s
coded speech, but can also be used for other data rates.
Hence, the conditions are that TDM is used and that in the
normal case each connection makes of a use of a fixed "time
slot" for transmission of ATM cells. A frame is defined as n
time slots, one for each connection. A connection uses the
"same" time slot in all frames. The period for the frames is
constant.
It is also assumed that the uplink and downlink direction on the
common medium and the generation of uplink CBR cells (implicitly
also the reference: frequency for the speech coding, normally 8
kHz) have a common synchronization source.
The waiting time can then be reduced to almost zero if two
conditions are fulfilled. Firstly the generation of ATM cells
has to be synchronous with the corresponding time slot, i.e. the
same number of ce:Lls as time slots per time unit. Secondly the
ATM cells must be ;packed and ready for transmission just before
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transmission in the allocated time slot. The latter requires
that either the time (the phase) when the ATM cell is ready for .
transmission relative the used time slot can be controlled/
chosen or that a "'suitable" free time slot can be allocated to
the connection. The latter method should be avoided since it is
dependent on other setup connections.
The rate of ATM ce~_ls is determined by the AAL being used and by
the 8 kHz signal which is normally used for analogue/digital
conversion of a speech signal, i.e. that the nominal frequency/
generation of cells is fixed and cannot be controlled. The
consequence hereof is that it is the nominal period of time for
the allocated time slots that must be adapted to the generation
of ATM cells, and n.ot vice versa.
IS Here we describe speech signals and since such high quality
standards are put .on them that they must be sent by CBR, it is
preferably speech signals which benefit from the invention. It
is however not restricted to speech signals, but other types of
signals having a constant bit rate can of course be transmitted
using the same basic concept of the invention.
When a number of user nodes are to transmit cells towards a
common node the term uplink direction is used and when the
common node transmits towards the user node the term downlink
direction is used. Given that the used time slot in the uplink
channel and the ATM cell generation have the same nominal period
of time the ATM cell generation can be synchronized to the
uplink channel in the following manner: First the frequency. is
synchronized, i.e. the period of time for the generated ATM
cells is adapted/synchronized to the period of time for the
allocated time slot:, giving a random phase between the time slot
and the ATM cell gE~neration. Next, the phase is adjusted so that
each generated ATM cell becomes ready/packed just in time before
it is to be transmitted in "its" time slot, i.e. just before or
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at the same time as the time slot is transmitted so that it can
be placed in the time slot.
The method implies that regardless of in which node an ATM cell
is generated the phase is adapted to the allocated time slot.
The result is that: the waiting time can be made arbitrarily
short.
Figure 2 shows an example of an environment where the present
invention is of great use. The figure shows a HPC (Hybrid-Fiber-
l0 Coax) cable television system 1 where a multitude of
subscribers/nodes 2:-4 share a common physical medium, in this
case a bus 5. For reasons of simplicity only three nodes are
shown but it is to be understood that the number can be much
greater.
IS The cable television network is interactive which means that the
subscribers can transmit information in the uplink direction 6.
This embodiment gives a good example of the use of the invention
and is intended to show a practical implementation. In reality
the invention can of course be used in all systems with simplex
20 or duplex communication in a network where a multitude of
nodes/subscribers share a common physical medium and where
communication with a common unit takes place.
In this example the: bus 5 can be a conventional coax cable, but
which often in the uplink direction 6 is electrically/optically
25 converted in a converter 7.
The bus 5 is as mentioned connected to a number of subscribers
or nodes 2-4. One node 4 has been enlarged and is shown in the
figure in more detail and one can here see an example of how the
connection can loolc like. A network terminal 8 is connected as
30 an interface towards the bus 5. The network terminal will be
described in more detail below. If this is used as a cable
television network, a TV 9 is suitably connected as an external
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unit for reception of downlink data 10. In order to transmit
data in the uplinh direction 6, a circuit emulator 11 can be
connected to the neawork terminal 8 according to the figure. The
5 circuit emulator J!1 is then e.g. used for packing the data
generated by the subscriber in ATM cells. In order to transmit
data in the uplink direction 6 the subscriber can use a computer
12 or an ordinary telephone 13 which then is connected to the
node 4. The telephone then of course requires an A/D-conversion
10 2 9 .
The system has an antenna 14 for reception of the TV-signals
which then are transmitted to the subscribers. The module 15
illustrates a head-end in a HFC network and provides the
telephony function in an interface towards PSTN 16 or the like.
The module comprises a MAC (Medium Access Control). In this it
is decided which subscriber who is allowed to transmit and when.
In the same module 15, modulation of the carrier can also take
place. It is common to use a downlink QAM modulation and a QPSK
uplink demodulation. The signals from the module 15 and the
signals from the antenna 14 can be transmitted on the same
medium 19 by means of combining them using a combiner 17.
Figure 3 shows the network terminal 8 and the circuit emulator
11 in more detail. The circuit emulator 11 comprises, besides an
ATM cell receiver 20 and a PLL 21 (Phase Locked Loop), also a
transmitter 22 where the ATM cells are generated and an A/D
converter 29. In this embodiment the A/D converter 29 receives
data from a telephone 13 (see figure 2) via the link 18. The
emulator 11 is thug connected to the network terminal 8 which in
turn has a connection to the bus 5.
When one then without imposing a delay adjusts the phase of the '
generated digital speech signal so that each generated ATM cell
is ready for transmission at an optimal time in order for the
waiting time in a buffer 23 in the network terminal 8 is to be
minimal, the starting point is always information from a phase
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<ietector 26 at the buffer 23 in the network terminal 8. The
detector 26 detects the phase between incoming ATM cells from
i=he transmitter 22 and the allocated time slot on the bus 5. In
i:he circuit according to the solution a signal thus arrives
which it takes some time for, let us say x ms to pack. A
t=ransmission permission with downlink data also arrives each x
ms. Thus, the object is to make certain that the generated ATM
cell is packed and transmitted to the buffer 23 just before
l~ransmission is to take place in the common medium 5, using the
phase control of the downlink traffic.
'.here are many altes:native ways to control the phase of the ATM
cell generation. The' alternative which is illustrated in figure
:3 is when the clock: in the downlink direction 10 is used as a
IS :reference for the uplink traffic 6 as well (here illustrated
with the network reference 31). Then the phase in the downlink
direction 10 to the circuit emulator 11 can be adjusted using a
~~ontrollable phase delay unit 24, using the information from the
.above mentioned phase detector 26 via the connection 28, until
'the uplink cells obtain a suitable phase (become ready/packed in
.an optimal time according to above), relative their allocated
time slots. This is obtained by means of phase delaying the
traffic in the downlink direction in the module 25 in the
network terminal 8 :>o that the PLL 21 in the circuit emulator 11
25' preferably clocks the A/D converter 29 so that the cells can be
packed in the transmitter 22 and that the transmission to the
buffer 26 is ready just before the transmission permission
arrives for the transmission to the time slot in the common
physical medium.
This is called an indirect method since the circuit emulator 11
cannot "see" that the phase changes. It just follows the phase
in the donwlink direction. This indirect. method can also be used
in an embodiment where the uplink ATM cells are generated in a
unit integrated in t:he network terminal B. In this case there is
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no need for a separate PLL 21 but the output signal from the
phase delay 24 can be used as a clock signal. ,
In another embodiment which is illustrated in figure 9, a
control channel 30~ can be used for controlling the phase from
the PLL 21'. Then one must let the phase detector 26' transmit
the same phase control information as above via the control
channel 30 to, for example, the PLL 21' from cell generation so
that the phase to be controlled to a desired position according
to the above. This method is called a direct method since in
this case necessary information is given directly to the PLL 21'
of the circuit emulator.
The methods illustrated above are not limited to the shown
preferred embodiment but can of course be used in every case
where ATM cells from many sources located at different locations
are to be transmit=ted via a common TDM based physical medium,
for example coax, fibre or radio. It should also be noted that
the embodiments in figures 3 and 4 only show possible ways to
implement the invention as such. The essence of the invention
lies in that one provides control of the generation of ATM cells
to a respective allocated time slot in order to minimize the
waiting time for the cells.
