Note: Descriptions are shown in the official language in which they were submitted.
: CA 02223231 1997-12-02
WO 96142149 PCTIUS96/09684
FAST AND EFFICIENT PACKET TRANSMISSION SYST]_M
AND METHOD
I~ACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to the field of digital
communications. More particularly, the present invention relates to a
system and method for transmitting and routing packets in a d igital
communications network which utilizes both variable length packets and
fixed length packets for achieving optimum efficiency and speed through
the network.
II. Description of the R~late-l Art
In packet switching networks, information is tran~mitteri in packets,
each of which contains a portion (or all for short messages) of the
2 0 subscriber's information plus some control information. The control
information at a minimum includes the destination address of the packet
which enables the network to route the packet through the network and
deliver it to its intended destination. At each node en route the packet is
received, stored briefly, and passed on to the next node.
In packet switching networks, network resources are shared by
multiple subscribers on an as-needed basis. In other words, a packet is
transmitted as it becomes available, but no transmission facilities are held
by a source when it has nothing to send. The connection between
subscribers is logical rather than physical. A primary advantage of this
3 0 type of switching network is its optirni7e~ use of the network resources by
ensuring that needed physical channels are never idle, except in the
absence of traffic.
In addition, since packet switching operates in a burst-oriented
access mode, a single network resource can accommodate multiple
~ 3 5 subscribers transmitting and receiving information at different or variable
data rates. This ability to simultaneously accommodate variable rate
information from multiple subscribers allows packet switching networks
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to achieve maximum bandwidth efficiencies. Since the length of variable
length packets are directly proportional to the rate of information being
transmitted within a given time period, variable length packets contain a
minimum number of "idle bits." This allows variable length packets to
5 utilize the bandwidth capacity of a transmission medium with maximum
efficiency.
Many of the advantages of variable length packet switching come
with a cost, however. As a result of the overhead involved in storing and
processing control information, a variable length packet may experience
10 delay times and other overhead inefficiencies during routing that do not
meet the low delay requirements of certain types of information such as
voice communications.
To decrease the delay times involved in processing control
information, fixed length packets, or cells (a cell is defined as a packet with
15 a fixed size), may be used to route information through a network. There
are two advantages in using cells when compared with using variable
length packets to route information. First, cells simplify network routing
and/or switch design due to synchronized flow of cells from all inputs
through switching ~l~m~nt.s. Second, cells reduce processing of control
20 information at network nodes because no cell length calculations are
required. Both of these advantages allow cells to be transmitted and
routed through a communications network with minim~l delay making
fixed length cells particularly well suited for low-delay communications
such as voice communications. Asynchronous Transfer Mode (ATM) has
2 5 become the accepted standard for cell relay. ATM is a low-delay, high-
bandwidth, fixed cell size, packet switching and multiplexing technique.
Using cells involves one obvious disadvantage, however. For
communication systems transmitting and receiving variable rate
information such as voice information, cells do not exhibit the bandwidth
3 0 efficiencies achieved by variable length packets. The length of variable
length packets will vary proportionally with the rate of the variable rate
information. Therefore, when using variable length packets, there is a
minimum number of "idle bits" for each packet. In contrast, each cell
must be long enough to accommodate the fastest variable rate
35 information. Therefore, slower rate information will occupy only a
portion of the bit space available on a fixed length cell and the rest of the
cell will consist of "idle bits" or "filler bits."
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Therefore, there is a tradeoff between network efficiency and
network speed. Although variable length packets maximize network
efficiency they exhibit greater switching and routing delays than that
exhibited by cells. Conversely, fixed length cells can be switched and
S routed much faster than variable length packets but are less efficient
because they contain a greater number of "idle bits."
Network efficiency is of paramount importance in the
communications industry because corresponding to network efficiency is
the ability to handle more subscribers simultaneously which equates to
10 more revenues from paying customers. (~n the other hand, the quality of
transmission is very important to the customer. Therefore, minimizing
transmission delays is also very important in the communications
industry.
With these two competing inleresLs in mind (i.e. network efficiency
15 and network speed), it is apparent that there is a need for a digital
communications network which will optimally maximize transmission
efficiency and minimize tran~mi~sion delays.
SUMMARY OF THE INVENTION
The present invention is a novel packet trar1~mi~si-)n system and
method for transmitting packets through a digital communications
network. The packet trar~mi~sion system utilizes both variable length
packets and fixed length packets, or cells, for optimally achieving
2 5 maximum efficiency and minimum delay during the transmission of
information through the digital communications network. Network
efficiency is maximized by using variable length packets when
information is transmitted across transmission lines. Network delay is
minimized by converting the variable length packets into cells at the
3 0 switch or router interface and then routing the cells to their appropriate
destination ports in accordance with the destination address of each cell.
The present invention can operate in two modes to provide two-
way communications between end subscribers. These two modes are
referred to as the forward link and the reverse link. On the reverse link,
3 5 subscriber information signals are received by a first signal processor,
routed through the system, and then transmitted by a second signal
processor to their next destinations. On the forward link, subscriber
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information signals are received by the second signal processor, routed
through the system, and then transmitted by the first signal processor to
their next destinations.
In one embodiment of the present invention, on the reverse link, a
S first signal processor receives subscriber information signals and processes
them into a variable length packet format. The variable length packets are
then transmitted across a transmission line to a first packet converter
which converts the variable length packets into cells. The cells are then
routed by a system router which routes each cell to a particular destination
10 port in accordance with the destination address of each cell. The cells may
be routed directly to an Asynchronous Transfer Mode (ATM) Network or
other network capable of receiving cells, routed back to a first packet
converter in the forward link direction (see forward link discussion
below), or routed to a second packet converter. At the second packet
15 converter, the cells are converted back into variable length packets. The
variable length packets are then processed by a second signal processor into
their next tr~n~ s;on formats and transmitted to their next destinations.
On the forward link, a second signal processor receives subscriber
information signals and processes them to produce variable length
2 0 packets. The variable length packets are then converted into cells by a
second packet collvelLeI-. The cells are routed by the system router to a first
packet converter. At the first packet converter, the cells are converted back
into variable length packets. These variable length packets are then
transmitted across a transmission line to a first signal processor where they
2 5 are processed into their next transmission formats and transmitted to their
next destinations.
The system router may receive cells directly from an Asynchronous
Transfer Mode (ATM) network or other fixed length packet source. These
cells can then be routed in either the forward link or reverse link direction
3 0 to their next destinations. As mentioned above, the system router may
also transmit cells directly to an ATM Network or other network capable
of receiving information signals in a fixed length packet (cell) format.
BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of the present invention will become
more apparent from the detailed description set forth below when taken in
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conjunction with the drawings in which like reference characters
correspond throughout and wherein:
FIG. 1 is a schematic overview of an exemplary packet trancmi~sion
system; and
FIG. 2 is a schematic overview of an exemplary Code ]Division
Multiple Access (CDMA) packet transmission system.
DETAILED DESCRIPTION OF THE PREFERRED
EMB ODIMENTS
1 0
An exemplary packet transmission system in which the present
invention is embodied is illustrated in FIG. 1. The packet transmission
system operates in two modes to provide two-way communications
between end-subscribers. These two modes are referred to as the forward
link and the reverse link. On the reverse link, subscriber information
signals are received by a first signal processor, routed through the system,
and then transmitted by a second signal processor to their next
destinations. On the forward link, subscriber information signals are
received by the second signal processor, routed through the system, and
2 0 then transmitted by the first signal processor to their next destinations.
On the reverse link, first signal processors 101a-lOln receive
subscriber information signals and process them into a variable length
packet format. The variable length packet format may be one of various
formats which are known in the prior art (i.e. HDLC, LAPB, LAPD, etc.).
2 5 The variable length packets are then transmitted across transmission lines
102a-102n. These transmission lines are typically E1 or T1 lines which are
commercial lines with standard bandwidths. However, other types of
lines may be used depending on the capacity requirements of the particular
system. Variable length packets utilize the maximum bandwidth capacity
3 0 of the transmission line because such packets contain a minimum number
of "idle bits." Therefore, more users can send information over the line
simultaneously and line efficiency is maximized.
Upon completion of their transit across transmission lines 102a-
102n, the variable length packets are converted into fixed length packets, or
3 5 cells, by first packet converters 103a-103n. In contrast to variable lengthpackets, the routing of cells does not require time-consuming packet
length calculations. Therefore, switching and routing is done using cells
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which can be switched or routed much faster than variable length packets.
After the variable length packets have been converted into cells, a system
router 104 routes each cell to its appropriate destination port in accordance
with the destination address of each cell.
Standard packet switches and routers are relatively inexpensive and
do not exhibit the bandwidth limitations inherent in transmission lines.
Therefore, achieving bandwidth efficiency at the switch or router is of
relatively little concern when compared to achieving the low delay
requirements of many types of communications. In addition, since
1 0 transmission lines are typically leased from local telephone carriers at a
considerable cost they must be utilized to their maximum capacities to
achieve cost efficiency. The system router, on the other hand, is typically
not leased. If one system router cannot handle the amount of information
traffic in its area, it may be upgraded or a second router can be built at
1 5 relatively little long-term cost to handle the additional information traffic.
Therefore, achieving bandwidth efficiencies at the system router is not as
important as achieving bandwidth efficiencies across leased transmission
lines. The main concern of the system router is to route information to
their appropriate destinations with minimal delay.
2 0 The cells may be routed either directly to an Asynchronous Transfer
Mode (ATM) Network or other network capable of lec~iviLLg cells, or back
to the first packet converters 103a-103m (see forward link discussion
below), or to the second packet converters 105a-105m. The second packet
converters 105a-105m convert the cells back into variable length packets.
2 5 The variable length packets are then processed by second signal processors
106a-106n into their next transmission formats and transmitted to their
next destinations.
On the forward link, second signal processors 106a-106n receive
subscriber information signals and process them into a variable length
3 0 packet format. The variable length packets are then converted into cells by
second packet converters 105a-105m The cells are then routed by a system
router 104 to first packet converters 103a-103n in accordance with the
destination address of each cell. The first packet converters 103a-103m
then convert the cells back into variable length packets. The variable
3 5 length packets are then transmitted across transmission lines 102a-102n to
first signal processors 101a-lOln where they are processed into their next
tran~mi~sion formats and transmitted to their next destinations.
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The system router 104 may receive cells directly from an
Asynchronous Transfer Mode (ATM) network or other fixed length packet
source. These cells can then be routed in either the forward link or reverse
link direction to their next destinations. As mentioned above, the system
router may also transmit cells directly to an ATM Network c r other
network capable of receiving information signals in a fixed length packet
format.
An exemplary Code Division Multiple Access (CDMA) packet
transmission system in which the present invention is embodied is
1 0 illustrated in FIG. 2. The use of CDMA modulation techniques is one of
several techniques for facilitating communications in which a large
number of system subscribers are present. The use of CDMA techniques in
a multiple access communication system is disclosed in U.S. Patent No.
4,901,307, entitled "SPREAD SPECTRUM MULTIPLE A.CCESS
1 5 COMMUNICATION SYSTEM USING A SATELLITE OR TERRESTRIAL
REPEATERS," and assigned to the assignee of the present invention. This
disclosure is hereby incorporated by reference.
The relevant features of a CDMA packet transmission system
(otherwise known as a CDMA Cellular Landbase Network (CCLN)) are
2 0 shown in FIG. 2. The CDMA packet tr~n~mi~sion system is composed of
two parts: a number of base station transceiver subsystems (BTS) 201a-201n
and a single base station controller (BSC) 200. In a CDMA
communications system, CDMA variable rate packets are transmitted over
the air between a subscriber and a base station transceiver subsystem 201a-
201n. The base station transceiver subsystems 201a-201n are the links
between subscribers and the base station controller 200 and provide the
common air interface to the subscribers. The base station controller 200
contains the resources for setting up and maintaining traffic channels and
routing information between the base station transceiver subsystems 201a-
3 0 201n and other networks such as a public switched telephone network
(PSTN).
The CDMA packet transmission system operates in two modes to
provide two-way communications between end-subscribers. These two
modes are referred to as the forward link and the reverse link. On the
3 5 reverse link, subscriber information signals are received by base station
transceiver subsystems 201a-201n, routed through the base station
controller 200, and then transmitted by selector bank subsystems (SBS)
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206a-206n to their next destinations. On the forward link, subscriber
information signals are received by the selector bank subsystems 206a-
206n, routed through the base station controller 200, and then transmitted
by the base station transceiver subsystems 201a-201n to their next
5 destinations.
On the reverse link, base station transceiver subsystems 201a-201n
receive incoming CDMA signals and process them into High-level Data
Link Control (HDLC) variable length packets. The HDLC format is a flag
synchronization transmission control format in which information
1 0 having an arbitrary bit length is regarded as a transfer unit called a variable
length packet (sometimes called a frame). The HDLC format enables
transfer of continuous information in variable length packets. After the
base station transceiver subsystems 201a-201n have processed the
incoming CDMA signals, the resulting HDLC variable length packets are
1 5 then transmitted across transmission lines 202a-202n to a base station
controller 200. At the base station controller 20~, the HDLC packets are
received by first packet converters 203a-203m which convert the HDLC
variable length packets into Asynchronous Transfer Mode (ATM) cells.
ATM is a low delay, high bandwidth, fixed packet size, packet switching
2 0 and multiplexing technique which has become an accepted standard for
cell relay. After the first packet converters 203a-203m convert the HDLC
packets to ATM cells, the ATM cells are routed by a system packet router
204 to their appropriate destination ports in accordance with the
destination address of each cell. The ATM cells may be routed either
2 5 directly to an ATM network, back to the first packet converters 203a-203m
in the forward link direction, or to the second packet converters 205a-
205m. At the second packet converters 205a-205m, the ATM cells are
converted back into HDLC variable length packets. The HDLC packets are
then sent to the selector bank subsystems 206a-206n which process the
3 0 HDLC packets into their next transmission formats and transmit them to
their next destinations.
The selector bank subsystems 206a-206n are responsible for the call
processing requirements of the base station controller 200 and provide an
interface between the base station controller 200 and other networks such
3 5 as the public switched telephone network (PSTN). Typically the HDLC
packets will contain voice information and the selector bank subsystems
will process the HDLC packets into pulse code modulated (PCM) voice
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signals which are then tirne-division multiplexed and sent to the public
switched telephone network. Each of the selector bank subsystems 206a-
206n contain a plurality of selector elements which provide the resources
for allocating a unique transmission channel, using time division
multiplexing techniques, for the individual subscriber information signals
transmitted between the selector bank subsystems 206a-206n and a public
switched telephone network or other interfacing network. The public
switched telephone network is usually a local telephone carrier. In
addition to voice communications the selector bank subsystems 206a-206n
1 0 may process other types of data to interface with a variety of networks.
On the forward link, selector bank subsystems 206a-206n receive
subscriber information signals and time division multiplex these signals
such that each subscriber signal is provided with a unique trar~mic~ion
channel between a selector bank subsystem and the public switched
1 5 telephone network, or other interfacing network. Typically the subscriber
information signals will be pulse code modulated (PCM) voice
communications transmitted by the public switched telephone r,etwork.
However, other types of signals may be received and processed by the
selector bank subsystems 206a-206n enabling them to interface with a
2 0 variety of networks. The selector bank subsystems 206a-206n then process
the subscriber information signals into HDLC variable length packets. The
HDLC packets are then sent to second packet converters 205a-205m which
convert the HDLC packets into ATM cells. The ATM cells are then routed
by the system router 204 to their appropriate destination ports in
2 5 accordance with the destination address of each cell. After the A [M cells
have been routed, they are converted back into HDLC variable length
packets by the first packet converters 203a-203m. The HDLC packets are
then transmitted across transmission lines 202a-202n to the base station
transceiver subsystems 201a-201n. The base station transceiver subsystems
3 0 201a-201n then process the variable length packets into CDMA signals and
transmit them to their next destinations.
The system router 204 may receive ATM cells directly from an ATM
network, bypassing the selector bank subsystems 206a-206n and the second
packet converters 205a-205m. At the system router 204, the ATM cells may
3 S then be routed in either the forward link or reverse link directions to their
next destinations.
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The system described above is not limited to either HDLC variable
length packet or ATM cell formats. Other types of variable length packet
formats and cell formats may be utilized to achieve the advantages of the
present invention.
S The previous description of the ~lefelLed embodiments is provided
to enable any person skilled in the art to make or use the present
invention. The various modifications to these embodiments will be
readily apparent to those skilled in the art, and the generic principles
defined herein may be applied to other embodiments without the use of
the inventive faculty. Thus, the present invention is not intended to be
limite~l to the embodiments shown herein but is to be accorded the widest
scope consistent with the principles and novel features disclosed herein.
We claim:
