Multiplex communications – Pathfinding or routing – Combined circuit switching and packet switching
Reexamination Certificate
1998-06-16
2002-11-26
Marcelo, Melvin (Department: 2663)
Multiplex communications
Pathfinding or routing
Combined circuit switching and packet switching
C370S395520, C370S466000, C370S543000
Reexamination Certificate
active
06487198
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to emulating a T
1
communications link over an Asynchronous Transfer Mode (ATM) communications network. In particular, the present invention relates to a method for unloading T
1
payloads from an ATM cell using a cell buffer and an elastic store buffer.
2. Related Art
A communications network serves to transport information among a number of locations. The information is usually presented to the network in the form of time-domain electrical signals and may represent any combination of voice, video, or computer data. A typical communication network consists of various physical sites called “nodes”, interconnected by conduits called “links”. Each link serves to carry information from one site to another site. Individual sites contain data terminating equipment (DTE) for combining, separating, and transforming data.
T
1
(also known as DS
1
) is one type of digital communications link. T
1
is a synchronous link capable of carrying 24 64 Kbps channels (or DS
0
channels) which are time domain multiplexed (TDM) and transmitted over a single physical line. T
1
was originally developed to carry digitized voice communication which leads to the 64 Kbps channel standard. This occurs because an analog voice signal can be adequately represented by a digital data stream if sampled at a rate of 8000 samples per second. If each voice sample is digitized using 8 bits, this results in a digital data stream of 64 kbps.
A T
1
link transmits one T
1
frame 8000 times per second (or one frame every 125 &mgr;s). Each T
1
frame contains 24 DS
0
timeslots, with 8 bits in each timeslot for a total of 192 bits, plus one additional frame bit that identifies the start of a T
1
frame. This results in a data stream of 1.544 Mbps (8000 frames/sec·193 bits/frame).
A T
1
link may process T
1
frames in groups known as T
1
superframes or T
1
extended superframes (ESF). A T
1
superframe is composed of 12 T
1
frames, such that each superframe contains a frame bit section composed of 12 frame bits, and a payload section of 12 samples for each of the 24 channels. A T
1
ESF comprises an ESF frame bit section including 24 frame bits, and an ESF payload section including 24 samples of each of the 24 channels.
Although T
1
was developed for real time voice communications, it is not limited to voice communications. The physical line carrying the 24 channels is capable of carrying real time digitized voice communications or non-real time data communications. Thus, a broader definition of a T
1
link is a digital TDM transmission link with a capacity of 1.544 Mbps.
Since T
1
is a synchronous TDM link, once a channel connection has been set up between two users, that channel is dedicated until the connection is torn down. For example, if channel #
5
of the 24 T
1
channels is set up between user A and user B, channel #
5
will carry all communication between user A and user B. If there is a pause in the communication between user A and user B during the transmission of a particular T
1
frame, then that particular T
1
frame will carry an empty channel #
5
timeslot. It can be seen that even a short pause of one minute can lead to 480,000 T
1
frames being transmitted with an empty channel #
5
timeslot. This is so even if channel #
6
is being fully utilized by computer data at 64 kBs, but the channel #
6
user would like to send data over two channels (e.g. #
5
and #
6
) for an effective rate of 128 kbps. This results in an inefficient use of the 1.544 Mbps of T
1
link capacity.
Asynchronous Transfer Mode (ATM) is a type of communications network that makes more efficient use of channel capacity than a synchronous T
1
link. The basic unit sent over an ATM network is an ATM cell. A DTE with an ATM port into the ATM cloud sends time domain multiplexed cells at whatever rate is needed to satisfy all of the demand from users using the DTE, but will buffer excess cells to be transmitted if demand temporarily exceeds capacity.
For example, assume user A wants to send data at a rate of 500 Kbps over a physical line into the ATM cloud with 1000 Kbps capacity for 10 seconds, and then not send anything for 10 seconds. Another user, user B, wants to send a large block of data, for example, a 100 million byte file of geological data used for oil field analysis. User B's priority is lower than that of user A.
Users A and B each start transmitting at the same time. The DTE would give user A as much of the 1000 kbps line capacity as it can use, since it has a higher priority. In this case, it's 50%. User B gets the remaining 50% since its priority is lower than user A. After 10 seconds user A stops sending data to the DTE, and the DTE allocates 100% of the line capacity to user B for the remainder of the time that user A is not sending data. When user A starts sending again, its higher priority traffic will force the DTE to scale back what user B sends. User A and user B would correspond to the users of channel #
5
and #
6
in the example three paragraphs back. The inherent efficiencies of ATM, with its ability to assign priorities such that intelligent decisions can be made on the spot, are one reason for its appeal to the telecommunications industry.
One way to improve the efficiency of a T
1
link is to emulate or terminate a T
1
line at an ATM DTE with private branch exchange (PBX) capabilities and package the T
1
frames in ATM cells that are sent over an ATM network. At the ATM destination node, the T
1
frames are unpackaged (or unloaded) and sent to a switch matrix. The switch matrix de-multiplexes the DS
0
channels and sends them to their particular DS
0
user destinations or to a T
1
line to another PBX. This approach is referred to as T
1
emulation over an ATM network.
As discussed earlier, T
1
was originally developed to carry real time voice communications, but is also capable of carrying non-real time data communications. Voice communications requires that system delay be kept to a minimum. In other words, users of a T
1
voice link will not tolerate noticeable transmission delay.
Real time voice is tolerant of an occasional deletion or duplication of a single frame of data. This typically occurs when the two end nodes carrying the T
1
traffic are not synchronized to each other. They are then operating at different clock rates, with one clock rate slightly higher than the other is. When this happens, the faster running end node will send more frames than the slower running end node can receive, and the slower running end node will send fewer frames than the faster running end node expects. To accommodate this, a frame of data is deleted or duplicated, depending on whether too many or too few, respectively, are received by each end node. This frame deletion or duplication is called a frame slip.
Current methods for unloading T
1
frames from ATM cells are configured for real time voice communications as opposed to non-real time data communications. What is needed is a method that may be configured for real time voice communications with little delay that can also be configured for non-real time data communications with little additional effort.
SUMMARY OF THE INVENTION
The present invention generally relates to emulating a T
1
communications link over an Asynchronous Transfer Mode (ATM) communications network. In particular, the present invention relates to a method and system for unloading one or more T
1
payloads from a target ATM cell. The invention can be customized to provide low delay for real time voice communications with occasional frame slips. Alternatively, the invention can be customized to provide high delay for non-real time data communications with virtually no frame slips. The modification to switch from serving real-time to non-real time communications is relatively easy to implement.
According to the present invention, a plurality of incoming ATM cells is stored in a cell buffer at a first rate, wherein the first rate is the incoming
Marcelo Melvin
MCI Communications Corporation
Palmehra Inder
LandOfFree
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