Method and system for emulating a T1 link over an ATM network

Multiplex communications – Pathfinding or routing – Combined circuit switching and packet switching

Reexamination Certificate

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Details

C370S395430, C370S466000, C370S474000, C370S528000

Reexamination Certificate

active

06272128

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to digital communications over an Asynchronous Transfer Mode (ATM) communications network.
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 can represent any combination of voice, video, or computer data. A typical communications network consists of various physical sites called “nodes”, interconnected by conduits called “links”. Each link carries 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 DS
0
channels which are time domain multiplexed (TDM) and transmitted over a single physical line. A DS
0
channel is a 64 kilobites per second (64 Kbps) channel, which is the world wide standard for digitizing voice conversation. 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 a T
1
payload with 24 DS
0
timeslots, one for each DS
0
channel with 8 bits in each timeslot. Each T
1
frame also has T
1
frame bit that identifies the start of the T
1
frame, so that a T
1
frame has a total size of 193 bits. This results in a data stream of 1.544 Mbps (8000 frames/sec·193 bits/frame).
A T
1
superframe is a group of 12 T
1
frames. Each superframe contains a frame bit section composed of 12 frame bits, and a payload section composed of 12 samples for each of the 24 DS
0
channels. A T
1
extended superframe (ESF) is a group of 24 T
1
frames. Each ESF frame is composed of an ESF frame bit section that contains 24 frame bits, and an ESF payload section that contains 24 samples of each of the 24 DS
0
channels.
Although T
1
was developed for voice communications, it is not limited to voice communications. The physical line can carry digitized voice samples, digital computer data, or any other type of data in any combination in the 24 channels. Thus, a broader definition of a T
1
link is a digital transmission link with a capacity of 1.544 Mbps.
Information concerning whether a channel is active, idle, ringing, etc., may be passed through the voice channel by borrowing, or robbing, one bit every 6
th
frame. This process is called robbed bit signaling. Robbed bit signaling does not noticeably affect the quality of voice connections in a telephone network.
When robbed bit signaling is used and a voice channel is used to carry digital data, only 7 of the 8 bits in each frame can be counted upon to pass data through the network from one end to the other, as the 8
th
bit is frequently modified as the robbed bit. This gives rise to a single DS
0
channel carrying only 56 Kbps of data. An entire T
1
link carrying digital data would carry 1.340 Mbps using this method. This is inefficient and for this reason a new standard for T
1
transmission called Primary Rate ISDN (PRI) was developed to more efficiently move digital data through a T
1
link.
The PRI format calls for a T
1
link to not have robbed bit signaling. Instead, one of the 24 voice channels is dedicated for channel management (e.g. active, ringing, etc.) and is called the “D” channel. The other 23 channels, called bearer channels or “B” channels, may now use all 64 Kbps to carry digital data. An entire T
1
link using PRI format can carry 1.430 Mbps. Industry standards frequently call this form of PRI 23B+D. Further efficiencies can arise when multiple T
1
links between two end nodes exist. (An end node is a node where a call is originated or terminated; all other nodes along the entire link are used to only route the traffic through the telephone network). A single D channel in one link can carry all the necessary information for several T
1
links. Two T
1
links with a single D channel would be called 47B+D, and four T
1
links would be called 95B+D. Some versions also carry a spare D channel in case the T
1
link with the active D channel goes down.
The D channel carries High level Data Link Control (HDLC) messages about the B channels in all the T
1
link(s) covered by that particular D channel. When a D channel carries an HDLC message it becomes known as the HDLC channel. Typically, the HDLC channel is the 24
th
channel on a T
1
link and occupies the 24
th
timeslot in a T
1
frame. The HDLC channel is used by the DTE equipment at the two end nodes to transmit link management messages. Examples of these link management messages are call setup and call tear-down.
Since T
1
is a synchronous TDM link, once a channel connection has been setup between two users, that channel is dedicated until the connection is torn down. This channel dedication is an inefficient use of the 1.544 Mbps of T
1
link capacity. For example, assume 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 (such as user A putting user B on hold) during the transmission of a particular T
1
frame, then that particular T
1
frame will carry an empty channel #5 timeslot. 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 Kbps. Because channel #5 is dedicated, the channel #6 user cannot send data over two channels (e.g. #5 and #6) for an effective rate of 128 Kbps.
Asynchronous Transfer Mode (ATM) is an asynchronous type of communications protocol. It is designed to be carried over the emerging fiber optical network, called the Synchronous Optical NETwork (SONET), although it can be carried over almost any communications link. The basic unit of ATM is the ATM cell. Each cell contains two parts; a header, which contains routing information, and a payload, which contains the data to be transported from one end node to another.
ATM is considered asynchronous because each node in the network does not know until after a cell arrives where it is intended to go. In a synchronous network, each timeslot is assigned a certain time when it is to arrive at each node. When it arrives will determine where a timeslot goes. Thus, the individual timeslots do not need to have routing information within them. The arrival of a particular ATM cell at a node, on the other hand, is not guaranteed to occur at a particular point in time.
There are a number of factors which makes ATM attractive to the telecommunications industry. One is the cost of the SONET transport mechanism. On a bit per bit basis, it is significantly less expensive than using metallic links by several factors often. The theoretical capacity of fiber is in excess of 20 tera bits per second (20 million million bits per second). Current technology is at 40 thousand million bits per second, and will soon increase to 160 thousand million bits per second. As technology improves, more information can be sent over each fiber optic buried in the ground.
On the other hand, metallic links that can span long distances and are reasonable to manufacture, have long ago reached their theoretical limits of roughly under 500 million bits per second, and are much bulkier than fiber optic links. The metallic link is also susceptible to rust and corrosion, whereas the fiber is relatively chemically inert. Because of signal attenuation (loss of signal strength as a signal travels down a link) on either type of link, repeaters which re-amplify the signal are needed. Metallic links attenuate the signals more than do fiber

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