Multiplex communications – Pathfinding or routing – Combined circuit switching and packet switching
Reexamination Certificate
1998-06-16
2001-02-27
Rao, Seema S. (Department: 2732)
Multiplex communications
Pathfinding or routing
Combined circuit switching and packet switching
C370S395430, C370S466000, C370S474000, C370S522000
Reexamination Certificate
active
06195346
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to voice and data communications, and in particular, to emulating a T1 communications link 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 node to another node. End user nodes contain data terminating equipment (DTE) for combining, separating, and transforming data and voice. As such, a DTE can be a voice switch, data switch, or a combination of the two. A series of non-DTE nodes interconnected to each other with ATM links is often referred to as an ATM cloud. A DTE is typically connected to the ATM cloud at one point, and may be connected with an ATM link, other types of data links including frame relay, or digital time domain multiplexed (TDM) communications links such as T1. When an ATM cloud node is connected to a DTE over a non-ATM link, it is typically called circuit emulation.
T1 (also known as DS
1
) is one type of a TDM communications link. T1 is a synchronous link capable of carrying 24 DS0 channels which are TDM and transmitted over a single physical line. A DS0 channel is a 64 kilobits per second (64 Kbps) channel, which is the world wide standard for digitizing voice conversation. The 64 Kbps bit rate is chosen 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 T1 link transmits one T1 frame 8000 times per second (or one frame every 125 &mgr;s). Each T1 frame contains a T1 payload with 24 DS0 timeslots, one for each DS0 channel with 8 bits in each timeslot. Each T1 frame also has a T1 frame bit that identifies the start of the T1 frame, so that a T1 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 T1 superframe is a group of 12 T1 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 DS0 channels. A T1 extended superframe (ESF) is a group of 24 T1 frames. Each ESF 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 DS0 channels.
Although T1 was developed for voice communications, it is not limited to voice communications. The physical line carrying the 24 channels does not care whether the channel timeslot contains digitized voice samples or digital computer data. Thus, a broader definition of a T1 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 DS0 channel carrying only 56 Kbps of data. An entire T1 link carrying digital data would carry 1.340 Mbps using this method. This is inefficient and for this reason a new standard for T1 transmission called Primary Rate ISDN (PRI) was developed to more efficiently move digital data through a T1 link.
The PRI format calls for a T1 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 T1 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 T1 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 T1 links. Two T1 links with a single D channel would be called 47B+D, and four T1 links with a single D channel would be called 95B+D. Some versions also carry a spare D channel in case the T1 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 T1 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 T1 link and occupies the 24
th
timeslot in a T1 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 teardown. When user A at node
1
wants to setup a call with user B at node
2
, DTE
1
and DTE
2
communicate system information to setup the call. This is done over the HDLC channel. Likewise, when either user A or user B terminates the call, DTE
1
and DTE
2
must communicate to tear down the call setup so the channel can be reused.
Another link management example is call blocking. If user B had previously notified the link management to block calls from user A, then the message from DTE
2
back to DTE
1
that user A is blocked would be carried over the HDLC channel. A further example is caller ID. If user B has caller ID, the phone number of user A will be carried over the HDLC channel and displayed to user B.
Since T1 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 T1 link capacity. For example, assume channel #5 of the 24 T1 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 T1 frame, then that particular T1 frame will carry an empty channel #5 timeslot. Even a short pause of one minute can lead to 480,000 T1 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
MCI Communications Corporation
Rao Seema S.
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