Chaper 12: Asynchronous Transfer Mode

What is ATM?

                Asynchronous Transfer Mode (ATM) is a technology that has the potential of revolutionizing data communications and telecommunications. Based on the emerging standards for Broadband Integrated Services Digital Networks (B-ISDN), ATM offers the economically sound "bandwidth on demand" features of packet-switching technology at the high speeds required for today's LAN and WAN networks -- and tomorrow's.

                       ATM is a member of the fast packet−switching family called cell relay. As part of its heritage, it is an evolution from many other sets of protocols. In fact, ATM is a statistical time−division multiplexed

(TDMed) form of traffic that is designed to carry any form of traffic and enables the traffic to be delivered asynchronously to the network. When traffic in the form of cells arrives, these cells are mapped onto the network and are transported to their next destination. When traffic is not available, the network will carry empty (idle) cells because the network is synchronous.

              ATM is a cell-relay technology that divides upper-level data units into 53-byte cells for transmission over the physical medium. It operates independently of the type of transmission being generated at the upper layers AND of the type and speed of the physical-layer medium below it.The technology was designed for the high-speed transmission of all forms of media from basic graphics to full-motion video. Because the cells are so small, ATM equipment can transmit large amounts of data over a single connection while ensuring that no single transmission takes up all the bandwidth. It also allows Internet Service Providers (ISPs) to assign limited bandwidth to each customer. While this may seem like a downside for the customer, it actually improves the efficiency of the ISP's Internet connection, causing the overall speed of the connection to be faster for everybody.

                    This allows the ATM technology to transport all kinds of transmissions (e.g, data, voice, video, etc.) in a single integrated data stream over any medium, ranging from existing T1/E1 lines, to SONET OC-3 at speeds of 155 Mbps, and beyond.

ATM Standards

                  The following are some of the basic ATM standards documents available from the International Telecommunications Union (ITU).

ITU-T I.361 - Defines the ATM Layer functions.

ITU-T I.363 - Defines the ATM Adaptation Layer protocols.

ITU-T I.610 - Defines the ATM Operation and Maintenance (OAM) functions.

Why the Interest in ATM?

                                                           Summary of speeds for various technologies

             When one considers the disappointing capacities of past technologies, we can see why there is the hype for ATM. ATM will be the basis of many of our future broadband communications systems; as such, it starts where other technologies stop. Many organizations have escalated their demands and needs for raw bandwidth, yet no single entity has emerged as a clear−cut winner to deliver the services necessary to support the demands of today's multimedia applications. Table 12−2 compares the capacities of ATM to the other techniques we used in the past. This will give the reader a chance to see what the excitement is all about.

ATM Protocols

                It takes many protocols to support an ATM network, which is one of the issues that continually

comes up as a negative from the supporters of the gigabit Ethernet crowd. To develop the necessary interfaces in support of the various points within a network, different protocols are necessary. The actual protocols needed depend on where the traffic originates, what transport mechanisms must be traversed, and where the traffic will terminate.

Graphic representation of the ATM protocol interfaces

The ATM Cell

                      Each individual ATM cell consists of a 5-byte cell header and 48 bytes of information encapsulated within its payload. The ATM network uses the header to support the virtual path and the virtual channel routing, and to perform a quick error check for corrupted cells.

The 5-byte header is structured as shown below:

Generic Flow Control (GFC)

               The GFC field of the header is only defined across the UNI. It is intended to control the traffic flow across the UNI and to alleviate short-term overload conditions. It is currently undefined and these 4 bits must be set to 0's.

Virtual Path Identifier (VPI)

                The VPI, an 8-bit field for the UNI and a 12-bit field for the NNI, is used to identify virtual paths. In an idle cell, the VPI is set to all 0's. (Together with the Virtual Channel Identifier, the VPI provides a unique local identification for the transmission.)

Virtual Channel Identifier (VCI)

             This 16-bit field is used to identify a virtual channel. For idle cells, the VCI is set to all 0's. (Together with the Virtual Path Identifier, the VCI provides a unique local identification for the transmission.)

Payload Type Identifier (PTI)

              The three bits of the PTI are used for different purposes. Bit 4 is set to 1 to identify operation, administration, or maintenance cells (i.e., anything other than data cells). Bit 3 is set to 1 to indicate that congestion was experienced by a data cell in transmission and is only valid when bit 4 is set to 0. Bit 2 is used by AAL 5 to identify the data as Type 0 (beginning of message, continuation of message; bit = 0) or Type 1 (end of message, single-cell message; bit = 1) when bit 4 is set to 0. It may also be used for management functions when bit 4 is set to 1. This bit is currently carried transparently through the network and has no meaning to the end user when AAL 5 is NOT in use.

Cell Loss Priority (CLP)

              The 1-bit CLP field is used for explicit indication of the priority of the cell. It may be set by the AAL Layer to indicate cells to discard in cases of congestion, or by the network as part of the traffic management on commercial subscriber networks.

Header Error Control (HEC)

              This is an 8-bit cyclical redundancy check computed for all fields of the first 4 bytes of the ATM cell header ONLY. It is capable of detecting all single-bit errors and some multiple-bit errors. The HEC is compared by each switch as the ATM cell is received and all cells with HEC discrepancies (errors) are discarded. Cells with single-bit errors may be subject to error correction (if supported or discarded. When a cell is passed through the switch and the VPI/VCI values are altered, the HEC is recalculated for the cell prior to being passed out the port.

Source: http://www.techfest.com/networking/atm/atm.htm
            http://www.techterms.com/definition/atm

            Broadband Telecommunications Handbook (VPN 3GW GPRS MPLS VoIP SIP).pdf