Asynchronous Transfer Mode (ATM)

Asynchronous Transfer Mode is a high-speed network technology that supports the transportation of voice, data, and video signals over a single stream. ATM combines both circuit and packet switching methods into one flexible technology that makes for simple network processing functions. That digital data is encoded in the form of small fixed size cells instead of the variable sized packets used by Internet Protocol or Ethernet. This ensures that the packets can be sent quickly and easily.

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 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 connection oriented, which means that data sent through the ATM network will always follow the same pre-defined path with the data arriving in the order it was sent.

ATM Cells

An ATM cell is 53 bytes long with a 5-byte header possessing information for control and signaling, and 48 bytes of data payload. Having fixed-size cells may reduce queuing delays for high priority cells. Because one knows the size of a cell beforehand, it becomes easier to implement the switching mechanism in hardware for efficient switching. The header information is generated in the ATM Layer, while the ATM Adaptation Layer (AAL) breaks the entire message into 48-byte data chunks. The cell header contains fields to help deal with congestion, maintenance, and error control problems. It is broken up into the following fields:

• Generic Flow Control (GFC), a mechanism used to alleviate shortterm overload conditions in the network. It is intended to provide efficient and equal utilization of the link between all the users.
• Virtual Path Identifier (VPI), which allows for more virtual paths to be supported within the network.
• Virtual Channel Identifier (VCI), which functions as a service access point as it is used for routing to and from the end user.
• Payload Type (PT), which is used to distinguish between user information and connection-associated layer management information.
• Cell Loss Priority (CLP), which is used to provide guidance to the network to discard the cell in case of congestion.
• Header Error Control (HEC), which contains the information that can be used by the physical layer for error detection or correction. It is calculated from the first 32 bits of the header.

VCI/VPI Connections

The entire ATM network is based on virtual connections set up by the switches upon initialization of a call. Virtual Channel Identifiers (VCI) and Virtual Path Identifiers (VPI) are used to identify these virtual connections. They are used to route information from one switch to another. VCI and VPI are not addresses; they are explicitly assigned to each segment within a network.

A Virtual Channel Connection (VCC) is set up between two end users through the network and used for full-duplex flow of cells. They are also used for user-network exchange (control signaling) and network-network exchange (network management and routing). The VCI label identifies a VCC between two ATM switches and may change at intermediate nodes within a route.

Virtual channels having the same endpoints are often grouped together to form a Virtual Path Connection (VPC). This grouping of channels makes the task of network management easier without losing flexibility.

Layers and Their Functions

ATM is a layered architecture allowing multiple services—voice, data, and video—to be carried over the network. It consists of three layers: the physical layer, the ATM layer, and the ATM adaptation layer.

The physical layer of ATM is similar to layer 1 of the Open Systems Interconnections (OSI) model and performs bit level functions. It defines electrical characteristics and network interfaces. It is further divided into two layers: Physical Medium (PM) and Transmission Convergence (TC) sub-layer.

The PM sublayer contains physical medium dependent functions and provides bit transmission capability including bit alignment.

The TC sublayer performs five primary functions. The lowest function is the generation and recovery of the transmission frame. Transmission frame adaptation adapts the cell flow according to the used payload structure of the transmission system in the sending direction, and extracts the cell flow from the transmission frame in the receiving direction.

ATM Layer

The ATM layer is next above the physical layer. The ATM layer takes the data to be sent and adds the 5-byte header information. It performs the following four actions:

• Cell header generation/extraction, which adds the appropriate ATM cell header to the received cell information field from the upper layer in the transmit direction. It does the opposite in the receive direction.
• Cell multiplex and demultiplex function, which multiplexes cells from individual virtual channels and virtual paths into one resulting cell stream in the transmit direction. It divides the arriving cell stream into individual cell flows to VCs or VPs in the receive direction.
• VPI and VCI translation, which is performed at the ATM switching and/or cross-connect nodes.
• Generic Flow Control (GFC), which supports control of the ATM traffic flow in a customer network.

ATM Adaptation Layer

The AAL performs the adaptation of OSI higher layer protocols, as most applications cannot deal directly with cells. The Adaptation Layer assures the appropriate service characteristics, and divides all types of data into the 48-byte payload that will make up the ATM cell. AAL is further divided into two sublayers: Segmentation and Reassembly (SAR) and Convergence Sublayer (CS).

The SAR sublayer performs segmentation of the higher layer information into a size suitable for the payload of the ATM cells of a virtual connection and, at the receiving side, it reassembles the contents of the cells of a virtual connection into data units to be delivered to the higher layers. The CS sublayer performs functions like message identification and time/clock recovery.

Mapping Circuits Through an ATM Network

ATM uses one of two connection types. The protocol is connection−oriented, so the two choices are a PVC or a SVC. There is actually no permanency to the circuits. They are logically mapped through the network and are used when needed for PVC or dial−connected when using the SVC. The concept is that the network provider will provide a committed bandwidth available to the user on demand whenever the user wants to use it. The connection is built into a routing table in each of the switches involved with the connection from end to end.

ATM Traffic Management

ATM must be flexible. It must meet the constantly changing demands of the user population.

These goals mean that the demands for traffic will rise or fall as necessary, and therefore

managing this traffic is of paramount importance.

ATM must meet the diverse needs of the end−user population. Many users will have varying

demands for both high− and low−speed traffic across the network. Using a QoS capability

throughout the ATM network, a user can determine the performance and the capabilities of

how the ATM network will meet their demands. These demands must be met in terms of the

delay or the actual delivery of the cells across the network.

Cost efficiency is a must. If ATM is truly to succeed, traffic management must also include

the effective usage of all of the circuitry available. ATM is designed to reduce the inefficient

circuit usage by efficiently mapping cells into dead spaces, particularly when data is

involved.

Robustness in the event of failure or in the event of excess demand is a requirement of the

traffic management goals. If the network is to be readily available for all users to be able to

transmit information on demand, then the network must be very robust to accommodate

failures, link downtime, and so on. Through this process, the managing of traffic must

accommodate such diverse needs on a WAN.

Key Benefits of ATM

ATM offers significant benefits to users and those who design and maintain communications networks. Because network transport functions can be separated into those related to an individual logical connection and those related to a group of logical connections, ATM simplifies network management. ATM also allows for the integration of networks, improving efficiency and manageability and providing a single network for carrying voice, data, and video.

ATM increases network performance and reliability because the network is required to deal with fewer aggregated entities. There is also less processing needed and it takes less time to add new virtual channels because capacity is reserved beforehand on a virtual path connection. Finally, ATM offers a high degree of infrastructure compatibility. Because ATM is not based on a specific type of physical transport, it can be transported over twisted pair, coaxial, and fiber optic cables.

Two additional features of ATM that warrant discussion are its asynchronous operation and its connection-oriented operation. ATM cells are intermixed via multiplexing, and cells from individual connections are forwarded from switch to switch via a single-cell flow. However, the multiplexing of ATM cells occurs via asynchronous transfer, in which cells are transmitted only when data is present to send. In comparison, in conventional time division multiplexing, keep-alive or synchronization bytes are transmitted when there is no data to be sent. Concerning the connection-oriented technology used by ATM, this means that a connection between the ATM stations must be established before data transfer occurs. The connection process results in the specification of a transmission path between ATM switches and end stations, enabling the header in ATM cells to be used to route the cells on the required path through an ATM network.

Sources:

http://www.telecomdictionary.com/telecom_dictionary_ATM_definition.html

http://www.javvin.com/protocolATMLayer.html

Cisco - Creating ATM VLANs and Configuring LANE Services

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/atm.htm