Asynchronous Transfer Mode (ATM)
Asynchronous Transfer Mode (ATM)
The physical infrastructure supporting data communications has improved its ability to transmit data quickly with advances such as optical fibers. As this physical capacity increases, there is a need to utilize effectively the bandwidth to carry a variety of traffic (voice, video, data) in an efficient manner. Traditionally, circuit switching is used to support the real-time delivery needed for voice and video. Packet switching is used to support intermittently heavy data traffic. Asynchronous Transfer Mode (ATM) has emerged as a technology that efficiently utilizes the bandwidth while carrying one or more traffic types. ATM is a high-speed packet switching technology that is capable of supporting both real-time voice and video and the kind of data traffic that has peaks and plateaus in its transmission.
ATM uses fixed size packets (called cells) to reduce processing and switching delays. The cell size is kept small, at 53 bytes, to allow for fast preparation and transmission. ATM allows different users to request varying amounts of resources to support the desired quality of transmission. It supports several traffic classes with differing quality-of service-requirements.
A user requests a connection to another user with a desired quality of service. The ATM switches use signaling protocols to communicate with one another about the availability of resources needed for the requested connection. ATM allocates bandwidth dynamically, so if some users are not transmitting their cells for some time, lower priority traffic with higher tolerance for delays can be transmitted.
History of ATM
ATM has grown out of the need for a worldwide standard to allow inter-operability of information, regardless of the end-system or type of information. There have been separate methods used for the transmission of information among users on a local area network (LAN) , versus users on a wide area network (WAN) . This has added to the complexity of networking as users' needs for connectivity expand from the LAN to metropolitan, national, and finally worldwide connectivity.
Today separate networks are being used to carry voice, data, and video information due to their different characteristics. Data traffic tends to be "bursty"—not needing to communicate for an extended period of time and then needing to communicate large quantities of information as fast as possible. Voice and video, on the other hand, tend to be more even in the amount of information required, but are very sensitive to when and in what order the information arrives. With ATM, separate networks are not required. ATM is the only standards-based technology that has been designed from the beginning to accommodate the simultaneous transmission of data, voice, and video. Although some technologies today are scalable in terms of one of the factors (size or bit-rate or number of users), only ATM is truly "scalable" in terms of bit-rate, network size, and number of users.
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.
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. Usually many virtual channels share a physical link at the same time, allowing asynchronous interweaving of cells from multiple connections. VPI connections share a common path through the network and thus network management actions need to be applied to only a single virtual path as opposed to all of the individual virtual channels.
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. Layers are as shown in Figure 1 and their functionality is summarized in Figure 2.
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 (e.g. optical fiber, coaxial, or twisted pair) dependent functions and provides bit transmission capability including bit alignment.
The TC sublayer performs five primary functions as shown in Figure 2. 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.
The cell delineation function enables the receiver to recover the cell boundaries. The Header Error Control (HEC) sequence generation is done in the transmit direction and its value is recalculated and compared with the received value. Cell rate decoupling inserts the idle cells in the transmitting direction in order to adapt the rate of the ATM cells to the payload capacity of the transmission system. It suppresses all idle cells in the receiving direction. Only assigned and unassigned cells are passed to the 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.
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 (virtual connection) and those related to a group of logical connections (virtual path), 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.
see also Internet; Network Design; Networks; World Wide Web.
Radhika Jain and Upkar Varshney
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