Asynchronous Transfer Mode

Asynchronous Transfer Mode (ATM) is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing,and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that use variable sized packets or frames. ATM provides data-link layer services that run over a wide range of OSI physical layer links. ATM has functional similarity with both circuit switch networking and small packet switched networking. It was designed for a network that must handle both traditional high-throughput data traffic (e.g., file transfers), and real time, low latency content such as voice and video. ATM uses a connection oriented model in which a virtual circuit must be established between two endpoints before the actual data exchange begins.ATM is a core protocol used over the SONET/SDH backbone of the Public Switch Telephone Network (PSTN) and Intergrated Service Digital Network (ISDN), but its use is declining in favour of All IP. 

The Rationale and Underlying Technology

 ATM can be considered to represent a unifying technology because it was designed to transport voice, data, and video (including graphics images) on both local and wide area networks. Until the development of ATM, networks were normally developed based on the type of data to be transported. Thus, circuit-switched networks, which included the public switched telephone network and high-speed digital transmission facilities, were primarily used to transport delay-sensitive information, such as voice and video. In comparison, on packet-based networks, such as X.25 and Frame Relay, information can tolerate a degree of delay. Network users can select a networking technology to satisfy a specific communications application, but most organizations support a mixture of applications. Thus, most organizations are forced to operate multiple networks, resulting in a degree of inefficiency and escalating communications costs. By combining the features from both technologies, ATM enables a single network to support voice, data, and video.

Architecture

ATM is based on the switching of 53-byte cells, in which each cell consists of a 5-byte header and a payload of 48 bytes of information. Figure 14.1 illustrates the format of the ATM cell, including the explosion of its 5-byte header to indicate the fields carried in the header.

 

         Advantage of ATM:

         

         - Universal Switching Standard

         - Full support of Multimedia

         - Single Network Access

         - Reduction in network delay

         - True bandwith-on-demand

         - Optimization of network resources

         - Technical Long Life

          Disadvantage of ATM:

         

         - Cost, although it will decrease with time.

         - New customer premises hardware and software are 
           required:

           Competition on other technologies - 100Mbps FDDI,
           100Mbps Ethernet and fast Ethernet.

         - Presently the appilications that can benefit from 
           ATM as multimedia  are rare. 

• 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. 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.
  
  
  ATM consists of three layers:
  
  
  1.Physical 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. 
  
  
  
  
  2.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.
  -Cell multiplex and demultiplex function.
  -VPI and VCI translation.
  -Generic Flow Control (GFC). 
  
  
  
  
  3.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).
  
  Key issues in ATM:
   
  - Multiple logical connections over single physical  
    interface.
  - Flow on each logical connections is in fixed sized packets
    called cells.
  - Minimal error and flow control.
  - Data rates (physical layer)25.6Mbps to 622.08Mbps
  
  
  ATM is designed to support:
  
  
  1.Bussiness and institutions who:
  - connect LANS with fiber optic facilities to support  
    specific applications.
  - often send high volumes of data between several of their 
    locations.
  - have linked sites using applications such as CAD/CAM or  
    image processing.
   
  How does ATM differ from Frame Relay?
  
  
  - Atm makes use of a 53byte fixed length cell while the  
    frame in frame relay is much longer, and may vary in  
    length.
  - Error cheking is only done on the header in ATM rather  
    than on the whole cell or frame.
  - Virtual channels of ATM that follow the same route  
    through the network are bundled into paths. A similar  
    mechanism is not used in frame relay.
  
  
  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.
  
  
  Sources: 
  
  
  

  http://www.bookrags.com/research/asynchronous-transfer-mode-atm-csci-04/

  http://en.wikipedia.org/wiki/Asynchronous_Transfer_Mode
  http://technet.microsoft.com/en-us/library/bb726929.aspx
  http://www.scribd.com/doc/12468863/Asynchronous-Transfer-Mode-ATM-wwwstudentcenterin