ASYNCHRONOUS TRANSFER MODE

ATM (asynchronous transfer mode) a high-speed networking standard designed to support both voice and data communications. ATM is normally utilized by Internet service providers on their private long-distance networks. ATM operates at the data link layer (Layer 2 in the OSI model) over either fiber or twisted-pair cable. ATM is a dedicated-connection switching technology that organizes digital data into 53-byte cell units and transmits them over a physical medium using digital signal technology. Individually, a cell is processed asynchronously relative to other related cells and is queued before being multiplexed over the transmission path. Because ATM is designed to be easily implemented by hardware (rather than software), faster processing and switch  speeds are possible. The prespecified bit rates are either 155.520 Mbps or 622.080 Mbps. Speeds on ATM networks can reach 10 Gbps. Along with Synchronous Optical Network (SONET) and several other technologies, ATM is a key component of broadband ISDN (BISDN).

ATM provides the opportunity for both end users and communications carriers to transport virtually to any type of information using a common format. It allows variety of different applications and services (video, data, voice etc) to be supported on a single network. It has been adapted as the transmission mechanism for B-ISDN which is a digital network standard which will replace many existing network standards. 

ATM NETWORK

The technology allows both public (i.e., RBOC or carrier) and private (i.e., LAN or LAN-to-internal switch) ATM networks. This capability gives a seamless and transparent (to the user) connection from one end user to another end user, whether in the same building or across two continents. The basic network structure is as shown on the following page.

                  .^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^.
.-----------.     | .--------.   2   .--------.   |
|End User 1 |-----|-|  ATM   |-------|  ATM   |   |
`-----------'   1 | | Switch |       | Switch |---|-------+
                  | `--------'       `--------'   |       |
                  |        ATM Network  1         |       |
                  `vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv'       |
                                                        3 |
                                                          |
                  .^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^.        |
.-------------.   | .--------.   2   .--------.  |        |
| Private ATM |---|-|  ATM   |-------|  ATM   |  |        |
|    Switch   | 1 | | Switch |       | Switch |--|--------+
`------+------'   | `--------'       `--------'  |
     1 |          |         ATM Network 2        |
 .-----+------.   `vvvvvvvvvvvvvvvvvvvvvvvvvvvvvv'
 | End User 2 |
 `------------'

Three types of interfaces exist in this diagram:

1. User-to-Network Interface (UNI)
2. Network-to-Network Interface (NNI)
3. Inter-Carrier Interface (ICI)

The UNI exists between a single end user and a public ATM network, between a single end user and a private ATM switch, or between a private ATM switch and the public ATM network of an RBOC.

The NNI exists between switches in a single public ATM network. NNIs may also exist between two private ATM switches.

The ICI is located between two public ATM networks (an RBOC and an interexchange carrier).

All of these interfaces are very similar. The major differences between these types of interfaces are administrative and signalling related. The only type of signalling exchanged across the UNI is that required to set up a VIRTUAL CHANNEL for the transmission. 

ADVANTAGES OF ATM

• ATM Advantages
  
• ATM supports voice, video and data allowing multimedia and mixed services over a
• single network.
  
• High evolution potential, works with existing, legacy technologies
  
• Provides the best multiple service support
  
• Supports delay close to that of dedicated services
  
• Supports the broadest range of burstiness, delay tolerance and loss performance through the implementation of multiple QoS classes
  
• Provides the capability to support both connection-oriented and connectionless traffic using AALs
  
• Able to use all common physical transmission paths like SONET.
  
• Cable can be twisted-pair, coaxial or fiber-optic
  
• Ability to connect LAN to WAN
  
• Legacy LAN emulation
  
• Efficient bandwidth use by statistical multiplexing
  
• Scalability
  
• Higher aggregate bandwidth
  
• High speed Mbps and possibly Gbps 

DISADVANTAGES OF ATM

• Flexible to efficiency’s expense, at present, for any one application it is usually possible to find a more optimized technology
  
• Cost, although it will decrease with time
  
• New customer premises hardware and software are required
  Competition from other technologies -100 Mbps FDDI, 100 Mbps Ethernet and fast Ethernet
  
• Presently the applications that can benefit from ATM such as multimedia are rare
  The wait, with all the promise of ATM’s capabilities many details are still in the standards process

 ATM differs from more common data link technologies like Ethernet in several ways. For example, ATM utilizes no routing. Hardware devices known as ATM switches establish point-to-point connections between endpoints and data flows directly from source to destination. Additionally, instead of using variable-length packets as Ethernet does, ATM utilizes fixed-sized cells. ATM technology is designed to improve utilization and Quality of service (QoS) on high-traffic networks. Without routing and with fixed-size cells, networks can much more easily manage bandwidth under ATM than under Ethernet, for example. The high cost of ATM relative to Ethernet is one factor that has limited its adoption to "backbone" and other high-performance, specialized networks. 

In a data communications environment, the network can range in scope from a token-ring LAN to an X.25 or Frame Relay WAN. Thus, although some features are common to both LAN and WAN environments, there is also some variability. In general, a data communications network transports data by using variable-length packets. Although many WAN protocols are connection-oriented, some are connectionless. Similarly, many LAN protocols are connectionless, whereas others are connection-oriented. Because data communications networks were designed to transport files, records, and screens of data, transmission delay or latency, if small, does not adversely affect users. In comparison, in a telecommunications network, a similar amount of latency that is acceptable on a data network could wreak havoc with a telephone conversation. Recognizing the differences among voice, video, and data transportation, ATM was designed to adapt to the time sensitivity of different applications. It includes different classes of service that enable the technology to match delivery to the time sensitivity of the information it transports.


Sources:

http://searchnetworking.techtarget.com/definition/ATM

http://www.techfest.com/networking/atm/atm.htm

http://compnetworking.about.com/od/networkprotocols/g/bldef_atm.htm

http://homepages.uel.ac.uk/u0124452/MyPage/Advantages%20and%20Disadvantages%20of%20ATM.htm