Asynchronous Transfer Mode (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.  It is also 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 switched 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 switched telephone network (PSTN) and Integrated Services Digital Network (ISDN), but its use is declining in favour of All IP.

Table 1: Comparing Network Features

Feature	Data Communications	Telecommunications	ATM
Traffic support	Data	Voice	Data, voice, video
Transmission unit	Packet	Frame	Cell
Transmission length	Variable	Fixed	Fixed
Switching type	Packet	Circuit	Cell
Connection type	Connectionless or Connection-oriented	Connection-oriented	Connection-oriented
Time sensitivity	None to some	All	Adaptive
Delivery	Best effort	Guaranteed	Defined class or guaranteed
Media and operating rate	Defined by protocol	Defined by class	Scalable
Media access	Shared or dedicated	Dedicated	Dedicated

           

        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.

Advantages of ATM

• High evolution potential, works with existing, legacy technologies 
• ATM supports voice, video and data allowing multimedia and mixed services over a
• Single network. 
• 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 

ATM disadvantages

• 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 

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References:

http://technet.microsoft.com/en-us/library/bb726929.aspx

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

http://en.wikipedia.org/wiki/Asynchronous_Transfer_Mode

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