Asynchronous Transfer Mode

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Asynchronous Transfer Mode (ATM) technology is capable of transferring voice, video, and data through private and public networks. It is built on a cell-based architecture rather than on a frame-based architecture. ATM cells are always a fixed length of 53 bytes. The ATM cell contains a 5 byte ATM header followed by 48 bytes of ATM payload. Small, fixed-length cells are well suited for carrying voice and video traffic because this traffic is intolerant of delay. Video and voice traffic do not have to wait for a larger data packet to be transmitted.

The 53 byte ATM cell is less efficient than the bigger frames and packets of Frame Relay and X.25. Furthermore, the ATM cell has at least 5 bytes of overhead for each 48-byte payload. When the cell is carrying segmented network layer packets, the overhead is higher because the ATM switch must be able to reassemble the packets at the destination. A typical ATM line needs almost 20 percent greater bandwidth than Frame Relay to carry the same volume of network layer data.

ATM was designed to be extremely scalable and can support link speeds of T1/E1 to OC-12 (622 Mb/s) and higher. 

ATM offers both PVCs and SVCs, although PVCs are more common with WANs. And as with other shared technologies, ATM allows multiple VCs on a single leased-line connection to the network edge.?????

Similarities and differences between frame relay and ATM:



Virtual circuit based technology

Frame relay has a variable sized frame whereas ATM has a fixed sized cell.

?Allow establishment and release of? SVCs/PVCs.????

Frame relay supports moderate data rates ( ~2mbps ) while ATM supports very high data rates (~gbps )

Assume reliable links, smart end systems

Frame relay is designed for data while ATM is designed for data as well as voice, video and multimedia applications.

No flow control

Reference model in frame relay is two-dimensional while it is three-dimensional in ATM.

Congestion notification and discard eligibility feature

Standardization is more elaborated in ATM as compared to frame relay.


Frame relay is a virtual leased line technology for WAN inter-connectivity. ATM has a much wider scope.


Frame relay uses single label DLCI, whereas ATM uses two labels VPI/VCI.

Various applications of ATM network.
Ans:- Various applications are:-

  • Wireless ATM (WATM)

With the advent of wireless LANs, wireless ATM looms on the horizon. Several companies are on the verge of announcing products compliant with today's ATM switches. In the wireless arena, ATM switches set up low latency VCCs among voice switches to provide call and service type signaling. In this manner, ATM switches provide a front end to the wireless services, eliminating the need to connect every wireless switch to every voice switch, and saving lots of money. ATM switching will also play a major role in supporting wireless multimedia services and as Internet gateways for wireless subscribers.

  • ATM and 3G Networks:

In recent times, there has been a gradual migration of voice users from fixed wireline networks to wireless mobile cellular networks. The wireless networks too have evolved from Second Generation (2G) networks like Global System for Mobile Communication (GSM) and General Packet Radio Service (GPRS) to newer Third Generation (3G) networks like Universal Mobile Telecommunications System (UMTS). As the networks evolve, there is associated requirement for higher bandwidth? and? better service capabilities. Towards this end, the ability of ATM to provide speed transfer with service guarantees emerges as a suitale choice for transfer of voice and data in wireless networks.

  • ATM and MPLS Networks:

A recent industry development for the internetworking between IP and ATM is Multi-Protocol Label Switching (MPLS). MPLS is a new industry development standardized by the IETF. MPLS is being standardized as a technology that can best be described as a synergistic approach between IP and ATM. MPLS merges the flexibility of the IP routing protocols with the speed that ATM switches provide to introduce fast packet switching in frame based IP networks.
IETF started the MPLS standardization process with the intention of introducing label switching within IP networks. While the initial focus was on IP networks alone, the MPLS framework is flexible enough enough to work with multiple protocols. Due to its close resemblance to the ATM protocol, MPLS has many possible applications when used in conjunction with ATM. Amongst the two main areas of research involving ATM and MPLS is the support of IP over ATM using MPLS, and the concept of ATM-MPLS network internetworking.


ATM was always cited and, indeed designed, as a suitable technology for the communication of multimedia traffic including voice. There are many ways in which voice can be carried over ATM, and this fact is reflected in the many standards that have been developed by the ATM Forum and the ITU-T. We shall look at the various standard methods available, but concentrate on the support of voice over ATM Adaptation Layer 2 (AAL2).

  • Circuit emulation using both the unstructured and structured modes of AAL1
  • DBCES - Dynamic Bandwidth Circuit Emulation Service using AAL1
  • An in-depth look at the support of voice using AAL2

??????????????????????????????? ATM in the 3rd generation - 3G - UMTS mobile environment
Voice over DSL (Digital Subscriber Line)


The following example shows in detail how an SVC connection is established in an ATM network.

  • ATM and DSL Networks:

For decades, residential users have been using the dial-up modem to access the Internet via the Public Switched Telephone Network (PSTN). However. With the theoretical upper bound of 56 kbps on the bandwidth available using dial-up modems, other broadband access technologies are being explored. Not all solutions proposed for broadband access are easily deployable. Most solutions for broadband access involve huge infrastructure costs in the installation of new network elements and the networking of these elements by cables. This means that the solutions are not cost effective and are beyond the reach of the common users.
Amongst the proposed solutions for broadband access, Digital Subscriber line (DSL) is being seen as a technology that will over the above limitations of cost to offer a cost



In the academic arena, ATM technology facilitates fast, reliable, and dependable access to an expanding array of Web initiatives and institutional resources. ATM enables tele-education, telementoring, and real-time interactions with subject experts in remote locations; multimedia applications; and curricular enhancement and enrichment. ATM also promotes deployment of virtual schools, virtual universities, virtual museums, and virtual communities.

ATM is a connection-oriented virtual network transmission and switching technology that combines the low-delay of circuit-switched networks with the bandwidth flexibility and high-speed of packet-switched networks. ATM is an enabler of basic and advanced applications such as remote sensing, 3-D (three-dimensional) interactive simulations, tele-instruction, biological tele-research, and medical tele-consultations. Edge devices at the boundary of an ATM network convert non-ATM traffic streams into standard ATM cells.
ATM technology is implemented in backbone, enterprise, and edge switches as well as hubs, routers, bridges, multiplexers, servers, server farms, and NICs (Network Interface Cards) in high-end Internet appliances. The ATM Data Exchange Interface (DXI) enables fast access to public network services. A flexible and extendible networking solution, ATM technology supports network configurations that include DANs (Desk Area Networks), LANs, MANs (Metropolitan Area Networks), WANs (Wide Area Networks), and GANs (Global Area Networks).


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