Chapter 12: ATM

Asynchronous Transfer Mode (ATM)

Overview:

                                     

o   What is Asynchronous Transfer Mode (ATM)?

o   What are the advantages of asynchronous transfer mode (ATM)?

o   What are the disadvantages of asynchronous transfer mode (ATM)?

History:

            In 1992, a group of interested parties developed a set of standards−based specifications called the Asynchronous Transfer Mode (ATM). This was a step at developing a single set of standards for the integration of voice, data, video, and multimedia traffic on a single backbone network. Prior to this Development, the industries offered separate standards and networks for voice, others for data, and Still others for video communications. Above and beyond that, data networks were also treated differently with separate data networks for point−to−point needs, dialup data, and packet−switched data transmissions.

 

 

ATM defined

Asynchronous transfer mode (ATM) refers to a number of related technologies, including software, hardware, and connection media. ATM differs from other existing LAN and WAN technologies and was specifically designed to support high-speed communication. ATM allows networks to use bandwidth resources at maximum efficiency while maintaining Quality of Service (QoS) for users and programs with strict service requirements.

The basic components of ATM are the computers that are connected to the ATM network and the devices that connect these computers and ensure that data is transferred successfully. The computers that are connected to the ATM network are called end stations. Routers, DSLAMs, and ATM switches are examples of devices that connect end stations and ensure that data is transferred successfully.

Asynchronous means the available network bandwidth is not divided into fixed channels or slots synchronized by a timing mechanism or clock. Devices that communicate using asynchronous communication are not bound by design in terms of their ability to send and receive information at a precise transmission rate. Instead, the sender and receiver negotiate

ATM Protocols

It takes many protocols to support an ATM network, which is one of the issues that continually

comes up as a negative from the supporters of the gigabit Ethernet crowd. To develop the

necessary interfaces in support of the various points within a network (networks are pretty complex

in themselves), different protocols are necessary. The actual protocols needed depend on where

the traffic originates, what transport mechanisms must be traversed, and where the traffic will

terminate. a summary of protocols for the ATM user is shown in Figure.

What is Asynchronous Transfer Mode (ATM)?

ATM is a member of the fast packet−switching family called cell relay. As part of its heritage, it is an

evolution from many other sets of protocols. In fact, ATM is a statistical time−division multiplexed

(TDMed) form of traffic that is designed to carry any form of traffic and enables the traffic to be

delivered asynchronously to the network. When traffic in the form of cells arrives, these cells are

mapped onto the network and are transported to their next destination. When traffic is not available,

the network will carry empty (idle) cells because the network is synchronous.

What are the advantages of asynchronous transfer mode (ATM)?

The principle advantage is it's flexibility; it can be used to carry data, voice and video over the same physical medium. Traditionally, separate media was used for the different type of traffic, so a single medium solution saves money.

Benefits of ATM

ATM provides a flexible and scalable solution to the increasing need for quality of service in networks where multiple information types (such as data, voice, and real-time video and audio) are supported. With ATM, each of these information types can pass through a single network connection.

ATM can provide the following benefits:

• High-speed communication
• Connection-oriented service, similar to traditional telephony
• Fast, hardware-based switching
• A single, universal, int \/roperable network transport
• A single network connection that can reliably mix voice, video, and data
• Flexible and efficient allocation of network bandwidth

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:

• 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

Mapping Circuits Through an ATM Network

ATM uses one of two connection types. The protocol is connection−oriented, so the two choices are a PVC or a SVC. There is actually no permanency to the circuits. They are logically mapped through the network and are used when needed for PVC or dial−connected when using the SVC. In either case, the carriers promise only to make a best attempt to serve the needs of the end user when the time is appropriate. With no true guarantees, the consumer is at risk (sort of). However, the concept is that the network provider will provide a committed bandwidth available to the user on demand whenever the user wants to use it. This forms the basis of what ATM networks are all about: on−demand, high−speed communications networks. The connection is built into a routing table in each of the switches involved with the connection from end to end. As such, the switches only need to look up a table for the incoming port and channel and then determine the mapping (in the same table) for the output port and channel.





Figure :  ATM table lookup maps the input and output channels.

Note that the connection from the end user to the network may be on a T1, T3, or OC−n. From the first switch out, the network will use Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH) capabilities possibly mapped onto a Dense Wave Division Multiplexer (DWDM). The network carrier will use whatever services and bandwidth is available at the connection points.

Figure shown: Virtual path switching remaps the path, but keeps the channel the same.

Integrated Access at the Local Loop

Because DSL links are ready−made for voice and data, and ATM excels at carrying varied traffic, using VoATM over DSL over the local loop to the customer is a natural extension of these services. To enable the combination, equipment that supports VoATM is needed at each end of the local loop: a next−generation integrated access device (IAD) at the customer premises and a voice gateway at the CO as shown in Figures .

                                   Figure : The IAD will add services for the future.

 

                                      Figure : Combining the ATM and DSL at the local loop

 

These devices make it possible for the local exchange provider or the competitive DSL provider to use the existing facilities and still satisfy the needs of voice and data over the existing local loop. Using voice over the DSL circuit enables up to 16 simultaneous VoIP calls and Internet access to simultaneously run over the bandwidth on the single cable pair.

the network switches handle the mapping on the basis of VPI switching. VPI switching means that the switches use the virtual path for mapping through the network and will remap from one virtual path to another, while the virtual channel number is held consistent through the entire network.

the rate at which they will communicate based on physical hardware limitations and the ability to maintain a reliable flow of information within the network.

Transfer mode refers to the way the information is transferred between sender and receiver. In ATM, the concept of small, fixed-length cells is used to structure and parcel data for transfer. By using cells that contrast directly with the variable-length packet mechanism used by most existing network technologies, ATM assures that connections can be negotiated and managed so that no single data type or connection can monopolize the transfer path.

A Carrier's Transport

ATM succeeded in the carriers' networks, being deployed by major telephone companies and ISPs and sizable private enterprises. It was always installed for mission critical backbones because of its quality of service (QoS).

This shows how ATM is used as a network backbone or "switch fabric" within the enterprise. The edge device is an Ethernet workgroup switch with a high-speed ATM link. It converts LAN packets into ATM cells and vice versa.



source:

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

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

Broadband Telecommunications Handbook