Asynchronous transfer mode - the future network

ON A global level, data communications is moving toward a single, public networking environment. This will probably use an infrastructure with multi-giga bit rate of transmission using optical fibre links on the SDH (Synchronous Digital Hierarchy). The development in Switching technology for data communication has been phenomenal in the past-first using X2.25 protocol where store and forward technique is employed and then the advanced `frame-relay' technique where variable frame structure signalling and data transfer mechanism across intelligent user-end devices is used and now advanced further to met high end-users.

With the progress in the field of fast switching logic circuitry to cater to speeds in Gigabits per second, a method based on fast packet- switching using short fixed-length packets, known as Asynchronous Time Division (ATD) multiplexing has been introduced in data communication network and named popularly as ATM - Asynchronous Transfer Mode.

Asynchronous Transfer mode is a technology that can support a wide variety of applications in several different network environments. ATM is a connection-oriented network using virtual paths and virtual channel connections. Salient features of ATM include

- a flexible platform that makes efficient use of common network resources

- functions as common denominator for both Access Network and the Backbone Network

- facilitates end-user access to service networks and efficient interconnection of modes and equipments within these service networks,

- improves and simplifies Wide Area Networks (WAN) services

-serves as foundation for third generation mobile access networks

Voice and telephony form larger content in telecom transmission networks. Mostly they are transported in a switched path as circuit switched connection where the hardware is totally held up for each voice call. After the emergence of Internet that has evolved a new protocol called IP, the packet switching technology has found many applications at reduced cost. A new option for voice networking is being heralded, as transport of voice moves from circuit-switching to packet-switching technique. Moving voice networking to ATM benefits the network operator as also the end user. The obvious benefit of voice over ATM is its potential for lowering transmission costs.

In the existing STM (Synchronous Transfer Mode) based networks trunk interfaces are dedicated to specific routes and trunking transport capacity is provisioned according to busy hour traffic needs. But ATM uses on demand virtual channel connections for transporting groups of telephony trunks of optimum sizes enabling sharing of brandwidth between different routes. The trunk group resources are controlled by powerful processors and signalling procedures.

By mapping trunk groups on individual ATM virtual channel connections, voice delay due to ATM cell packetisation can be kept at minimum. ATM technology is very well switched for voice traffic as it provides on demand flexible bandwidth connection on large scale. Building a multiservice network with ATM switching for voice, video and data provides the operator with a number of cost advantages.

The circuit switched 64 kbps pulse-code modulated (PCM) voice can be transported over ATM using ATM Adaptation Layer (AAL) type 1 or type 2 with the introduction of compressed voice. The bit rate becomes lower, say 8 kbps, and AAL2 on ATM can provide bandwidth efficient transport of low bit rate real time services such as variable bit rate (VBR) voice. Voice transport over ATM can serve as a path over which parts of the PSTN/ ISDN can migrate to become multi-service network.

ATM switch can be designed for a capacity of 10 GB or 20 GB with scaleable features. User connectivity is given according to the requirement of the customer in different bandwidths.

Technical features of ATM that gives a cutting-edge over other networks are:

- Transmission over fibre optic cables, whether local or trunk connections.

- Parallel Transmission Capability, viz., switching and connecting any node to any other node in a dedicated manner.

- Maximum speed of operation with a required throughput.

- Usage of fixed cell lengths permitting incorporation of error correction and routing.

- Switching capabilities to enable establishing multiple virtual circuits between transmitter and receiver.

- Ability to simultaneously transmit many signals like audio, video and data for multimedia applications.

ATM cell format

The fundamental structural and functional unit of ATM is the cell. The ATM Cell consists of two parts called the Header (3 to 8 octets long in size) and Information Fields (32 to 120 octets long in size). The packets of information to be transferred from one user to another are in fixed-sized blocks called Cell. As ATM is a connection oriented switching technique, the header must contain a virtual channel identifier (VCI) or virtual path identifier (VPI).All cells of same connection are transferred through the same route using VCI or VPI. This ensures that cells are received in the same order as transmitted. Signalling information is transported on separate virtual channels.

Any of the two methods of transmission plans can be used in ATM i.e., using ATM cells of the synchroous payload frame or pure. ATM where there are no frames. In the frameless case an error- check algorithm placed in the cell header achieves synchronization.

The cell connection and bandwidth allocations are made by a system called `connection-admission - control (CAC). This function translates the incoming connection parameter into bandwidth value.

This function has a specially designed algorithm for each service category. To meet requirement of efficiency and quality of service (QoS) each algorithm is tailored to the switch architecture, buffering and scheduling principles for a particular service. There are some standardised class of service categories in ATM.

Constant Bit Rate (CBR): This is used for transmitting voice and video over circuit switched networks. Here cell loss and cell delay conditions are clearly defined with constant bit rate.

-Variable bit Rate (VBR): Real time VBR is used for packetized voice and video transmission. Non-real time VBR is the basic ATM service for all data application. Here bit rate is variable and requires a defined delay for delivery which is characterized by Peak Cell Rate and sustainable cell rate.

-Available Bit Rate (ABR): This is used for bursty traffic and characterized by Peak Cell Rate and Minimum Cell Rate. ATM Forum has standardized ABR service for multimedia applications.

-Unspecified bit Rate: This class of service is a connectionless data service. Data gram traffic is an example of this.

Internet over ATM

Internet uses IP protocol for transporting informations in the form of `data gram' unit, which is route independent across a single or several networks. This transmission is not guaranteed for reliability or sequence of delivery but only a best - effort - made to deliver safely. Integrating the ATM technology into the existing internet technology requires schemes for managing the transmission of IP datagrams over ATM networks.

Such schemes ideally will take advantage of the strengths of ATM such as its capacity for supporting quality of service guarantees while effectively bridging the gap between the data forwarding model of ATM (virtual - circuits) and the internet (data grams) ATM networks have the potential to provide real-time performance guarantees, such as bounds on bandwidth and packet loss. The performance guarantees are likely to improve the internetworking to a greater extend.

ATM technology, which offers much higher aggregate bandwidth and traffic engineering capabilities, needs to be utilized to meet the IP traffic growth because of the increase in number of real- time multimedia applications and demand for meeting the requirement to introduce Value Added Services with QoS.

Congestion control in ATM networks:

The effect of congestion in the packet switched networks is performance degradation in ATM. The congestion is caused due to saturation of the network resources such as communication links, processor speed and buffers. Congestion will reflect in delay in delivery, wastage of system resources and possibly a total network failure.

The problem of congestion becomes particularly critical in ATM networks due to diverse service requirements, high link speed and diverse characteristics of traffic. Simply increasing the resources cannot solve this problem in ATM. But a scheme for avoiding, preventing and recovering from a state of network congestion is required and it is known as congestion control.

Two methods of congestion control are possible in ATM:

- Preventive Control: This is a open loop control where parameters are specified. If there is available bandwidth then the connection request is accepted by the network and desired QoS (quality of service) is guaranteed throughout the session.

The preventive control takes necessary action to prevent congestion by restricting the admission of calls to the network. This preventive control is basically an admission control mechanism, which is resource allocation mechanism maintaining balance between QoS and network resource utilization by limiting the number of calls into the network.

- Reactive control: Closed loop control or reactive control schemes dynamically regulate cell emission rates of the various sources by using feedback information from the network. This type of control takes necessary action to recover from the state of congestion.

A. Ganesan, Divisional Engineer BSNL, Chennai Telephones

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