Asynchronous Transfer Mode (ATM) introduction

Today we are going to talk about the protocol Asynchronous Transfer Mode (ATM). ATM is one of the legacy protocol used for the WAN network by service providers apart from Frame Relay and these days very rare enterprise networking using these technologies.

What is Asynchronous Transfer Mode (ATM) ?
Asynchronous Transfer Mode (ATM) is a technology designed for the high-speed transfer of voice, video, and data through public and private networks using cell relay technology. 

ATM is an International Telecommunication Union Telecommunication Standardization Sector (ITU-T) standard. 

Ongoing work on ATM standards is being done primarily by the ATM Forum, which was jointly founded by Cisco Systems, NET/ADAPTIVE, Northern Telecom, and Sprint in 1991. 

A cell switching and multiplexing technology, ATM combines the benefits of circuit switching (constant transmission delay, guaranteed capacity) with those of packet switching (flexibility, efficiency for intermittent traffic). To achieve these benefits, ATM uses the following features:

• Fixed-size cells, permitting more efficient switching in hardware than is possible with variable-length packets 
• Connection-oriented service, permitting routing of cells through the ATM network over virtual connections, sometimes called virtual circuits, using simple connection identifiers 
• Asynchronous multiplexing, permitting efficient use of bandwidth and interleaving of data of varying priority and size

The combination of these features allows ATM to provide different categories of service for different data requirements and to establish a service contract at the time a connection is set up. 

This means that a virtual connection of a given service category can be guaranteed a certain bandwidth, as well as other traffic parameters, for the life of the connection.

ATM Cell Basic Format
The basic unit of information used by ATM is a fixed-size cell consisting of 53 octets, or bytes. The first 5 bytes contain header information, such as the connection identifier, while the remaining 48 bytes contain the data, or payload because the ATM switch does not have to detect the size of a unit of data, switching can be performed efficiently. 

The small size of the cell also makes it well suited for the transfer of real-time data, such as voice and video. Such traffic is intolerant of delays resulting from having to wait for large data packets to be loaded and forwarded.

Fig 1.1- Asynchronous Transfer Mode (ATM)

ATM Device Types:
An ATM network is made up of one or more ATM switches and ATM endpoints. An ATM endpoint (or end system) contains an ATM network interface adapter. Workstations, routers, data service units (DSUs), LAN switches, and video coder-decoders (CODECs) are examples of ATM end systems that can have an ATM interface. 

The above mentioned figure illustrates several types of ATM end systems router, LAN switch, workstation, and DSU/CSU, all with ATM network interfaces connected to an ATM switch through an ATM network to another ATM switch on the other side.

ATM Network Interface Types
There are two types of interfaces that interconnect ATM devices over point-to-point links: the User-Network Interface (UNI) and the Network-Network Interface (NNI), sometimes called Network-Node Interface. A UNI link connects an ATM end-system (the user side) with an ATM switch (the network side). An NNI link connects two ATM switches in this case, both sides are network. 

UNI and NNI are further subdivided into public and private UNIs and NNIs, depending upon the location and ownership of the ATM switch. A private UNI connects an ATM endpoint and private ATM switch; a public UNI connects an ATM endpoint or private switch to a public switch. 

A private NNI connects two ATM switches within the same private network; a public NNI connects two ATM switches within the same public network. A third type of interface, the Broadband Inter-Carrier Interface (BICI) connects two public switches from different public networks.

ATM Cell Header Formats:
The ATM cell includes a 5-byte header. Depending upon the interface, this header can be in either UNI or NNI format. The UNI cell header, as depicted in Figure 1-4, has the following fields: 

• Generic flow control (GFC): Provides local functions, such as flow control from endpoint equipment to the ATM switch. This field is presently not used. 
• Virtual path identifier (VPI) and virtual channel identifier (VCI):VPI identifies a virtual path leg on an ATM interface. VPI and VCI together identify a virtual channel leg on an ATM interface. Concatenating such legs through switches forms a virtual connection across a network. 
• Payload type (PT): indicates in the first bit whether the cell contains user data or control data. If the cell contains user data, the second bit indicates whether congestion is experienced or not, and the third bit indicates whether the cell is the last in a series of cells that represent a single AAL5 frame. (AAL5 is described in the “Service-dependent ATM Adaptation Layers” section on page 1-14.) If the cell contains control data, the second and third bits indicate maintenance or management flow information. 
• Cell loss priority (CLP): indicates whether the cell should be discarded if it encounters extreme congestion as it moves through the network. 
• Header error control (HEC): contains a cyclic redundancy check on the cell header.

ATM Services:
There are three general types of ATM services: 

• Permanent virtual connection (PVC) service: connection between points is direct and permanent. In this way, a PVC is similar to a leased line. 
• Switched virtual connection (SVC) service: connection is created and released dynamically. Because the connection stays up only as long as it is in use (data is being transferred), an SVC is similar to a telephone call. 
• Connectionless service: similar to Switched Multi-megabit Data Service (SMDS)

Advantages of PVCs
They are the guaranteed availability of a connection and that no call setup procedures are required between switches. Disadvantages include static connectivity and that they require manual administration to set up. 

Advantages of SVCs include connection flexibility and call setup that can be automatically handled by a networking device. Disadvantages include the extra time and overhead required to set up the connection.