What Is ATM?

Asynchronous Transfer Mode, a network technology based on transferring data in cells or packets of a fixed size.
The cell used with ATM is relatively small compared to units used with older technologies.
The small, constant cell size allows ATM equipment to transmit video, audio, and computer data over the same network, and assure that no single type of data hogs the line.
The cost of maintaining separate, specialized networks for computer, voice, and video is high.
To reduce networking costs, ATM enables integration of all of these services on a single network and the combination of existing networks into a single infrastructure.
In particular, Windows operating systems provide rich connectivity using Asynchronous Transfer Mode (ATM) while maintaining support for legacy systems.

ATM Overview

ATM is a connection-oriented, unreliable (does not acknowledge the receipt of cells sent), virtual circuit packet switching technology.
Unlike most connectionless networking protocols, ATM is a deterministic networking system — it provides predictable, guaranteed quality of service.
From end to end, every component in an ATM network provides a high level of control. ATM technology includes:

Scalable performance. ATM can send data across a network quickly and accurately, regardless of the size of the network. ATM works well on both very low and very high-speed media.
Flexible, guaranteed Quality of Service (QoS). ATM allows the accuracy and speed of data transfer to be specified by the client. This feature distinguishes ATM from other high-speed LAN technologies such as gigabit Ethernet. The QoS feature of ATM also supports time dependent (or isochronous) traffic. Traffic management at the hardware level ensures that quality service exists end-to-end. Each virtual circuit in an ATM network is unaffected by traffic on other virtual circuits. Small packet size and a simple header structure ensure that switching is done quickly and that delays due to high traffic are minimized.
Unobstructed speed. ATM imposes no architectural speed limitations. Its pre-negotiated virtual circuits, fixed-length cells, message segmentation and re-assembly in hardware, and hardware-level switching all help support extremely fast forwarding of data.
Integration of different traffic types. ATM supports integration of voice, video, and data services on a single network. ATM over Asymmetric Digital Subscriber Line (ADSL) enables residential access to these services.

ATM Fixed-Length Cells

ATM uses small (53-byte), fixed-length cells that require less logic to process, the network spends no time determining where a particular cell begins and ends.
The small cell size ensures that delays in forwarding cells are minimized. Because the cell size is so predictable, buffer usage and analysis algorithms can be simplified and optimized.
In LAN technologies, such as Ethernet, have inherent speed limitations Either the underlying infrastructure (the cable) or the segment length must be changed to support fast traffic.

It can invent a faster physical layer — if you can design a quicker method of transmitting data from one place to another over one wire or many wires — ATM can work over that physical layer and at those new speeds.
ATM allows information with different requirements and from different nodes to be transmitted nearly simultaneously without conflict.
ATM places fixed-length cells on the media when the data is produced according to the parameters of a negotiated connection.
ATM can simultaneously handle the needs of isochronous (time-dependent) traffic, such as voice and video, and non-isochronous traffic, such as LAN data.
It can carrymultiple types of traffic with end-to-end quality of service

ATM Cell Basics

• ATM carries information based on fixed length cells

– Compare this to the other packet switching technologies such as Frame Relay etc. where each packet may be of a different length

• The length of each cell is 53 Bytes

– First 5 bytes are used as the cell header

– Next 48 bytes are used as the payload carrying the data

The primary function of this header is the physical access control, it is often used to reduce cell jitters in CBR services, assign fair capacity for VBR services, and to control traffic for VBR flows.
Such functionality requires the power to control any UNI structure, be it a ring, a star, some bus configuration, or any combination of these.

The role of the VPI/VCI fields is to indicate Virtual Path or Virtual Channel identification numbers, so that the cells belonging to the same connection can be distinguished.
A unique and separate VPI/VCI identifier is assigned in advance to indicate which type of cell is following, unassigned cells, physical layer OAM cells, metasignalling channel or a generic broadcast signaling channel.

The Payload Type Indicator (PTI) is a 3-bit field. Its bits are used as follows:
The first bit indicates the type of ATM cell that follows. A first bit set to 0 indicates user data; a bit set to 1 indicates operations, administration & management (OA&M) data.
The second bit indicates whether the cell experienced congestion in its journey from source to destination. This bit is also called the Explicit Forward Congestion Indication (EFCI) bit. The second bit is set to 0 by the source; if an interim switch experiences congestion while routing the cell, it sets the bit to 1. After it is set to 1, all other switches in the path leave this bit value at 1.
Destination ATM endpoints can use the EFCI bit to implement flow control mechanisms to throttle back on the transmission rate until cells with an EFCI bit set to 0 are received.
The third bit indicates the last cell in a block for AAL5 in user ATM cells. For non-user ATM cells, the third bit is used for OA&M functions.

The Cell Loss Priority (CLP) field is a 1-bit field used as a priority indicator. When it is set to 0, the cell is high priority and interim switches must make every effort to forward the cell successfully.
When the CLP bit is set to 1, the interim switches sometimes discard the cell in congestion situations. The CLP bit is very similar to the Discard Eligibility (DE) bit in Frame Relay.
An ATM endpoint sets the CLP bit to 1 when a cell is created to indicate a lower priority cell.
The ATM switch can set the CLP to 1 if the cell exceeds the negotiated parameters of the virtual channel connection.
This is similar to bursting above the Committed Information Rate (CIR) in Frame Relay.

The Header Error Check (HEC) field is an 8-bit field that allows an ATM switch or ATM endpoint to correct a single-bit error or to detect multi-bit errors in the first 4 bytes of the ATM header.
Multi-bit errored cells are silently discarded. The HEC only checks the ATM header and not the ATM payload.
Checking the payload for errors is the responsibility of upper layer protocols.