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(ATM) Asynchronous Transfer Mode. Asynchronous Transfer Mode (ATM) Definition / Architecture Switches Virtual Connections Setup Cell Formats Adaptation Layer LAN over ATM (Protocol issue). ATM Definition / Architecture.

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Asynchronous Transfer Mode

Asynchronous Transfer Mode (ATM)

Definition / Architecture


Virtual Connections Setup

Cell Formats

Adaptation Layer

LAN over ATM (Protocol issue)

ATM Definition / Architecture

Asynchronous Transfer Mode (ATM) is the cell relay protocol designed by the ATM Forum and adopted by ITU-T. It is a wide area protocol featuring high data rates and equal-sized packets (cells). ATM is suitable for transferring text, audio and video data.

Some of the key concepts involved in ATM include:

The basic unit of transmission is the cell. A cell is 53 bytes in length - 5 bytes of headerand 48 bytes of payload. These small, fixed-format cells can be switched very quickly.

ATM networks are based on connection-oriented cell relay - cells are multiplexed onto higher bit rate channels, but there is no timing relationship between individual cells within a particular connection. Cells may be dropped but are never delivered out of order.

ATM Definition / Architecture (cont….)

The base data rate for an ATM data link is 155 Mbps, with higher rates of 622 Mbps and 1200 Mbps. It's fast.

The telecommunications industry sees ATM as the basis for an entirely new class of service, Broadband ISDN (B-ISDN), which will permit seamless integration of voice (telephony), HiFi audio, video (broadcast, video-on demand, conferencing, etc) and data.

ATM is complex.

ATM Virtual Connection

The existence of a VC is based on entries in routing tables in each of the switches (which are really routers), set up at VC establishment time.

Each switch compares its table entries to a field in the cell header to decide where the cell is to be sent.

ATM Virtual Connection Setup

All cells relating to a connection (call, circuit) follow the same path through the network. During call setup, a separate Protocol Connection Identifier is assigned to the connection on each link through the network.

The numeric value of the PCI identifier has only local significance at each link, and changes from one link to another as cells relating to a particular call (or connection) pass through the network.

Associated with each incoming link/port at a switch is a routing table that contains, for each incoming PCI, the corresponding outgoing link/port and the new PCI to be used.

ATM Virtual Connection Setup (Cont……)

The PCI is made up of two subfields: the virtual path identifier (VPI) and the virtual channel identifier (VCI). Routing can be on the basis of the value of either, or both, of these subfields.

The VPI can be used where multiple connections are set up between the same pair of hosts. The VCI field is used where connections are not between the same pair of hosts.

ATM Cell Format


Generic flow control, only present in cells passing over the user-network interface. On network-network interfaces (NNI), this is part of the VPI field.


Virtual path and virtual channel identifier


payload type identifier. All data cells have a zero in the MSB position.


Cell loss priority. 0 == HIGH, 1 == LOW


Header Error Correction, or checksum.

ATM Adaptation Layer

ATM was designed to replace all existing digital and analog telecommunication

systems. The underlying lightweight packet stream had to be adapted to provide a number of different types of service.

This was achieved by placing an adaptation layeron top of the lightweight packet stream. The adaptation layer hides the underlying packet stream and presents an appropriate interface to the higher layer(s). Various adaptation layers have been defined viz.

The ATM Adaptation Layer (AAL) creates and receives 48-byte payloads through the lower layers of ATM on behalf of different types of applications.

ATM Adaptation is necessary to link the cell-based technology at the ATM Layer to the bit-stream technology of digital devices (such as telephones and video cameras) and the packet-stream technology of modern data networks (such as frame relay, X.25 or LAN protocols such as TCP/IP or Ethernet).

ATM Adaptation Layer

AAL1 defines a constant bit rate stream between the ends of the ATM virtual circuit (VC). AAL1 emulates a traditional phone call. AAL1 steals one byte from the payload field to extend the cell header.

AAL2 as for AAL1, except the bit rate can vary. Nevertheless, the ends of the

VC still have a fixed timing relationship. This is appropriate for transfer of, eg,

compressed video. AAL2 steals 3 bytes from the payload field for


AAL3/4 originally specified as two distinct adaptation layers, this is now a

complex specification for a connectionless data service over ATM. Higher

layer data packets (up to 64 Kbytes) have 4 bytes each of header and trailer

information added, then are diced into ATM cells from which 4 bytes of

payload are stolen for admin purposes.

AAL5 (designed to replace AAL3/4) accepts data packets of up to 64Kbytes,

adds 8 trailer bytes, then dices this up to fit in ATM cells (without any further


ATM Adaptation Layer (AAL5 and encapsulation of Internet data)

Send IP datagrams over

an ATM service

using AAL5

LANs over ATM

Where ATM is to be used to form a LAN, fundamental differences between ATM and IP becomes troublesome:

IP is connectionless and has provision for broadcasting built in.

Everything is carried by IP

ATM is connection oriented. (broadcasting is difficult)

ATM and IP addressing are incompatible

A number of different approaches have been proposed to solve these problems.

· CLIP - Classical IP over ATM

· LANE - LAN Emulation

· NHRP - Next Hop Resolution Protocol

· MPOA - Multi-protocol over ATM

· MPLS - label switching

LANE - LAN Emulation

Standard defined by the ATM Forum

Stations attached via ATM have the same capabilities that they normally obtain from legacy LANs, such as Ethernet and Token Ring.

Emulate a LAN on top of an ATM network. Specifically, the LANE protocol defines mechanisms for emulating either an IEEE 802.3 Ethernet or an 802.5 Token Ring LAN.

LANE - LAN Emulation

LANE consists of two primary components:

The LANE client allows LAN protocols and applications that use LAN to function as if they were communicating with a traditional LAN. It exposes LAN functionality at its top edge (to users) and native ATM functionality at its bottom (to the ATM protocol layers).

The LANE services are a group of native ATM applications that hide the connection-oriented nature of ATM from connectionless legacy protocols. These services maintain the databases necessary to map LAN addresses to ATM addresses, thus allowing the LANE clients to create connections and send data.

CLIP - Classical IP over ATM

Standard defined by the ATM Forum

IP treats ATM as another subnetwork to which routers and hosts are attached

Multiple Logical IP subnetworks (LIS) are overlaid on top of the ATM network.

CLIP - Classical IP over ATM

IP over ATM is handled by two main components:

The IP over ATM server is composed of an ATMARP server and Multicast Address Resolution Service (MARS).

The ATMARP server provides services to map network layer IP unicast addresses to ATM addresses, while MARS provides similar services for broadcast and multicast addresses. Both services maintain IP address databases just as LANE services do.

IP over ATM clients in the same logical IP subnet (LIS) need to be configured to use the same ATMARP server. Traditional routing methods are used to route between logical IP subnets, even if they are on the same physical network.


IP over ATM is often preferred to LANE because it is faster than LANE.

A primary reason for this performance advantage is that IP over ATM adds almost no additional header information to packets as they are handed down the stack.

After it has established a connection, the IP over ATM client typically can transfer data without modification.

NHRP - Next Hop Resolution Protocol

NHRP uses “Next Hop” clients to initiate connections to “Next Hop” servers.

Together, these act to discover an ATM destination address corresponding

To an IP address.

The originating client then establishes an ATM connection directly to the

Destination host. Intermediate routers act as both NHRP servers and NHRP

Clients during call set-up.

MPOA - Multi-protocol over ATM

Design by the ATM forum

Integrates LANE and NHRP

Uses LANE to establish layer 2 connections (ie. Within the same LIS)

Uses NHRP to establish layer 3 connections (to another LIS)

Constant setting up and tearing down of ATM virtual circuits stresses the

ATM switches.

MPLS - Label Switching

An IETF initiative, currently under development.

The big problem with all the previous approaches has been the duplicated Network infrastructures (IP and ATM) with their incompatible addressing Models and routing techniques

Label switching routers are intended to integrate layer 2 switching

Functions with layer 3 addressing.

The intention is that the destination Ip address would be replaced by a small “label”. The label would identify the virtual circuit between adjacent LSRs. When Delivered by ATM, the label becomes equivalent to the ATM Virtual Circuit Identifier.

IP Switching

Introduced by a company called “lpsilon”

A combination of IP router and ATM switch which combines the best of both


This technique relies on the fact that where classical IP ATM is in use, the

First cell of an encapsulated datagram will contain the entire IP and TCP

or UDP) headers, presuming minimum length headers.

This allows the ATM switch to make a routing decision (on the basis of the IP destination address) and a stream decision (on the basis of the TCP header) on the basis of the contents of the first cell.

If the ATM switch is also a router (ie, connected to more than one LIS), it can immediately begin sending the cells of a datagram to its destination,