ATM

Asynchronous Transfer Mode (ATM)

Switching

Background

Asynchronous Transfer Mode (ATM) is an International Telecommunication Union–

Telecommunication Standardization Sector (ITU-T) standard for cell relay wherein information for

multiple service types, such as voice, video, or data, is conveyed in small, fixed-size cells. ATM

networks are connection oriented. This chapter provides summaries of ATMprotocols, services, and

operation. Figure 20-1 illustrates a private ATMnetwork and a public ATMnetwork carrying voice,

video, and data traffic.

Standards

ATMis based on the efforts of the ITU-T Broadband Integrated Services Digital Network (BISDN)

standard. It was originally conceived as a high-speed transfer technology for voice, video, and data

over public networks. The ATMForum extended the ITU-T’s vision of ATMfor use over public and

private networks. The ATM Forum has released work on the following specifications

ATM Devices and the Network Environment

20-2 Internetworking Technology Overview

• User-to-Network Interface (UNI) 2.0

• UNI 3.0

• UNI 3.1

• Public-Network Node Interface (P-NNI)

• LAN Emulation (LANE)

ATM Devices and the Network Environment

ATMis a cell-switching and multiplexing technology that combines the benefits of circuit switching

(guaranteed capacity and constant transmission delay) with those of packet switching (flexibility and

efficiency for intermittent traffic). It provides scalable bandwidth from a few megabits per second

(Mbps) to many gigabits per second (Gbps). Because of its asynchronous nature, ATM is more

efficient than synchronous technologies, such as time-division multiplexing (TDM).

With TDM, each user is assigned to a time slot, and no other station can send in that time slot. If a

station has a lot of data to send, it can send only when its time slot comes up, even if all other time

slots are empty. If, however, a station has nothing to transmit when its time slot comes up, the time

slot is sent empty and is wasted. Because ATM is asynchronous, time slots are available on demand

with information identifying the source of the transmission contained in the header of each ATM

cell.

ATM Cell Basic Format

ATM transfers information in fixed-size units called cells. Each cell consists of 53 octets, or bytes.

The first 5 bytes contain cell-header information, and the remaining 48 contain the “payload” (user

information). Small fixed-length cells are well suited to transferring voice and video traffic because

such traffic is intolerant of delays that result from having to wait for a large data packet to download,

among other things. Figure 20-2 illustrates the basic format of an ATM cell.

Figure 20-2 An ATM network comprises ATM switches and endpoints.

ATM Devices

An ATMnetwork is made up of an ATMswitch and ATMendpoints. An ATM switch is responsible

for cell transit through an ATM network. The job of an ATM switch is well defined: it accepts the

incoming cell from an ATM endpoint or another ATM switch. It then reads and updates the

cell-header information and quickly switches the cell to an output interface toward its destination.

An ATM endpoint (or end system) contains an ATM network interface adapter. Examples of ATM

endpoints are workstations, routers, digital service units (DSUs), LAN switches, and video

coder-decoders (CODECs). Figure 20-3 illustrates an ATM network made up of ATM switches and

ATM Network Interfaces

An ATM network consists of a set of ATM switches interconnected by point-to-point ATM links or

interfaces. ATMswitches support two primary types of interfaces: UNI and NNI. The UNI connects

ATM end systems (such as hosts and routers) to an ATM switch. The NNI connects two ATM

switches.

Depending on whether the switch is owned and located at the customer’s premises or publicly owned

and operated by the telephone company, UNI and NNI can be further subdivided into public and

private UNIs and NNIs. A private UNI connects an ATM endpoint and a private ATM switch. Its

public counterpart connects an ATM endpoint or private switch to a public switch. A private NNI

connects two ATM switches within the same private organization. A public one connects two ATM

switches within the same public organization.

An additional specification, the Broadband Interexchange Carrier Interconnect (B-ICI), connects

two public switches from different service providers. Figure 20-4 illustrates the ATM interface

specifications for private and public networks.

ATM Cell-Header Format

An ATM cell header can be one of two formats: UNI or the NNI. The UNI header is used for

communication between ATM endpoints and ATM switches in private ATM networks. The NNI

header is used for communication between ATM switches. Figure 20-4 depicts the basic ATM cell

format, the ATM UNI cell-header format, and the ATM NNI cell-header format.

Unlike the UNI, the NNI header does not include the Generic Flow Control (GFC) field.

Additionally, the NNI header has a Virtual Path Identifier (VPI) field that occupies the first 12 bits,

allowing for larger trunks between public ATM switches.

ATM Cell-Header Fields

In addition to GFC and VPI header fields, several others are used in ATM cell-header fields. The

following descriptions summarize the ATM cell-header fields illustrated in Figure 20-5:

• Generic Flow Control (GFC)—Provides local functions, such as identifying multiple stations

that share a single ATM interface. This field is typically not used and is set to its default value.

• Virtual Path Identifier (VPI)—In conjunction with the VCI, identifies the next destination of a

cell as it passes through a series of ATM switches on the way to its destination.

• Virtual Channel Identifier (VCI)—In conjunction with the VPI, identifies the next destination of

a cell as it passes through a series of ATM switches on the way to its destination.

• 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 congestion, and the third bit indicates

whether the cell is the last in a series of cells that represent a single AAL5 frame.

• Congestion Loss Priority (CLP)—Indicates whether the cell should be discarded if it encounters

extreme congestion as it moves through the network. If the CLP bit equals 1, the cell should be

discarded in preference to cells with the CLP bit equal to zero.

• Header Error Control (HEC)—Calculates checksum only on the header itself.

ATM Services

Three types of ATM services exist: permanent virtual circuits (PVC), switched virtual circuits

(SVC), and connectionless service (which is similar to SMDS).

A PVC allows direct connectivity between sites. In this way, a PVC is similar to a leased line.

Among its advantages, a PVC guarantees availability of a connection and does not require call setup

procedures between switches. Disadvantages of PVCs include static connectivity and manual setup.

An SVC is created and released dynamically and remains in use only as long as data is being

transferred. In this sense, it is similar to a telephone call. Dynamic call control requires a signaling

protocol between the ATM endpoint and the ATM switch. The advantages of SVCs include

connection flexibility and call setup that can be handled automatically by a networking device.

Disadvantages include the extra time and overhead required to set up the connection.

ATM Virtual Connections

ATM networks are fundamentally connection oriented, which means that a virtual channel (VC)

must be set up across the ATM network prior to any data transfer. (A virtual channel is roughly

equivalent to a virtual circuit.)

Two types of ATM connections exist: virtual paths, which are identified by virtual path identifiers,

and virtual channels, which are identified by the combination of a VPI and a virtual channel

identifier (VCI).

A virtual path is a bundle of virtual channels, all of which are switched transparently across the ATM

network on the basis of the common VPI. All VCIs and VPIs, however, have only local significance

across a particular link and are remapped, as appropriate, at each switch.

ATM Switching Operations

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A transmission path is a bundle of VPs. Figure 20-6 illustrates how VCs concatenate to create VPs,

which, in turn, concatenate to create a transmission path.

ATM Switching Operations

The basic operation of an ATM switch is straightforward: The cell is received across a link on a

known VCI or VPI value. The switch looks up the connection value in a local translation table to

determine the outgoing port (or ports) of the connection and the new VPI/VCI value of the

connection on that link. The switch then retransmits the cell on that outgoing link with the

appropriate connection identifiers. Because all VCIs and VPIs have only local significance across a

particular link, these values are remapped, as necessary, at each switch.

ATM Reference Model

The ATM architecture uses a logical model to describe the functionality it supports. ATM

functionality corresponds to the physical layer and part of the data link layer of the OSI reference

model.

The ATM reference model is composed of the following planes, which span all layers:

• Control—This plane is responsible for generating and managing signaling requests.

• User— This plane is responsible for managing the transfer of data.

• Management— This plane contains two components:

— Layer management manages layer-specific functions, such as the detection of failures and

protocol problems.

— Plane management manages and coordinates functions related to the complete system.

The ATM reference model is composed of the following ATM layers:

• Physical layer—Analogous to the physical layer of the OSI reference model, the ATM physical

layer manages the medium-dependent transmission.

• ATM layer—Combined with the ATM adaptation layer, the ATM layer is roughly analogous to

the data link layer of the OSI reference model. The ATM layer is responsible for establishing

connections and passing cells through the ATM network. To do this, it uses information in the

header of each ATM cell.

• ATM adaptation layer (AAL)—Combined with the ATMlayer, the AAL is roughly analogous to

the data data-link layer of the OSI model. The AAL is responsible for isolating higher-layer

protocols from the details of the ATM processes.

Finally, the higher layers residing above the AAL accept user data, arrange it into packets, and hand

it to the AAL. Figure 20-7 illustrates the ATM reference model.

The ATM Physical Layer

The ATMphysical layer has four functions: bits are converted into cells, the transmission and receipt

of bits on the physical medium are controlled, ATM cell boundaries are tracked, and cells are

packaged into the appropriate types of frames for the physical medium.

The ATM physical layer is divided into two parts: the physical medium-dependent (PMD) sublayer

and the transmission-convergence (TC) sublayer.

The PMD sublayer provides two key functions. First, it synchronizes transmission and reception by

sending and receiving a continuous flow of bits with associated timing information. Second, it

specifies the physical media for the physical medium used, including connector types and cable.

Examples of physical medium standards for ATM include Synchronous Optical

Network/Synchronous Digital Hierarchy (SONET/SDH), DS-3/E3, 155 Mbps over multimode fiber

(MMF) using the 8B/10B encoding scheme, and 155 Mbps 8B/10B over shielded twisted-pair (STP)

cabling.

The TC sublayer has four functions: cell dilineation, header error-control (HEC) sequence

generation and verification, cell-rate decoupling, and transmission-frame adaptation. The cell

delineation function maintains ATMcell boundaries, allowing devices to locate cells within a stream

of bits. HEC sequence generation and verification generates and checks the header error-control

code to ensure valid data. Cell-rate decoupling maintains synchronization and inserts or suppresses

idle (unassigned) ATM cells to adapt the rate of valid ATM cells to the payload capacity of the

transmission system. Transmission frame adaptation packages ATM cells into frames acceptable to

the particular physical-layer implementation.

ATM Adaptation Layers: AAL1

AAL1, a connection-oriented service, is suitable for handling circuit-emulation applications, such

as voice and video conferencing. Circuit-emulation service also accommodates the attachment of

equipment currently using leased lines to an ATM backbone network. AAL1 requires timing

synchronization between the source and destination. For this reason, AAL1 depends on a medium,

such as SONET, that supports clocking. The AAL1 process prepares a cell for transmission in three

steps. First, synchronous samples (for example, 1 byte of data at a sampling rate of

125 microseconds) are inserted into the Payload field. Second, Sequence Number (SN) and Sequence

Number Protection (SNP) fields are added to provide information that the receiving AAL1 uses to

verify that it has received cells in the correct order. Third, the remainder of the Payload field is filled

with enough single bytes to equal 48 bytes. Figure 20-8 illustrates how AAL1 prepares a cell for

transmission.

ATM Adaptation Layers: AAL3/4

AAL3/4 supports both connection-oriented and connectionless data. It was designed for network

service providers and is closely aligned with Switched Multimegabit Data Service (SMDS). AAL3/4

is used to transmit SMDS packets over an ATM network.

AAL3/4 prepares a cell for transmission in four steps. First, the convergence sublayer (CS) creates

a protocol data unit (PDU) by prepending a beginning/end tag header to the frame and appending a

length field as a trailer. Second, the segmentation and reassembly (SAR) sublayer fragments the

PDU and prepends a header to it. Then, the SAR sublayer appends a CRC-10 trailer to each PDU

fragment for error control. Finally, the completed SAR PDU becomes the Payload field of an ATM

cell to which the ATM layer prepends the standard ATM header.

An AAL 3/4 SAR PDU header consists of Type, Sequence Number, and Multiplexing Identifier

fields. Type fields identify whether a cell is the beginning, continuation, or end of a message.

Sequence number fields identify the order in which cells should be reassembled. The Multiplexing

Identifier field determines which cells from different traffic sources are interleaved on the same

virtual circuit connection (VCC) so that the correct cells are reassembled at the destination.

ATM Adaptation Layers: AAL5

AAL5 is the primary AAL for data and supports both connection-oriented and connectionless data.

It is used to transfer most non-SMDS data, such as classical IP over ATM and LAN Emulation

(LANE). AAL5 also is known as the simple and efficient adaptation layer (SEAL) because the SAR

sublayer simply accepts the CS-PDU and segments it into 48-octet SAR-PDUs without adding any

additional fields.

AAL5 prepares a cell for transmission in three steps. First, the CS sublayer appends a variable-length

pad and an 8-byte trailer to a frame. The pad ensures that the resulting PDU falls on the 48-byte

boundary of an ATMcell. The trailer includes the length of the frame and a 32-bit cyclic redundancy

check (CRC) computed across the entire PDU. This allows the AAL5 receiving process to detect bit

errors, lost cells, or cells that are out of sequence. Second, the SAR sublayer segments the CS-PDU

into 48-byte blocks. A header and trailer are not added (as is in AAL3/4), so messages cannot be

interleaved. Finally, the ATM layer places each block into the Payload field of an ATM cell. For all

cells except the last, a bit in the Payload Type (PT) field is set to zero to indicate that the cell is not

the last cell in a series that represents a single frame. For the last cell, the bit in the PT field is set to

one.

ATM Addressing

The ITU-T standard is based on the use of E.164 addresses (similar to telephone numbers) for public

ATM (BISDN) networks. The ATM Forum extended ATM addressing to include private networks.

It decided on the subnetwork or overlay model of addressing, in which the ATMlayer is responsible

for mapping network-layer addresses to ATMaddresses. This subnetwork model is an alternative to

using network-layer protocol addresses (such as IP and IPX) and existing routing protocols (such as

IGRP and RIP). The ATM Forum defined an address format based on the structure of the OSI

network service access point (NSAP) addresses.

Subnetwork Model of Addressing

The subnetwork model of addressing decouples the ATM layer from any existing higher-layer

protocols, such as IP or IPX. Therefore, it requires an entirely new addressing scheme and routing

protocol. Each ATM system must be assigned an ATM address, in addition to any higher-layer

protocol addresses. This requires an ATM address resolution protocol (ATM ARP) to map

higher-layer addresses to their corresponding ATM addresses.

NSAP Format ATM Addresses

The 20-byte NSAP-format ATM addresses are designed for use within private ATM networks,

whereas public networks typically use E.164 addresses, which are formatted as defined by ITU-T.

The ATM Forum has specified an NSAP encoding for E.164 addresses, which is used for encoding

E.164 addresses within private networks, but this address can also be used by some private networks.

ATM Addressing

20-10 Internetworking Technology Overview

Such private networks can base their own (NSAP format) addressing on the E.164 address of the

public UNI to which they are connected and can take the address prefix from the E.164 number,

identifying local nodes by the lower-order bits.

All NSAP-format ATM addresses consist of three components: the authority and format identifier

(AFI), the initial domain identifier (IDI), and the domain specific part (DSP). The AFI identifies the

type and format of the IDI, which, in turn, identifies the address allocation and administrative

authority. The DSP contains actual routing information.

Three formats of private ATM addressing differ by the nature of the AFI and IDI. In the

NSAP-encoded E.164 format, the IDI is an E.164 number. In the DCC format, the IDI is a data

country code (DCC), which identifies particular countries, as specified in ISO 3166. Such addresses

are administered by the ISO National Member Body in each country. In the ICD format, the IDI is

an international code designator (ICD), which is allocated by the ISO 6523 registration authority

(the British Standards Institute). ICD codes identify particular international organizations.

The ATMForum recommends that organizations or private-network service providers use either the

DCC or ICD formats to form their own numbering plan.

Figure 20-9 illustrates the three formats of ATM addresses used for private networks.

ATM Address Fields

The following descriptions summarize the fields illustrated in Figure 20-9:

• AFI—Identifies the type and format of the address (DCC, ICD, or E.164).

• DCC—Identifies particular countries.

• High-Order Domain Specific Part (HO-DSP)—Combines the routing domain (RD) and area

indentifier (AREA) of the NSAP addresses. The ATM Forum combined these fields to support a

flexible, multilevel addressing hierarchy for prefix-based routing protocols.

• End System Identifier (ESI)—Specifies the 48-bit MAC address, as administered by the Institute

of Electrical and Electronic Engineers (IEEE).

• Selector (SEL)—Used for local multiplexing within end stations and has no network significance.

• ICD—Identifies particular international organizations.

• E.164—Indicates the BISDN E.164 address.

ATM Connections

ATM supports two types of connections: point-to-point and point-to-multipoint.

Point-to-point connects two ATM end systems and can be unidirectional (one-way communication)

or bidirectional (two-way communication). Point-to-multipoint connects a single-source end system

(known as the root node) to multiple destination end systems (known as leaves). Such connections

are unidirectional only. Root nodes can transmit to leaves, but leaves cannot transmit to the root or

each other on the same connection. Cell replication is done within the ATM network by the ATM

switches where the connection splits into two or more branches.

It would be desirable in ATM networks to have bidirectional multipoint-to-multipoint connections.

Such connections are analogous to the broadcasting or multicasting capabilities of shared-media

LANs, such as Ethernet and Token Ring. A broadcasting capability is easy to implement in

shared-media LANs, where all nodes on a single LAN segment must process all packets sent on that

segment. Unfortunately, a multipoint-to-multipoint capability cannot be implemented by using

AAL5, which is the most common AAL to transmit data across an ATM network. Unlike AAL3/4,

with its Message Identifier (MID) field, AAL5 does not provide a way within its cell format to

interleave cells from different AAL5 packets on a single connection. This means that all AAL5

packets sent to a particular destination across a particular connection must be received in sequence;

otherwise, the destination reassembly process will be unable to reconstruct the packets. This is why

AAL5 point-to-multipoint connections can be only unidirectional. If a leaf node were to transmit an

AAL5 packet onto the connection, for example, it would be received by both the root node and all

other leaf nodes. At these nodes, the packet sent by the leaf could be interleaved with packets sent

by the root and possibly other leaf nodes, precluding the reassembly of any of the interleaved

packets.

ATM and Multicasting

ATM requires some form of multicast capability. AAL5 (which is the most common AAL for data)

currently does not support interleaving packets, so it does not support multicasting.

If a leaf node transmitted a packet onto an AAL5 connection, the packet can get intermixed with

other packets and be improperly reassembled. Three methods have been proposed for solving this

problem: VP multicasting, multicast server, and overlaid point-to-multipoint connection.

Under the first solution, a multipoint-to-multipoint VP links all nodes in the multicast group, and

each node is given a unique VCI value within the VP. Interleaved packets hence can be identified by

the unique VCI value of the source. Unfortunately, this mechanism would require a protocol to

uniquely allocate VCI values to nodes, and such a protocol mechanism currently does not exist. It is

also unclear whether current SAR devices could easily support such a mode of operation.

ATM Quality of Service (QoS)

20-12 Internetworking Technology Overview

A multicast server is another potential solution to the problem of multicasting over an ATMnetwork.

In this scenario, all nodes wanting to transmit onto a multicast group set up a point-to-point

connection with an external device known as a multicast server (perhaps better described as a

resequencer or serializer). The multicast server, in turn, is connected to all nodes wanting to receive

the multicast packets through a point-to-multipoint connection. The multicast server receives

packets across the point-to-point connections and then retransmits them across the

point-to-multipoint connection—but only after ensuring that the packets are serialized (that is, one

packet is fully transmitted prior to the next being sent). In this way, cell interleaving is precluded.

An overlaid point-to-multipoint connection is the third potential solution to the problem of

multicasting over an ATM network. In this scenario, all nodes in the multicast group establish a

point-to-multipoint connection with each other node in the group and, in turn, become leaves in the

equivalent connections of all other nodes. Hence, all nodes can both transmit to and receive from all

other nodes. This solution requires each node to maintain a connection for each transmitting member

of the group, whereas the multicast-server mechanism requires only two connections. This type of

connection would also require a registration process for informing the nodes that join a group of the

other nodes in the group so that the new nodes can form the point-to-multipoint connection. The

other nodes must know about the new node so that they can add the new node to their own

point-to-multipoint connections. The multicast-server mechanism is more scalable in terms of

connection resources but has the problem of requiring a centralized resequencer, which is both a

potential bottleneck and a single point of failure.

ATM Quality of Service (QoS)

ATM supports QoS guarantees composed of traffic contract, traffic shaping, and traffic policing.

A traffic contract specifies an envelope that describes the intended data flow. This envelope specifies

values for peak bandwidth, average sustained bandwidth, and burst size, among others. When an

ATM end system connects to an ATM network, it enters a contract with the network, based on QoS

parameters.

Traffic shaping is the use of queues to constrain data bursts, limit peak data rate, and smooth jitters

so that traffic will fit within the promised envelope. ATMdevices are responsible for adhering to the

contract by means of traffic shaping. ATM switches can use traffic policing to enforce the contract

The switch can measure the actual traffic flow and compare it against the agreed-upon traffic

envelope. If the switch finds that traffic is outside of the agreed-upon parameters, it can set the

cell-loss priority (CLP) bit of the offending cells. Setting the CLP bit makes the cell discard eligible,

which means that any switch handling the cell is allowed to drop the cell during periods of

congestion.

ATM Signaling and Connection Establishment

When an ATM device wants to establish a connection with another ATM device, it sends a

signaling-request packet to its directly connected ATM switch. This request contains the ATM

address of the desired ATM endpoint, as well as any QoS parameters required for the connection.

ATM signaling protocols vary by the type of ATM link, which can be either UNI signals or NNI

signals. UNI is used between an ATM end system and ATM switch across ATM UNI, and NNI is

used across NNI links.

The ATMForum UNI 3.1 specification is the current standard for ATMUNI signaling. The UNI 3.1

specification is based on the Q.2931 public network signaling protocol developed by the ITU-T. UNI

signaling requests are carried in a well-known default connection: VPI = 0, VPI = 5.

Asynchronous Transfer Mode (ATM) Switching 20-13

The ATM Connection-Establishment Process

Standards currently exist only for ATM UNI signaling, but standardization work is continuing on

NNI signaling.

The ATM Connection-Establishment Process

ATM signaling uses the one-pass method of connection setup that is used in all modern

telecommunication networks, such as the telephone network. An ATM connection setup proceeeds

in the following manner. First, the source end system sends a connection-signaling request. The

connection request is propagated through the network. As a result, connections are set up through

the network. The connection request reaches the final destination, which either accepts or rejects the

connection request.

Connection-Request Routing and Negotiation

Routing of the connection request is governed by an ATM routing protocol (which routes

connections based on destination and source addresses), traffic, and the QoS parameters requested

by the source end system. Negotiating a connection request that is rejected by the destination is

limited because call routing is based on parameters of initial connection; changing parameters might,

in turn, affect the connection routing. Figure 20-10 highlights the one-pass method of ATM

connection establishment.

ATM Connection-Management Messages

Anumber of connection- management message types, including setup, call proceeding, connect, and

release, are used to establish and tear down an ATM connection. The source end end-system sends

a setup message (including the destination end-system address and any traffic QoS parameters) when

it wants to set up a connection. The ingress switch sends a call proceeding message back to the

source in response to the setup message. The destination end system next sends a connect message

if the connection is accepted. The destination end system sends a release message back to the source

end system if the connection is rejected, thereby clearing the connection.

LAN Emulation (LANE)

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Connection-management messages are used to establish an ATM connection in the following

manner. First, a source end system sends a setup message, which is forwarded to the first ATM

switch (ingress switch) in the network. This switch sends a call proceeding message and invokes an

ATM routing protocol. The signaling request is propagated across the network. The exit switch

(called the egress switch) that is attached to the destination end system receives the setup message.

The egress switch forwards the setup message to the end system across its UNI, and the ATM end

system sends a connect message if the connection is accepted. The connect message traverses back

through the network along the same path to the source end system, which sends a connect

acknowledge message back to the destination to acknowledge the connection. Data transfer can then

begin.

LAN Emulation (LANE)

LANE is a standard defined by the ATM Forum that gives to stations attached via ATM the same

capabilities they normally obtain from legacy LANs, such as Ethernet and Token Ring. As the name

suggests, the function of the LANE protocol is to 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. The current LANE protocol does not define a separate encapsulation

for FDDI. (FDDI packets must be mapped into either Ethernet or Token Ring emulated LANs

[ELANs] by using existing translational bridging techniques.) Fast Ethernet (100BaseT) and

IEEE 802.12 (100VG-AnyLAN) both can be mapped unchanged because they use the same packet

formats. Figure 20-11 compares a physical LAN and an ELAN.

The LANE protocol defines a service interface for higher-layer (that is, network layer) protocols thatis identical to that of existing LANs. Data sent across the ATM network is encapsulated in theappropriate LANMAC packet format. Simply put, the LANE protocols make an ATM network lookand behave like an Ethernet or Token Ring LAN—albeit one operating much faster than an actual Ethernet or Token Ring LAN network.

It is important to note that LANE does not attempt to emulate the actualMAC protocol of the specific

LAN concerned (that is, CSMA/CD for Ethernet or token passing for IEEE 802.5). LANE requires

no modifications to higher-layer protocols to enable their operation over an ATM network. Because

the LANE service presents the same service interface of existing MAC protocols to network-layer

drivers (such as an NDIS- or ODI-like driver interface), no changes are required in those drivers.

The LANE Protocol Architecture

The basic function of the LANE protocol is to resolve MAC addresses to ATM addresses. The goal

is to resolve such address mappings so that LANE end systems can set up direct connections between

themselves and then forward data. The LANE protocol is deployed in two types of ATM-attached

equipment: ATM network interface cards (NICs) and internetworking and LAN switching

equipment.

ATMNICs implement the LANE protocol and interface to the ATMnetwork but present the current

LAN service interface to the higher-level protocol drivers within the attached end system. The

network-layer protocols on the end system continue to communicate as if they were on a known

LAN by using known procedures. However, they are able to use the vastly greater bandwidth ofATM

networks.

The second class of network gear to implement LANE consists of ATM-attached LAN switches and

routers. These devices, together with directly attached ATM hosts equipped with ATM NICs, are

used to provide a virtual LAN (VLAN) service in which ports on the LAN switches are assigned to

particular VLANs independently of physical location. Figure 20-12 shows the LANE protocol

architecture implemented in ATM network devices.:

LANE Components

The LANE protocol defines the operation of a single ELAN or VLAN. Although multiple ELANs

can simultaneously exist on a single ATMnetwork, an ELAN emulates either an Ethernet or a Token

Ring and consists of the following components:

• LAN emulation client (LEC)—The LEC is an entity in an end system that performs data

forwarding, address resolution, and registration of MAC addresses with the LAN emulation

server (LES). The LEC also provides a standard LAN interface to higher-level protocols on

legacy LANs. An ATM end system that connects to multiple ELANs has one LEC per ELAN.

• LES—The LES provides a central control point for LECs to forward registration and control

information. (Only one LES exists per ELAN.)

• Broadcast and unknown server (BUS)—The BUS is a multicast server that is used to flood

unknown destination address traffic and to forward multicast and broadcast traffic to clients

within a particular ELAN. Each LEC is associated with only one BUS per ELAN.

• LAN emulation configuration server (LECS)—The LECS maintains a database of LECs and the

ELANs to which they belong. This server accepts queries from LECs and responds with the

appropriate ELAN identifier, namely the ATM address of the LES that serves the appropriate

ELAN. One LECS per administrative domain serves all ELANs within that domain.

LAN Emulation Connection Types

The Phase 1 LANE entities communicate with each other by using a series of ATM VCCs. LECs

maintain separate connections for data transmission and control traffic. The LANE data connections

are data-direct VCC, multicast send VCC, and multicast forward VCC.

Data-direct VCC is a bidirectional point-to-point VCC set up between two LECs that want to

exchange data. Two LECs typically use the same data-direct VCC to carry all packets between them

rather than opening a new VCC for each MAC address pair. This technique conserves connection

resources and connection setup latency.

Multicast send VCC is a bidirectional point-to-point VCC set up by the LEC to the BUS.

Multicast forward VCC is a unidirectional VCC set up to the LEC from the BUS. It typically is a

point-to-multipoint connection, with each LEC as a leaf.

LANE Operation

The operation of a LANE system and components is best understood by examining these stages of

LEC operation: intialization and configuration, ; joining and registering with the LES, ; finding and

joining the BUS, ; and data transfer.

Initialization and Configuration

Upon initialization, an LEC finds the LECs to obtain required configuration information. It begins

this process when the LEC obtains its own ATM address, which typically occurs through address

registration.

The LEC must then determine the location of the LECS. To do this, the LEC first must locate the

LECS by one of the following methods: by using a defined ILMI procedure to determine the LECS

address, by using a well-known LECS address, or by using a well-known permanent connection to

the LECS (VPI = 0, VCI = 17).

When the LECS is found, the LEC sets up a configuration-direct VCC to the LECS and sends a

LE_CONFIGURE_REQUEST. If a matching entry is found, the LECS returns a

LE_CONFIGURE_RESPONSE to the LEC with the configuration information it requires to

connect to its target ELAN, including the following: ATM address of the LES, type of LAN being

emulated, maximum packet size on the ELAN, and ELAN name (a text string for display purposes).

Joining and Registering with the LES

When an LEC joins the LES and registers its own ATM and MAC addresses, it does so by following

three steps:.

1 After the LEC obtains the LES address, the LEC optionally clears the connection to the LECS,

sets up the control-direct VCC to the LES, and sends an LE_JOIN_REQUEST on that VCC. This

allows the LEC to register its own MAC and ATM addresses with the LES and (optionally) any

other MAC addresses for which it is proxying. This information is maintained so that no two

LECs will register the same MAC or ATM address.

2 After receipt of the LE_JOIN_REQUEST, the LES checks with the LECS via its open

connection, verifies the request, and confirms the client’s membership.

3 Upon successful verification, the LES adds the LEC as a leaf of its point-to-multipoint

control-distribute VCC and issues the LEC a successful LE_JOIN_RESPONSE that contains a

unique LAN Emulation Client ID (LECID). The LECID is used by the LEC to filter its own

broadcasts from the BUS.

Finding and Joining the BUS

After the LEC has successfully joined the LECS, its first task is to find the BUS/s ATM address to

join the broadcast group and become a member of the emulated LAN.

First, the LEC creates an LE_ARP_REQUEST packet with the MAC address 0xFFFFFFFF. Then

the LEC sends this special LE_ARP packet on the control-direct VCC to the LES. The LES

recognizes that the LEC is looking for the BUS and responds with the BUS’s ATM address on the

control-distribute VCC.

When the LEC has the BUS’s ATM address, it joins the BUS by first creating a signaling packet with

the BUS’s ATM address and setting up a multicast-send VCC with the BUS. Upon receipt of the

signaling request, the BUS adds the LEC as a leaf on its point-to-multipoint multicast forward VCC.

The LEC is now a member of the ELAN and is ready for data transfer.

Data Transfer

The final state, data transfer, involves resolving the ATM address of the destination LEC and actual

data transfer, which might include the flush procedure.

When a LEC has a data packet to send to an unknown-destinationMAC address, it must discover the

ATM address of the destination LEC through which the particular address can be reached. To

accomplish this, the LEC first sends the data frame to the BUS (via the multicast send VCC) for

distribution to all LECs on the ELAN via the multicast forward VCC. This is done because resolving

the ATM address might take some time, and many network protocols are intolerant of delays.

The LEC then sends a LAN Emulation Address Resolution Protocol Request (LE_ARP_Request)

control frame to the LES via a control-direct VCC.

If the LES knows the answer, it responds with the ATM address of the LEC that owns the MAC

address in question. If the LES does not know the answer, it floods the LE_ARP_REQUEST to some

or all LECs (under rules that parallel the BUS’s flooding of the actual data frame, but over

control-direct and control-distribute VCCs instead of the multicast send or multicast forward VCCs

used by the BUS). If bridge/switching devices with LEC software participating in the ELAN exist,

they translate and forward the ARP on their LAN interfaces.

In the case of actual data transfer, if an LE_ARP is received, the LEC sets up a data-direct VCC to

the destination node and uses this for data transfer rather than the BUS path. Before it can do this,

however, the LEC might need to use the LANE flush procedure, which ensures that all packets

previously sent to the BUS were delivered to the destination prior to the use of the data-direct VCC.

In the flush procedure, a control cell is sent down the first transmission path following the last packet.

The LEC then waits until the destination acknowledges receipt of the flush packet before using the

second path to send packets.