27_atm.pdf

CHAPTER

Chapter Goals

• Understand the ATM cell structure.

• Identify the ATM model layers.

• Know the ATM connection types.

• Describe the call establishment process.

• Understand the purpose of each LANE component.

• Describe LANE operations.

• Know the purpose of MPOA.

Asynchronous Transfer Mode Switching

Asynchronous Transfer Mode (ATM) is an International Telecommunication

Union–Telecommunications Standards Section (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 ATM protocols, services, and

operation. Figure 27-1 illustrates a private ATM network and a public ATM network carrying voice,

video, and data traffic.

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Chapter 27

Asynchronous Transfer Mode Switching

Standards

Figure27-1 APrivateATMNetworkandaPublicATMNetworkBothCanCarryVoice,Video,andData

Trafﬁc

Data

Voice

AT M

switch

Video

Shared

h ub

WAN

Public

ATM network

Router

Private ATM network

Standards

ATM is based on the efforts of the ITU-T Broadband Integrated Services Digital Network (B-ISDN)

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

public networks. The ATM Forum extended the ITU-T’s vision of ATM for use over public and private

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

• User-to-Network Interface (UNI) 2.0

• UNI 3.0

• UNI 3.1

• UNI 4.0

• Public-Network Node Interface (P-NNI)

• LAN Emulation (LANE)

• Multiprotocol over ATM

ATM Devices and the Network Environment

ATM is 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) .

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Chapter 27 Asynchronous Transfer Mode Switching

ATM Devices and the Network Environment

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

has much data to send, it can send only when its time slot comes up, even if all other time slots are empty.

However, if 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 27-2 illustrates the basic format of an ATM cell.

Figure27-2 An ATM Cell Consists of a Header and Payload Data

Field length,

in bytes

Header

Payload

ATM Devices

An ATM network is made up of an ATM switch and ATM endpoints . 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 27-3

illustrates an ATM network made up of ATM switches and ATM endpoints.

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Chapter 27

Asynchronous Transfer Mode Switching

ATM Devices and the Network Environment

Figure27-3 An ATM Network Comprises ATM Switches and Endpoints

Router

LAN switch

ATM switch

Workstation

CSU/DSU

ATM endpoints

ATM Network Interfaces

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

interfaces. ATM switches 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 is 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 intercarrier interface (B-ICI) , connects two public switches

from different service providers. Figure 27-4 illustrates the ATM interface specifications for private and

public networks.

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Chapter 27 Asynchronous Transfer Mode Switching

ATM Cell Header Format

Figure27-4 ATM Interface Speciﬁcations Differ for Private and Public Networks

Private ATM

network

Public ATM

network A

Public ATM

network B

Private

NNI

Public

UNI

Public

NNI

B-ICI

Private UNI

ATM Cell Header Format

An ATM cell header can be one of two formats: UNI or 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 27-5 depicts the basic ATM cell format, the ATM UNI

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

Figure27-5 An ATM Cell, ATM UNI Cell, and ATM NNI Cell Header Each Contain 48 Bytes of Payload

GFC

VPI

Header

(5 bytes)

VCI

CLP

HEC

bytes

Payload

(48 bytes)

Payload

(48 bytes)

Payload

(48 bytes)

8 bits

ATM cell

ATM UNI cell

ATM NNI cell

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.

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