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atm
CHAPTER
27
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
To
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
5
48
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
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
VPI
VPI
Header
(5 bytes)
VCI
VCI
PT
CLP
PT
CLP
HEC
HEC
53
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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