Asynchronous Transfer Mode - 123SeminarsOnly

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26 Οκτ 2013 (πριν από 4 χρόνια και 8 μήνες)

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Er. Amit Mahajan



A transfer mode in which information is organized into
cells; it is asynchronous in the sense that the recurrence
of cells containing information from an individual user
is not necessarily periodic".

speed transfer technology for voice, video, and
data over public networks


As the speed and number of local area networks (LANs)
continue their relentless growth, increasing demand is
place on wide area packet
switching networks to support
the tremendous throughput generated by these LANs.

X.25 was designed to support direct connection of
terminals and computers over long distances.

X.25 packets may be of varying length,whereas ATM
packets are of fixed size.

X.25, with its substantial overhead, is being recognized
as an inadequate tool for wide area networking


ATM is normally utilized for high bandwidths of 34 Mbps
and upwards.

At speeds of 2 Mbps and below, Frame Relay is more
bandwidth efficient than ATM.

ATM transmits only fixed
size frames, called cells, not
sized frames as frame relay do.

There are two main drivers that caused businesses migrate
from frame relay service to ATM :

1. The need for greater capacity than frame relay can handle.

2. The need to support mixed
media traffic, especially voice
and video.

Both frame relay and ATM take advantage of the
reliability and fidelity of modern digital facilities
to provide faster packet
switching than X.25.

ATM is even more streamlined than frame relay in
its functionality, and can support data rates several
orders of magnitude greater than frame relay.

Frame relay was developed as part of the work of

ATM was developed as part of the work on
broadband ISDN


The industry offers two solutions for achieving a large integrated
network: SONET/SDH and ATM .

The SONET is a globally accepted, non
proprietary standard for
broadband transmission through fiber
optic cables.

It handles transmissions from 51 Mbps to 10 Gbps.

SONET/SDH is a physical transport medium that occupies the two
bottom layers of OSI model.

ATM is a high
speed packet switching technique suitable for LAN,
area network and broadband ISDN.

SONET is a physical structure, while ATM is a transmission

If the ATM protocol is used, a transport medium is still needed to
carry traffic over the network


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

ATM switch 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 contains an ATM network interface adapter.

Examples of ATM endpoints are workstations, routers, LAN switches, and
video coder
decoders (CODECs).

ATM Network Comprises ATM Switches and Endpoints

ATM virtual connections

A Transmission path (TP)
is the physical connection

so on) between an end point and a
switch or between two switches

A Virtual Path (VP)

transports ATM cells belonging to virtual
channels which share a common identifier, called the Virtual
Path Identifier VPI. Connects two switches.

A Virtual Channel (VC)
provides the transport of ATM cells
which have the same unique identifier, called the Virtual
Channel Identifier (VCI).

ATM virtual connections

ATM protocol reference model

The Physical Layer

This layer describes the physical transmission of information through an
ATM network.

The original design of ATM was based on SONET because high data rate of
SONET’s carrier , the boundaries of cells can be clearly defined .

SONET specifies the use of pointer to define the beginning of a payload.

Types of physical media specified for ATM include shielded and unshielded
pair, coaxial cable, and fiber
optic cable, which provide cell transport
capabilities ranging from a T1 rate of 1.544Mbps to a SONET range of

The ATM Layer

The ATM layer represents the physical interface between
the ATM Adaptation layer (AAL) and the Physical layer.
Thus, the ATM layer is responsible for relaying cells
from the AAL to the Physical layer for transmission, and
in the opposite direction from the Physical layer to the
AAL for use in an endpoint.

When transporting cells to the Physical layer, the ATM
layer is responsible for generating the five
byte cell
header for each cell. When receiving cells from the
Physical layer, the ATM layer performs a reverse
operation, extracting the five
byte header from each cell.

ATM cell header format

ATM transfer information in fixed
size units called cells.

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.

Each cell consists of 53 octets, or bytes. The first 5 bytes contain cell
header information, and the remaining 48 contain the payload (user

ATM Cell Header Fields

Generic Flow Control (GFC)

The 4
bit GFC field provides flow control at the
UNI level.The ITU
T has determined that this level of flow control is not necessary
at the NNI.

Virtual Path Identifier (VPI)

The VPI is an 8
bit field in a UNI cell & a 12
field in an NNI cell.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)

VCI is 16
bit field in both.

Payload Type (PT)

Indicates in the first bit whether the cell contains user data or
control data. If the cell contains user data, the bit is set to 0. If it contains control
data, it is set to 1. The second bit indicates congestion (0 = no congestion, 1 =
congestion), and the third bit indicates whether the cell is the last in a series of cells
that represent a single AAL5 frame (1 = last cell for the frame).

Cell 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

Header Error Control (HEC)

Calculates checksum only on the first 4 bytes of
the header. HEC can correct a single bit error in these bytes, thereby preserving the
cell rather than discarding it.

The ATM Adaptation Layer

Was developed to enable two ATM concepts.

This layer is responsible for providing an interface between higher
protocols and the ATM layer.

AAL maps the data stream originated by the higher
layer protocol into the
byte payload of ATM cells, with the header placement being assigned by
the ATM layer.

In the reverse direction, the AAL receives the payload of ATM cells in 48
byte increments from the ATM layer and maps those increments into the
format recognized by the higher
layer protocol

Class A services

are data streams with a constant bit rate, running over
established connections.

Class B services

are similar, but instead of being locked to a regular
data rate they send 'peaks' of data at some times, and little or none at
others. Examples include compressed video

Class C services

are those carrying data messages on established
connections. These are inherently variable bit
rate, as Class B.
Examples include X.25 and Frame Relay.

Class D

are the so called connectionless datagrams, where a
packet of data is sent into the network and contains its own destination
address. Examples include many traditional local
area networks such
as Ethernet, wide area networks and the new switched multimegabit


Timing Relationship

Bit Rate

Type of Connection





















In AAL1 cell payload the Sequence Number Protection (SNP) field protects
the Sequence Number (SN) field from the effect of bit errors occurring
during transmission, in effect providing a forward error detection and
correction capability.

AAL1 is designated for transporting constant bit rate (CBR) data, such
as real
time voice and video traffic.

First byte in the normal 48
byte cell payload is used for cell sequencing
and protection of the sequence number, limiting the actual payload to
47 bytes per AAL1
generated cell.

The AAL2 was intended to support a VBR .

But now used for low
rate traffic & short frame traffic such as
audio,video or fax .ex mobile telephony.

AAL3 is designed to transport delay
insensitive user data, such as Frame
Relay, X.25, or IP traffic.

AAL3/4 uses four additional bytes beyond the cell header. The use of those
bytes makes 44 bytes in the cell available for transporting the actual

In comparison, AAL5 uses all 48 bytes beyond the cell header to transport
the payload, providing a minimum 10% enhanced throughput in
comparison to AAL3/4.


ATM signalling uses the one
pass method of connection setup that is used
in all modern telecommunication networks, such as the telephone network.

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) is a standard defined by the ATM Forum that gives
to stations attached via ATM the same capabilities that they normally obtain
from legacy LANs.

The LANE protocol defines mechanisms for emulating either an IEEE 802.3
Ethernet or an 802.5 Token Ring LAN.

The LANE protocols make an ATM network look and behave like an Ethernet or
Token Ring

LANE requires no modifications to higher
layer protocols to enable their
operation over an ATM network.

LANE Components

LAN Emulation client (LEC)

It is an entity in an end system that
performs data forwarding, address resolution, and registration of MAC
addresses with the LAN Emulation Server (LES).

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


The LES provides a central control point for LECs to forward
registration and control information.When a station receives a frame to be
sent to another station using a physical address , LEC sends a special frame
to the LES.

The server creates a virtual circuit between the source & the destination
station.The source station can now use this virtual circuit(& the
corresponding identifier) to send the frame or frames to the destination.

Broadcast and Unknown Server (BUS)

Multicasting & Broadcasting
require the use of another server called the broadcast/unknown
server(BUS).If a station needs to send a frame to a group of stations or to
every station,the frame first goes to the bus;this server has permanent
virtual connnection to every station.The server creates copies of the
received frame & sends a copy to a group of stations or to all
stations,simulating a multicasting or broadcasting process.The server can
also deliver a unicast frame by sending the frame to every station.In this
case the destination address is unknown.This is sometimes more efficient
then getting the connection identifier from the LES

LAN Emulation Configuration Server (LECS)

This is used for the
initial connection between the client & LANE.

This server is always waiting to receive the initial contact.It has well
known ATM address that is known to every client in the system.

The LECS maintains a database of LECs and the ELANs to which they


service (QoS) that guarantees

traffic contract, traffic shaping,
and traffic policing is based on the service class , user related attributes,&
related attributes

Traffic contract

specifies an envelope that describes the intended data
flow. 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

ATM switches can use
traffic policing

to enforce the contract.

The switch
can measure the actual traffic flow and compare it against the agreed
traffic envelope. If the switch finds that traffic is outside of the agreed
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
for the multimedia applications and provide overall
optimization of network resources

Service Class

Quality of Service Parameter

constant bit rate (CBR)

It is designed for customers who need real time audio or video services.

The cell
rate is constant with time. CBR applications are quite sensitive to cell
variation. Examples of applications that can use CBR are telephone traffic ,
videoconferencing, and television.

variable bit rate

real time


This class allows users to send traffic at a rate that varies with time depending on
the availability of user

is designed for those users who do not need
real time services but use compression techniques to create a variable bit rate.
Multimedia e
mail is an example of VBR


variable bit rate

real time


This class is similar to VBR

NRT but is designed for applications that are
sensitive to cell
delay variation. It is designed for those users who need real time
services & use compression techniques to create a variable bit rate. Examples for
time VBR
interactive compressed video.

available bit rate (ABR)

This class of ATM services provides rate
based flow control and is aimed at data
traffic such as file transfer and e
mail. Although the standard does not require the
cell transfer delay and cell
loss ratio to be guaranteed or minimized, it is desirable
for switches to minimize delay and loss as much as possible. Depending upon the
state of congestion in the network, the source is required to control its rate. The
users are allowed to declare a minimum cell

more network capacity is
available, this minimum rate can be exceeded.ABR is particularly suitable for
applications that are

unspecified bit rate (UBR)

This class is a best effort delivery service that does not guarantee anything and is
widely used today for TCP/IP.

User related attributes

The Sustained cell rate is the average cell rate over a long time
interval.The actual cell rate may be lower or higher than this value, but the
average should be equal to or less than the SCR.

The peak cell rate defines the sender’s maximum cell rate. The
user’s cell rate can sometimes reach this peak,as long as the SCR is

The minimum cell rate defines the minimum cell rate acceptable to
the sender.For example,if the MCR is 50,000, the network must guarantee
that the sender can send atleast 50,000 cells per second.

The cell variation delay tolerance is a measure of the variation in
cell transmission times.For example,if the CVDT is 5 ns ,this means that
the difference between the minimum & the maximum delays in delivering
the cells should not exceed 5 ns.

Network related attributes

The network related attributes are those that define characteristics of the
network.The following are some network related attributes:

The cell loss ratio defines the fraction of cells lost(or delivered so
late that they are considered lost) during transmission.For example, if the
sender sends 100 cells & one of them is lost,the CLR is

CLR = 1/100 = 10

The cell transfer delay is the average time needed for a cell to travel
from source to destination. The maximum CTD & the minimum CTD also
considered attributes.

The cell delay variation is the difference between CTD maximum &
the CTD minimum.

The cell error ratio defines the fraction of cells delivered in error.

IP over ATM

When IP works with ATM , the IP packets are segmented into fixed length cells
of ATM, transmitted through the ATM network, & then reassembled into IP
packets at the receiving end.

Each entry/exit point is a router. An ATM backbone can span an entire continent
and may have tens or even hundreds of ATM switches.

Most ATM backbones have a permanent virtual channel (VC) between each pair
of entry/exit points.

entry points,

1) permanent VCs are needed to directly
entry/exit points. Each router interface that connects to the
ATM network will have two addresses. The router interface will have
an IP address, as usual, and the router will have an ATM address, which
is essentially a LAN address.

Consider now an IP datagram that is to be moved across the ATM
backbone To four IP routers, the backbone appears as a single logical

ATM interconnects these four routers just as Ethernet can be used
to connect four routers.

Let us refer to the router at which the datagram enters the ATM network
as the “entry router” and the router at which the datagram leaves the
network as the “exit router.”

The entry router does the following:

1. Examines the destination address of the datagram.

2. Indexes its routing table and determines the IP address of the exit

3. To move the datagram to the next router, the physical address of the
hop router must be determined.

4. IP in the entry router then passes down to the link layer (that is, ATM)
the datagram along with the ATM address of the exit router.

ATM must now move the datagram to the ATM
destination address.
This task
has two subtasks:

Determine the VCI for the VC that leads to the ATM destination address.

Segment the datagram into cells at the sending side of the VC (that is, at the
entry router), and reassemble the cells into the original datagram at the
receiving side of the VC (that is, at the exit router).

ATM uses AAL5 to provide a more efficient way to segment and reassemble a
datagram. Recall that IP in the entry router passes the datagram down to ATM
along with the ATM address of the exit router. ATM in the entry router indexes
an ATM table to determine the VCI for the VC that leads to the ATM destination
address. AAL5 then creates ATM cells out of the IP datagram:

The datagram is encapsulated in a CPCS
PDU using the format in fig.

PDU is chopped up into 48
byte chunks. Each chunk is placed in the
payload field of an ATM cell.

All of the cells except for the last cell have the third bit of the PT field set to 0.

The AAL_indicate bit is used to reassemble IP datagrams from ATM cells.

The last cell has the bit set to 1. AAL5 then passes the cells to the ATM
layer. ATM sets the VCI and CLP fields and passes each cell to the TC
sublayer. For each cell, the TC sublayer calculates the HEC and inserts it in
the HEC field. The TC sublayer then inserts the bits of the cells into the
PMD sublayer.

The ATM network then moves each cell across the network to the ATM
destination address. At each ATM switch between the ATM source and the
ATM destination, the ATM cell is processed by the ATM physical and ATM
layers, but not by the AAL layer. At each switch the VCI is typically
translated and the HEC is recalculated.

When the cells arrive at the ATM destination address, they are directed to
an AAL buffer that has been put aside for the particular VC. The CPCS
PDU is reconstructed using the AAL_indicate bit to determine which cell is
the last cell of the CPCS
PDU. Finally, the IP datagram is extracted out of
the CPCSPDU and is passed up the protocol stack to the IP layer

ATM Advantages:

Provides scalable bandwidth from a few megabits per second (Mbps) to
many gigabits per second (Gbps).

length cells enables low
cost hardware to be developed to perform
required cell switching based on the contents of the cell header, without
requiring more complex and costly software

size cells allow ATM to support quantifiable QoS

Because of its asynchronous nature, ATM is more efficient than
synchronous technologies, such as TDM.

Simplified Network Management.

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 ).

ATM disadvantages

Overhead of cell header (5 bytes per cell)

Complex mechanisms for achieveing QoS

Congestion may cause cell losses

ATM handles data traffic smoothly, but runs into delay
problems with voice transmissions.


ATM is a high
speed packet switching technique suitable for LAN, wide
area network and broadband ISDN (integrated services digital network)

The decision of when to use ATM and when to use frame relay largely
depends on the applications businesses want to run over their enterprise
networks, the amount of bandwidth they need and their performance

ATM is ideally suited for converged voice,data and video networks because
it assures quality of service.

It also provides the high amounts of bandwidth that businesses are
increasingly demanding for data and other applications.

Frame relay, on the other hand, continues to be a highly economical and
reliable choice, especially for medium
speed, data only applications.

It provides scalable bandwidth from a few megabits per second (Mbps) to
many gigabits per second (Gbps). ATM provides no retransmissions on a
link basis

Combining the ATM & SONET offers scalability and flexibility. While
Asynchronous Transfer Mode (ATM) and Synchronous Optical Network
(SONET) technologies are still emerging technologies, the combination of
the two will drastically alter future corporate LAN design


Martin De Prycker
, Asynchronous Transfer Mode. Solutions for
Broadband ISDN

, 1993)

William Stallings,ISDN & Broadband ISDN with frame relay & ATM(Prentice


P.S. Neelakanta

A Textbook on ATM Telecommunications, Principles and
implementation. CRC Press. 2000
18/3/09 for ATM standards document

Ginsburg, David. ATM: Solutions for Enterprise Internetworking. Boston: Addison
Wesley Publishing Co, 1996

Clark, Kennedy, and Kevin Hamilton. CCIE Professional Development: Cisco LAN
Switching.Indianapolis: Cisco Press, 1999.24/4/09

Behrouz A Forouzan,Fourth edition,Data communications & networking