High speed computer networks

Asynchronous transfer mode (ATM)

Sosina M.
Addis Ababa institute of technology (AAiT)
2012 E.C.
Overview
 Asynchronous transfer mode
Protocol architecture
Logical connection
ATM cells

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Asynchronous transfer mode (ATM)
Asynchronous Transfer Mode (ATM) is the cell relay protocol designed
by the ATM Forum and adopted by the ITU-T

Asynchronous vs. synchronous transfer

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ATM
Frames of varying size
Difficult to provide consistent data rate
E.g., multiplexing using different frame sizes

Audio and video frames ordinarily are small - mixing with data traffic
often creates unacceptable delays
Making the shared frame links unusable for audio and video information

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Cell networks
Many of the problems associated with frame internetworking are solved
by adopting a concept called cell networking

A cell is a small data unit of fixed size

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ATM Protocol architecture
ATM is a cell-switched network
Similar to packet switching using X.25 and frame relay
ATM involves the transfer of data in discrete chunks
Allows multiple logical connections to be multiplexed over a single physical
interface

ATM Protocol architecture

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Protocol architecture
Illustrates the basic architecture for an interface between user and network
 The physical layer involves the specification of a transmission medium and a signal
encoding scheme
Two layers of the protocol architecture relate to ATM functions
 ATM layer common to all services that provides packet transfer capabilities
 ATM adaptation layer (AAL) that is service dependent
ATM layer defines:
the transmission of data in fixed-size cells
the use of logical connections
AAL layer
Used to support information transfer protocols not based on ATM
maps higher-layer information into ATM cells to be transported over an ATM network
collects information from ATM cells for delivery to higher layers

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Protocol architecture

Three separate planes

User plane: for user information transfer
Control plane: Performs call control and connection control functions
Management plane:
 plane management - performs management functions related to a system as a whole and
provides coordination between all the planes
 layer management - performs management functions relating to resources and parameters
residing in its protocol entities

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ATM logical connection
The logical connection in ATM are referred as virtual channel connections
(VCCs)
VCC (Virtual Channel Connection): a logical connection analogous to virtual circuit in
X.25 or data link connection in FR
A VCC is set up between two end users through the network
VCCs are also used for user-network exchange (control signaling) and network-network
exchange (network management and routing)
virtual path connection (VPC)
A bundle of VCCs that have the same endpoints
The virtual path technique helps contain the control cost by grouping connections sharing
common paths through the network into a single unit
VCC and VPC are full-duplex connections
However, the channel or path bandwidth and other traffic parameters can be configured
to be different in the transmit and receive directions

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ATM logical connection
Transmission path: a physical connection between end points

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Advantages of Virtual Paths

Simplified network architecture

Network transport functions can be separated into virtual channel and virtual path
Increased network performance and reliability
The network deals with fewer, aggregated entities
Reduced processing and short connection setup time
The addition of new virtual channels to an existing virtual path involves minimal
processing
 new virtual channel connections can be established by executing simple control

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Call Establishment Using Virtual Paths

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Virtual channel connection uses

Between end users

to carry end-to-end user data
to carry control signaling between end users
Between an end user and a network entity
Used for user-to-network control signaling
Between two network entities
Used for network traffic management and routing functions

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VP/VC characteristics
ITU-T Recommendation I.150 lists the following as characteristics of virtual
channel connections
Quality of service (QoS): A user of a VCC is provided with a QoS specified by
parameters such as cell loss ratio and cell delay variation.
Cell sequence integrity: The sequence of transmitted cells within a VCC is preserved.
Traffic parameter negotiation and usage monitoring:
 Traffic parameters (such as average rate, peak rate, burstiness, and peak duration) can be
negotiated between a user and the network for each VCC
 monitors the input of cells to the VCC, to ensure that the negotiated parameters are not violated

Virtual connections
 Switched VCC: an on-demand connection, which requires a call control signaling for setup and
tearing down
 Semipermanent VCC is of long duration and is set up by configuration or network management
action.
VPI restriction – One or more virtual channel identifiers may be reserved for network
use (VCCs used for network management)
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Control signaling
A mechanism for the establishment and release of VPCs and VCCs
VCC establishment/release
Semi-permanent VCCs – no control signaling is required
Meta-signaling channel – used to set up VCCs that can be used for call control
signaling
The meta-signaling channel can also be used to set up
 A user-to-network signaling virtual channel
• can then be used to set up VCCs to carry user data
 A user-to-user signaling virtual channel
• can then be used to allow the two end users, without network intervention, to establish and release
user-to-user VCCs to carry user data
• Such a channel must be set up within a pre-established VPC

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Control signaling

VPC establishment/release
Semi-permanent VPC- no signaling
Customer controlled
 the customer uses a signaling VCC to request the VPC from the network
Network controlled
 The network establishes a VPC for its own convenience
 The path may be network-to-network, user-to-network, or user-to-user.

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ATM cells
The asynchronous transfer mode makes use of fixed-size cells

fixed-size cells
May reduce queuing delay for a high-priority cell
With fixed-size cells, it is easier to implement the switching mechanism in
hardware

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ATM headers
Generic flow control
 used for control of cell flow only at the local user-
network interface
 alleviates short-term overload conditions in the network
Virtual Path Identifier (VPI)
Virtual Channel Identifier (VCI)
Payload type
 user information, network management and maintenance
information
Cell loss priority (CLP)
 provide guidance to the network in the event of
congestion
Header error control
 Contains eight check bits which are used for checking
the header
 The payload of an ATM cell is not checked for errors

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ATM service categories

An ATM network is designed to be able to transfer many different types

of traffic simultaneously, including real-time flows such as voice and
video
Each is handled as a stream of 53-octet cells traveling through a virtual channel
But, the way in which each data flow is handled within the network depends on
the characteristics of the traffic flow and the requirements of the application

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ATM service categories
Real time service
Audio and video have a strict constraint on delay and the variability of the delay
Interactive applications have tight constraints on delay

Constant Bit Rate (CBR)

It is used by applications that require a fixed data rate that is continuously available during
the connection lifetime and a relatively tight upper bound on transfer delay
commonly used for uncompressed audio and video information
E.g. videoconferencing, interactive audio
Real-Time Variable Bit Rate (rt-VBR)
intended for time-sensitive applications; i.e., those requiring tightly constrained delay and
delay variation
For example, the standard approach to video compression results in a sequence of image
frames of varying sizes. Because real-time video requires a uniform frame transmission
rate, the actual data rate varies

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ATM service categories
Non-Real-Time Services
intended for applications that have bursty traffic characteristics and do not have
tight constraints on delay and delay variation
Non-Real-Time Variable Bit Rate (nrt-VBR)
The end system specifies a peak cell rate, a sustainable or average cell rate, and a
measure of how bursty or clumped the cells may be.
With this information, the network can allocate resources to provide relatively low
delay and minimal cell loss
nrt-VBR service can be used for data transfers that have critical response time
requirements. E.g., include airline reservations, banking transactions, and process
monitoring.

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ATM service categories
Unspecified Bit Rate (UBR)
suitable for applications that can tolerate variable delays and some cell losses
With UBR, cells are forwarded on a first-in-first-out (FIFO) basis using the
capacity not consumed by other services
Best effort service- e.g. Text/data/image transfer
Available Bit Rate (ABR)
An application using ABR specifies a peak cell rate (PCR) and a minimum cell
rate (MCR) that it requires
The network allocates resources so that all ABR applications receive at least their
MCR capacity
Any capacity not used by ABR sources remains available for UBR traffic
The ABR mechanism uses explicit feedback to sources to assure that capacity is
fairly allocated

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ATM vs. frame relay
ATM Frame relay
designed to be convenient for hardware software controlled - less
implementation - cost is higher compared expensive
to frame relay does not provide flow control or
error control. These functions
8 bit of error control field in header part of must be supported by upper
ATM cell is used for error control. layers.
Provides flow control at user to network Carry traffic in the form of data
interface (UNI) level only only
Supports quantifiable QoS Do not support quantifiable QoS
Frame size is fixed in ATM networks. Frame size is variable in frame
Therefore it gives less processing relay networks. Therefore it
overhead. gives medium processing
overhead
The cell transfer speed of ATM is high Frame transfer speed is low

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