atm advantages

7/27/2019 ATM Advantages

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"A transfer mode in which information isorganized into cells; it is asynchronous inthe sense that the recurrence of cellscontaining information from an individual

user is not necessarily periodic".

ATM – definition



What is it really?

Low-level network layer —above physicallayer, below AAL (ATM adaptation layer)

Single transport mechanism for differenttypes of traffic (voice, data, video, etc.)

Streamlined protocol, minimal error and

flow control capabilities Fixed packet size = ATM cell

Simplified processing, management

Fast

packetswitching,

high data

rates



Synchronous Transfer Mode

Pre-assigned ―slots,‖ frame boundaries,

global timing Slots identified by position from the start

of the frame

BW allocated in units of slots

Idle slots wasted

Efficient for Constant Bit Rate traffic



Contrast with STM ―Bandwidth on demand‖—Slots assigned

on demand, users take any empty slot

Nothing pre-assigned, no global timing Slot Cell, fixed size of 53 bytes

Arbitrary bit rates: can support T-1 usingCBR, voice/video using real-time VBR, IP-based traffic using ABR and UBR, etc.

Each cell must be self-identifying(overhead)



ATM cell contentsHeader – 5 bytes

General Flow Control (GFC) – traffic control for

different QoS, alleviates short-term overloads VPI – routing field for network

VCI – routing to/from user

Payload type

Cell loss priority (CLP) Header error control (HEC) – can correct single

bit errors in header

Information – 48 bytes



PT coding

000

001

010

011

100

101

110

111

______________Interpretation____________

User data cell, congestion not experienced, SDU type=0

User data cell, congestion not experienced, SDU type=1

User data cell, congestion experienced, SDU type=0

User data cell, congestion experienced, SDU type=1

OAM segment associated cell

OAM end-to-end associated cell

Resource management cell

Reserved for future function

SDU = Service Data Unit

OAM = Operations, Administration, and Maintenance

Stallings, Data & Computer

Communications, 6th ed., Table 11.2



ATM Cells Small size, may reduce queuing delay of high priority cells

Fixed size, more efficient switching

Info field,

48 bytes

Header error control

PT CLP

VC identifier

VP identifier

GFC VP identifier

5-byte

header

Info field,

48 bytes

Header error control

PL type CLP

VC identifier

VP identifier NNIUNI

53 bytecell



VC’s are not always VC’s Virtual Channel

Transmission is

connection-oriented VC set up by some

signaling protocol beforeany cells can be sent

Virtual Path Connection(VPC), bundle of VCC’s Virtual channels

Virtual path

Physical

channel



Logical connections

VPC = bundle of VCC’s with the same

endpoints all switched together Network management of group of

connections, not many individual ones

Setup time is for a VP, adding VC’s to itinvolves minimal processing



Request for

VCC originates

VPC exists?

Request for

VCC originates

Establish

new VPC

Can QoS be

satisfied?

Block VCC or

request more

capacity

Reject VCC

request

Makeconnection

Request

granted?Yes

YesYes

NoNo

No



ATM Adaptation Layer (AAL) Layer above ATM

Service dependent

Mask ATM specifics from user;universality

SAR: segmentation and reassembly. ―Translates‖ service data from a

non-ATM format into ATM cells,then back again at destination

CS: convergence sublayer. Takescare of delay jitter, error checking,remove corrupted cells

Physical layer

ATM layer

SAR sublayer

CS

AAL user

AAL



ATM Service Categories Real-time services

Constant bit rate – uncompressed audio/video info

Videoconferencing, TV, pay-per-view, VOD, etc. rt-Variable bit rate

Non-real-time services

nrt-VBR – high end system QoS, critical response time

Bank transactions, airline reservations, etc.

Unspecified bit rate – best-effort service

e.g. text/image messaging, telecommuting

Available bit rate – bursty apps requiring reliable end-to-end connection

e.g. LAN, router-to-router reliability



ATM advantages

Universality

Mixed traffic types, real-time and non-real-time Scalability

LANs, MANs, WANs, WLANs

Efficient use of network resourcesBandwidth on demand concept

Simplified network infrastructure



ATM challenges

In-network

mux/buffering canlead to cell delay orloss QoSguarantees

Many types of traffic

Large geographicdistribution

Traffic modeling,

control

atm advantages

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