An Overview of ATM Signalling

1
An Overview of ATM Signalling
2
Introduction

• ATM is a connection-oriented protocol, which
  means that virtual channels must be set up before
  any data cells can be sent on the channel
• Connection setup is done using a signalling
  protocol

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ATM Signalling Protocol

• Protocol consists of two parts
• User-Network Interface (UNI)
• defines how hosts talk to switches
• Network-Network Interface (NNI)
• defines how switches talk to other switches
• Cell formats slightly different

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ATM UNI Cell Format
GFC
VPI
VPI
VCI
VCI
PT
CLP
VCI
RES
HEC
Payload
(48 bytes)
5
ATM NNI Cell Format
VPI
VPI
VPI
VCI
VCI
PT
CLP
VCI
RES
HEC
Payload
(48 bytes)
6
ATM NNI Cell Format
VPI
VPI
VPI
VCI
VCI
PT
CLP
VCI
RES
HEC
Payload
(48 bytes)
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Basic Signalling Operation

• Connection requests proceed hop-by-hop through
  the switches of the network en route to
  destination
• Switches perform Call Admission Control (CAC)
  based on traffic descriptor, QOS requirements,
  and available resources at that switch

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Basic Operation (Contd)

• If connection is acceptable, then request is
  forwarded on, otherwise reject is returned
• If destination accepts connection, then accept
  is returned
• VPI and VCI assigned

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Example ATM Signalling
10
Example ATM Signalling
ATM Switch
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Example ATM Signalling
UNI
12
Example ATM Signalling
NNI
UNI
13
Example ATM Signalling
UNI
NNI
UNI
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Example 1 ATM Signalling
CR
CR Connection Request
15
Example 1 ATM Signalling
CR
CR Connection Request
16
Example 1 ATM Signalling
CR
CR Connection Request
17
Example 1 ATM Signalling
CR
CR Connection Request
18
Example 1 ATM Signalling
CR
CR Connection Request
19
Example 1 ATM Signalling
OK
(VCI 12)
CR
CR Connection Request
OK Connection Accept
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Example 1 ATM Signalling
OK
(VCI 12)
CR
(VCI 4)
CR Connection Request
OK Connection Accept
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Example 1 ATM Signalling
OK
(VCI 12)
CR
(VCI 104)
(VCI 4)
CR Connection Request
OK Connection Accept
22
Example 1 ATM Signalling
OK
(VCI 12)
CR
(VCI 104)
(VCI 4)
(VCI 4)
CR Connection Request
OK Connection Accept
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Example 1 ATM Signalling
OK
(VCI 12)
CR
(VCI 104)
(VCI 4)
OK
(VCI 4)
(VCI 77)
CR Connection Request
OK Connection Accept
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Example 2 ATM Signalling
CR
CR Connection Request
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Example 2 ATM Signalling
CR
NO
CR Connection Request
NO Connection Reject
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Example 3 ATM Signalling
CR
CR Connection Request
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Example 3 ATM Signalling
CR
CR Connection Request
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Example 3 ATM Signalling
CR
CR Connection Request
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Example 3 ATM Signalling
CR
NO
CR Connection Request
NO Connection Reject
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Example 3 ATM Signalling
CR
NO
CR Connection Request
NO Connection Reject
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Example 3 ATM Signalling
CR
NO
NO
CR Connection Request
NO Connection Reject
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Summary

• ATM UNI and NNI signalling specifications define
  the protocols for connection setup and teardown
  between ATM equipment (Version 4.0 soon)
• Can support point to point and multipoint
  connections

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Where to get more information

• ATM Forum
• Fore Systems SPANS protocol specification (Simple
  Protocol for ATM Network Signalling)