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ATM UNI: ATM Signaling User-to-Network Interface

Signalling is the process by which ATM users and the network exchange the control of information, request the use of network resources, or negotiate for the use of circuit parameters. The VPI/VCI pair and requested bandwidth are allocated as a result of a successful signalling exchange. These messages are sent over the Signalling ATM Adaptation Layer (SAAL), which ensures their reliable delivery. The SAAL is divided into a Service Specific Part and a Common Part. The Service Specific Part is further divided into a Service Specific Coordination Function (SSCF), which interfaces with the SSCF user; and a Service Specific Connection-Oriented Protocol (SSCOP), which assures reliable delivery.

The UNI signalling protocols within the SAAL are responsible for ATM call and connection control, including call establishment, call clearing, status enquiry, and point-to-multipoint control. This signalling message uses the Q.931 message format, which is made up of a message header and a variable number of Information Elements (IEs).

The VPI/VCI pair and requested bandwidth are allocated as a result of a successful signalling exchange. Two levels of virtual connections can be supported at the UNI:

  • A point-to-point or point-to-multipoint Virtual Channel Connection (VCC) which consists of a single connection established between two ATM VCC end-points.
  • A point-to-point or point-to-multipoint Virtual Path Connection (VPC) which consist of a bundle of VCCs carried transparently between two ATM VPC end-points.
Note: For VPC at the Public UNI, traffic monitoring and throughput enforcement will be performed across all cells carried on the same VPI independently of the VCI values.

Protocol Structure - ATM Signaling UNI: Signaling User-to-Network InterfaceSAAL protocol stacks illustrated below support UNI connection control signaling:

 

User-Network Signalling

SAAL

UNI SSCF

SSCOP

AAL Type 5 Common Part

 

ATM Layer

Physical Layer

UNI Signaling Message:

8

7

6

5

4

3

2

1

bit/Octet

Protocol discriminator

1

0

0

0

0

Length of call reference value

2

Flag

Call reference value

3

Call reference value (continued)

4

5

Message type

6

Message type (continued)

7

Message length

8

Message length (continued)

9

Variable length Information Elements as required

etc.
  • Protocol discriminator -Distinguishes Messages for user-network call control from other messages. (9 for Q.2931 messages)
  • Call reference - Unique number for every ATM connection which serves to link all signalling messages relating to the same connection. It is comprised of the call reference value and the call reference flag. The call reference flag indicates who allocated the call reference value.
  • Message type - The message may be of the following types:
    1. Call establishment messages: such as CALL PROCEEDING, sent by the called user to the network or by the network to the calling user to indicate initiation of the requested call. CONNECT, sent by the called user to the network and by the network to the calling user to indicate that the called user accepted the call. CONNECT ACKNOWLEDGE, sent by the network to the called user to indicate that the call was awarded and by the calling user to the network. And SETUP, sent by the calling user to the network and by the network to the calling user to initiate a call.
    2. Call clearing messages: such as RELEASE, sent by the user to request that the network clear the connection or sent by the network to indicate that the connection has cleared. RELEASE COMPLETE, sent by either the user or the network to indicate that the originator has released the call reference and virtual channel. RESTART, sent by the user or the network to restart the indicated virtual channel. RESTART ACKNOWLEDGE, sent to acknowledge the receipt of the RESTART message.
    3. Miscellaneous messages: such as STATUS, sent by the user or network in response to a STATUS ENQUIRY message. STATUS ENQUIRY, sent by the user or the network to solicit a STATUS message.
    4. Point-to-Multipoint messages: such as ADD PARTY, adds a party to an existing connection. ADD PARTY ACKNOWLEDGE, acknowledges a successful ADD PARTY. ADD PARTY REJECT, indicates an unsuccessful ADD PARTY. DROP PARTY, drops a party from an existing point-to-multipoint connection. DROP PARTY ACKNOWLEDGE, acknowledges a successful DROP PARTY.
  • Message length - The length of the contents of a message.
  • Information Elements - There are several types of information elements. Some may appear only once in the message; others may appear more than once. Depending on the message type, some information elements are mandatory and some are optional. The order of the information elements does not matter to the signalling protocol. The information elements in UNI 3.0 are listed in the following table:

IE

Description

Max. No.

Cause

Gives the reason for certain messages. For example, the Cause IE is part of the release message, indicating why the call was released.

2

Call state

Indicates the current state of the call.

1

Endpoint reference

Identifies individual endpoints in a point-to-multipoint call.

1

Endpoint state

Indicates the state of an endpoint in a point-to-multipoint call.

1

AAL parameters

Includes requested AAL type and other AAL parameters.

1

ATM user cell rate

Specifies traffic parameters.

1

Connection identifier

Identifies the ATM connection and gives the VPI and VCI values.

1

Quality of Service parameter

Indicates the required Quality of Service class for the connection.

1

Broadband high-layer information

Gives information about the high-layer protocols for compatibility purposes.

1

Broadband bearer capacity

Requests a service from the network (such as CBR or VBR link, point-to-point and point-to-multipoint link).

1

Broadband low-layer information

Checks compatibility with layer 2 and 3 protocols.

3

Broadband locking shift

Indicates a new active codeset.

-

Broadband non-locking shift

Indicates a temporary codeset shift.

-

Broadband sending complete

Indicates the competition of sending the called party number.

1

Broadband repeat indicator

Indicates how IEs which are repeated in the message should be handled.

1

Calling party number

Origin of the call.

1

Calling party subaddress

Subaddress of calling party.

1

Called party number

Destination of the call.

1

Called party subaddress

Subaddress of the called party.

1

Transit network selection

Identifies one requested transit network.

1

Restart indicator

Identifies which facilities should be restarted (e.g., one VC, all VCs).

1


Related Protocols
ATM , AAL0-AAL5 , LAN Emulation (LANE ), CES, PNNI , MPOA and Q.2931

Sponsor Source

The ATM protocol reference model is based on standards developed by the ITU.
http://www-comm.itsi.disa.mil/atmf/sig.html#af10.1 :UNI 4.0 Specification
http://www.atmforum.com/standards/approved.html : ATM Forum approved specifications



Reference

ITU-T Recommendation I.363, “B-ISDN ATM Adaptation Layer (AAL) Specification”
http://www.atmforum.com/standards/approved.html#uni ATM User-Network Interface Specification
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/atm.htm : ATM Overvie