ATM Signalling & Routing Protocols

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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.

The protocols illustrated below support connection control signalling. 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.

		User-Network Signalling
	SAAL	UNI SSCF
		SSCOP
		AAL Type 5 Common Part
		ATM Layer
		Physical Layer
ATM signalling protocol stack

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.

ftp://ftp.atmforum.com/pub/approved-specs/

A signalling message uses the Q.931 message format. It is made up of a message header and a variable number of Information Elements (IEs). This is shown in the following figure:

The message header is shown in the following diagram:

Protocol discriminator
Distinguishes Messages for user-network call control from other messages.

Call reference
Unique number for every ATM connection which serves to link all signalling messages relating to the same connection. It identifies the call at the local user network interface to which the particular message applies. The call reference 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:

Call establishment messages:
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.
SETUP, sent by the calling user to the network and by the network to the calling user to initiate a call.

Call clearing messages:
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.

Miscellaneous messages:
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.

Point-to-Multipoint messages:
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
Described below.

Example of UNI signalling decode

Types of 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:

For further reference about the exact structure and parameters of the IEs, refer to the ATM Forum, ATM User-Network Interface Specifications 3.0 and 3.1.

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ITU Q.2931 Signalling

http://www.itu.int/itudoc/itu-t/rec/q/q1000up/q2931_29825.html

This is the ITU version of signalling. The Q.2931 signalling protocol specifies the procedures for the establishment, maintenance and clearing of network connections at the B-ISDN user network interface. The procedures are defined in terms of messages exchanged. The basic capabilities supported by Q.2931 Signalling are as follows:

• Demand (switched virtual) channel connections.
• Point-to-point switched channel connections.
• Connections with symmetric or asymmetric bandwidth requirements.
• Single-connection (point-to-point) calls.
• Basic signalling functions via protocol messages, information elements and procedures.
• Class X, Class A and Class C ATM transport services.
• Request and indication of signalling parameters.
• VCI negotiation.
• Out-of-band signalling for all signalling messages.
• Error recovery.
• Public UNI addressing formats for unique identification of ATM endpoints.
• End-to-end compatibility parameter identification.
• Signalling interworking with N-ISDN and provision of N-ISDN services.
• Forward compatibility.

The message types for Q.2931 are the same as in UNI 3.0/3.1, with the exception of the point-to-multipoint messages which are not supported. The following are new signalling messages specific to Q.2931:

ALERTING, sent by the called user to the network and by the network to the calling user indicating that the called user alerting has been initiated.

PROGRESS, sent by the user or the network to indicate the progress of a call in the event of interworking.

SETUP ACKNOWLEDGE, sent by the network to the calling user or by the called user to the network to indicate that call establishment has been initiated.

INFORMATION, sent by the user or the network to provide additional information.

NOTIFY, sent by the user or by the network to indicate information pertaining to a call connection.

The information elements in Q.2931 are as follows:

• Called party number.
• Called party sub-address.
• Transit network selection.
• Restart indicator.
• Narrow-band low layer compatibility.
• Narrow-band high layer compatibility.
• Broadband locking shift.
• Broadband non-locking shift.
• Broadband sending complete.
• Broadband repeat indicator.
• Calling party number.
• Calling party sub-address.
• ATM adaptation layer parameters.
• ATM traffic descriptor.
• Connection identifier.
• OAM traffic descriptor.
• Quality of Service parameter.
• Broadband bearer capability.
• Broadband Low Layer Information (B-LLI).
• Broadband High Layer Information (B-HLI).
• End-to-end transit delay.
• Notification indicator.
• Call state.
• Progress indicator.
• Narrow-band bearer capability.
• Cause.

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ITU Q.2971 Signalling

ITU Q.2971 10/95

This is the ITU version of signalling. The Q.2971 signalling protocol specifies the procedures for the establishment, maintenance and clearing of point-to-multipoint virtual channel calls/connections by means of Digital subscriber signalling system 2 (DSS2) at the B-ISDN user network interface. The procedures are defined in terms of messages exchanged. Q.2971 uses the same message capabilities as Q.2931. However, in addition, it also supports point-to-multipoint unidirectional switched channel connections. A point-to-multipoint virtual channel connection is a collection of associated ATM virtual channel links connecting 2 or more endpoints. Q.2971 supports only unidirectional transport from the root to the leaves.

The following are the additional messages (not used in Q.2931) used with ATM point-to-mulitpoint call and connection control:
ADD PARTY, adds a party to an existing connection.
ADD PARTY ACKNOWLEDGE, acknowledges a successful ADD PARTY.
PARTY ALERTING
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.

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UNI 4.0 Signalling

http://www-comm.itsi.disa.mil/atmf/sig.html#af10.1

UNI 4.0 provides the signalling procedures for dynamically establishing, maintaining and clearing ATM connections at the ATM User-Network Interface. UNI 4.0 applies both to Public UNI (the interface between endpoint equipment and a public network) and private UNI (the interface between endpoint equipment and a private network).

The following new features are available within the UNI 4.0 signalling protocol:

• Leaf initiated join.
• Enhanced ATM traffic descriptor.
• Available bit rate capability.
• Individual QoS parameters.
• Narrow ISDN over ATM.
• AnyCast capability.
• New information elements.
• New VPI/VCI options.
• Proxy signalling capability.
• Virtual UNIs.
• Supplementary services such as direct dialing in, multiple subscriber number, calling line identification presentation, calling line identification restriction, connected line identification presentation, connected line identification rest, user-to-user signalling.
• Error handling for instruction indicators.
• Using setup for adding parties.
• Both NSAP and ASTM endsystem addresses.
• Network can support leaves that do not support P-PM.

The message types for UNI 4.0 are the same as in Q.2931, with the exception of the SETUP ACKNOWLEDGE and INFORMATION messages which are not supported. The following are new signalling messages specific to UNI 4.0: LEAF SETUP REQUEST and Leaf Setup Failure.

The following are the information elements contained in UNI 4.0:

• Narrowband bearer capability.
• Cause.
• Call state.
• Progress indicator.
• Notification indicator.
• End-to end transit delay.
• Connected number.
• Connected subaddress.
• Endpoint reference.
• Endpoint state.
• ATM adaptation layer parameters.
• ATM traffic descriptor.
• Connection identifier.
• Quality of service parameter.
• Broadband high layer information.
• Broadband bearer capability.
• Broadband low layer information.
• Broadband locking shift.
• Broadband non-locking shift.
• Broadband repeat indicator.
• Calling party number.
• Calling party subaddress.
• Called party number.
• Called party subaddress.
• Transit network selection.
• Restart indicator.
• Narrowband low layer compatibility.
• Narrowband high layer compatibility.
• Generic identifier transport.
• Minimum acceptable traffic descriptor.
• Alternative ATM traffic descriptor.
• ABR setup parameters.
• Leaf initiated join call identifier.
• Leaf initiated join parameters.
• Leaf sequence number.
• Connection scope selection.
• ABR additional parameters.
• Extended QoS parameters.

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Q.SAAL

Q. 2110 http://www.itu.int/itudoc/itu-t/rec/q/q1000up/q2110_27521.html
Q.2144 http://www.itu.int/itudoc/itu-t/rec/q/q1000up/q2144_33084.html
Q.2100 http://www.itu.int/itudoc/itu-t/rec/q/q1000up/q2100_27509.html
RFC1953 http://www.cis.ohio-state.edu/htbin/rfc/rfc1953.html

Following is the structure for each Q.SAAL message type.

BGN PDU (Begin)
The BGN PDU is used to initially establish an SSCOP connection or reestablish an existing SSCOP connection between two peer entities. The BGN requests the clearing of the peer’s transmitter and receiver buffers, and the initialization of the peer’s transmitter and receiver state variables.

Bytes
	1			2	3	4
1	N(UU)
2	Rsvd	S	PDU Type	N(MR)
	8 7 6	5	4 3 2 1
Begin PDU (BGN PDU)

BGAK PDU (Begin Acknowledge)
The BGAK PDU is used to acknowledge the acceptance of a connection request by the peer entity.

BGREJ PDU (Begin Reject)
The BGREJ PDU is used to reject the connection establishment of the peer SSCOP entity.

END PDU (End)
The END PDU is used to release an SSCOP connection between two peer entities.

Bytes
	1			2	3	4
1	N(UU)
2	Rsvd	S	PDU Type	Reserved
	8 7 6	5	4 3 2 1
End PDU (END PDU)

ENDAK PDU (End Acknowledge)
The ENDAK PDU is used to confirm the release of an SSCOP connection that was initiated by the peer SSCOP entity.

RS PDU (Resynchronization Command)
The RS PDU is used to resynchronize the buffers and data transfer state variables in the transmit direction of an SSCOP connection.

RSAK PDU (Resynchronization Acknowledgement)
The RSAK PDU is used to acknowledge the resynchronization of the local receiver stimulated by the received RS PDU.

SD PDU (Sequenced Data)
The SD PDU is used to transfer, across an SSCOP connection, sequentially numbered PDUs containing information fields provided by the SSCOP user.

Bytes
	1			2		3	4
1	Information (maximum k bytes)
. . .				PAD (0-3 Bytes)
n	PL	Rsd	PDU Type		N(S)
	8 7	6 5	4 3 2 1
Sequenced data PDU (SD PDU)

SDP PDU (Sequenced Data with Poll)
The SDP PDU is used to transfer, across an SSCOP connection, sequentially numbered PDUs containing information fields provided by the SSCOP user. The SDP PDU also contains a poll request that is used to stimulate the transmission of a STAT PDU. Therefore, an SDP PDU is the functional concatenation of an SD PDU and a POLL PDU.

POLL PDU (Status Request)
The POLL PDU is used to request, across an SSCOP connection, status information about the peer

SSCOP entity. It contains a sequence number for use in the retransmission of lost SD or SDP PDUs.

STAT PDU (Solicited Status Response)
The STAT PDU is used to respond to a status request (POLL PDU) received from a peer SSCOP entity. It contains information regarding the reception status of SD or SDP PDUs, credit information for the peer transmitter, and the sequence number of the POLL PDU to which it is in response.

USTAT PDU (Unsolicited Status Response)
The USTAT PDU is used to respond to a detection of a new missing SD or SDP PDU, based on the examination of the sequence number of the SD or SDP PDU. It contains information regarding the reception status of SD or SDP PDUs and credit information for the peer transmitter.

MD PDU (Management Data)
The MD PDU is used for unassured management data transfer between two SSCOP entities. When a management entity requests unacknowledged information transfer, the MD PDU is used to send information to the peer management entity without affecting SSCOP status or variables. MD PDUs do not carry a sequence number and therefore, the MD PDU may be lost without notification. (see UD PDU diagram above).

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IISP

IISP (Interim Interswitch Signalling Protocol) is a protocol that has been devised to provide signalling between switches from multiple vendors. It has been developed as a stopgap solution, until the more sophisticated PNNI (Private Network-to-Network Interface) specification is complete. There is no migration path from IISP to PNNI.

IISP uses UNI 3.1 signalling procedures as does PNNI. However, the switches using IISP are not peers, i.e., one switch functions as the network node and the other as an end-station. There is no support for dynamic distribution of routing information.

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PNNI Signalling and Routing

For more information on how to simulate a PNNI network with hundreds of nodes
in your lab

http://www-comm.itsi.disa.mil/atmf/pnni.html#af55

PNNI (Private Network-to-Network Interface), is a hierarchical, dynamic link-state routing protocol. It is designed to support large-scale ATM networks. The PNNI protocol uses VPI/VCI 0,18 for its messages. In addition, it uses signalling messages to support connection establishment across multiple networks. The signalling is based on UNI 4.0 and Q.2931. Specific information elements were added to UNI 4.0 in order to support the routing process of PNNI.

PNNI Signalling

PNNI Signalling contains the procedure to dynamically establish, maintain and clear ATM connections at the private network to network interface or network node interface between 2 ATM networks or 2 ATM network nodes. The PNNI signalling protocol is based on the ATM forum UNI specification and on Q.2931.

PNNI Messages include:
ALERTING, CALL PROCEEDING, CONNECT, SETUP, RELEASE, RELEASE COMPLETE, NOTIFY, STATUS, STATUS ENQUIRY, RESTART, RESTART ACKNOWLEDGE, STATUS, ADD PARTY, ADD PARTY ACKNOWLEDGE, PARTY ALERTING, ADD PARTY REJECT, DROP PARTY, DROP PARTY ACKNOWLEDGE.

The following messages for ATM call connection control for the support of 64 Kbits based ISDN circuit code services are transported without change across the PNNI:
ALERTING, CONNECT PROGRESS, SETUP, RELEASE.

The following message types supported by Q.2931 are not supported by PNNI:
CONNECT ACKNOWLEDGE, SETUP ACKNOWLEDGE, INFORMATION.

PNNI signalling decode

PNNI Routing

The structure of the PNNI header is shown in the following illustration:

Packet type	Packet length	Prot ver	Newest ver	Oldest ver	Reserved
2	2	1	1	1	1
PNNI header structure

Packet type
The following packet types are defined:

Packet