GSM 09.02 - Version 5.3.0 - Digital cellular telecommunications - ETSI

3.1.3 Congestion control for Signalling System No. 7
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GSM 09.02 version 5.3.0: August 1996
The requirements of SS7 Congestion control have to be taken into account as far as possible.
Means which could be applied to achieve the required traffic reductions are described in subclauses 3.1.1
and 3.1.2.
3.2 Compatibility
3.2.1 General
This ETS of the Mobile Application Part is designed in such a way that an implementation which conforms
to it can also conform to the Mobile Application Part operational version 1 specifications, except on the
MSC-VLR interface.
A version negotiation mechanism based on the use of an application-context-name is used to negotiate the
protocol version used between two entities for supporting a MAP-user signalling procedure.
When starting a signalling procedure, the MAP-user supplies an application-context-name to the
MAP-provider. This name refers to the set of application layer communication capabilities required for this
dialogue. This refers to the required TC facilities (i.e. version 1 or 2) and the list of operation packages
(i.e. set of operations) from which operations can be invoked during the dialogue.
A version one application-context-name may only be transferred to the peer user in a MAP-U-ABORT to an
entity of version two or higher (i.e. to trigger a dialogue which involves only communication capabilities
defined for MAP operational version 1).
If the proposed application-context-name can be supported by the responding entity the dialogue continues
on this basis otherwise the dialogue is refused an the initiating user needs to start a new dialogue which
involve another application-context-name which require less communication capabilities but provides similar
functionalities (if possible).
When a signalling procedure can be supported by several application contexts which differs by their
version number, the MAP-User need to select a name. It can either select the name which corresponds to
the highest version it supports or follow a more specific strategy so that the number of protocol fallbacks
due to version compatibility problems be minimized.
3.2.2 Strategy for selecting the Application Context (AC) version
A method should be used to minimize the number of protocol fall-backs which would occur sometimes if the
highest supported AC-Name were always the one selected by GSM entities when initiating a dialogue. The
following method is an example which can be used mainly at transitory phase stage when the network is
one of mixed phase entities.
3.2.2.1 Proposed method
A table (table 1) may be set up by administrative action to define the highest application context (AC)
version supported by each destination; a destination may be another node within the same or a different
PLMN, or another PLMN considered as a single entity. The destination may be defined by an E.164
number or an E.214 number derived from an IMSI. The table also includes the date when each destination
is expected to be able to handle at least one AC of the MAP version 2 protocol. When this date is reached,
the application context supported by the node is marked as "unknown", which will trigger the use of table 2.