Ss7

greydullNetworking and Communications

Oct 30, 2013 (3 years and 11 months ago)

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Signalling Systems


System which allows various network
components to exchange information


In particular, it supports


call / connection control


network management

Signalling Environments


Subscriber Signalling


Applied on Access Links


Tends to be only Call Control


Relatively Simple


e.g. DSS1 in ISDN



Network Signalling


In the network


Call Control and Network Control


Complex


e.g Signalling System 7 (SS7)

Signalling


Types of signalling


in
-
band signalling


signalling messages carried in same channel
as user information


e.g.


TCP/IP


analogue access links in telephone
networks


out
-
band signalling


Separate signalling channel for signalling
messages


e.g.


ISDN access links (Frame Relay)


SS7


Signalling


Within out
-
band signalling another
distinction can be made


Channel Associated Signalling (CAS)


Direct relationship between the signalling
channel and information channel


Common channel signalling (CCS)


Signalling channel is shared on a demand
basis

Signalling


Modern signalling systems use out
-
band
CCS


More secure (main reason why out
-
band
signalling developed)


Signalling can be carried out at the same time
as user information is exchanged


More efficient use of resources
-

allocated on a
demand basis


Separate signalling network means less
processing on messages
-

no need to separate
signalling from user information


Signalling protocol can evolve independently,
without effecting user application
communication stack, can also be more
complex.


At call set
-
up, signalling messages can be
transmitted in parallel with the establishment of
information channel (Not possible with CAS)


set
-
up times tend to be shorter

Signalling


Signalling network is a separate network


Two modes of operation are possible with CCS


Associated mode


Same topology as information network


Signalling switches located with
information switches


Non Associated mode


Combined signalling / information
switches


Also, separate signalling switches
called Signalling Transfer Points
(STPs)


Result is a separate signalling network
topology, with links and interfaces into
underlying information network

Signalling


Two signalling standards which operate


Subscriber signalling


e.g. Digital subscriber system 1


Network signalling


Signalling System 7 (SS7)

Signalling


Signalling


SS7


Concerned with connection set
-
up and network
management


Defined in Q700 series


Network components are


Signalling Points


Process signalling messages


Signalling Transfer Points


SPs with switching facilities


Signalling links

Signalling


SS7 Protocol architecture


Message Transfer Part


3 layers, which provide a reliable, but
connectionless service


similar in functionality to IP


i.e. provides carrier for higher layers


Signalling Data Link


typical physical layer functionality


Signalling Link Layer


provides typical layer 2 functionality, i.e.
error free link


Signalling Network Layer


Signalling Message Functions


Switching of signalling messages
across the network


Signalling Network Management Functions


traffic, link and route management
functions

Signalling


Signalling Connection Control Part (SCCP)


Equivalent to upper layer 3


(so concerned with routing)


Augments the basic service provided
by MTP


A range of services


2 connectionless services


2 connection
-
oriented services

Signalling


User Parts


Equivalent to layer 4


These typically relate to the control of
specific types of calls


e.g. ISDN User Part


Used to set
-
up ISDN calls

Signalling


Transaction Capabilities Part (TCAP)


Used to



execute remote processes


provide means for transfer of data
between nodes


Used extensively in Intelligent Networks
(INs)


Essentially application layers


Signalling


Consider how SS7 supports setting up a
call


Consider how SS7 supports the 0800
service