Download SS7 - De Montfort University

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
Operations,
maintenance and
administration part
Mobile
application
services
Transaction
services
Application
entity
Application
entity
Other application
entity
Application
service
element
Mobile
application
part
Application
service
elements
Transaction
capabilities
application
part
Transaction
capabilities
application
part
Transaction
capabilities
application
part
Signalling connection control part (SCCP)
(Level 4)
Message transferl part (MTP)
(Levels 1- 3)
Figure Q5
SS7 Protocol Architecture
Call control
application services
ISDN
user part
(Level 4)
Telephone
user part
(Level 4)
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