Signalling Systems

1
Signalling Systems

• System which allows various network components to
  exchange information
• In particular, it supports
• call / connection control
• network management

2
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)

3
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

4
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

5
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

6
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

7
Signalling

• Two signalling standards which operate
• Subscriber signalling
• e.g. Digital subscriber system 1
• Network signalling
• Signalling System 7 (SS7)

8
Signalling
9
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

10
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

11
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

12
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

13
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

14
Signalling

• Consider how SS7 supports setting up a call
• Consider how SS7 supports the 0800 service