Version 1'0

1
Objectives

• Understanding
• Problems with the PSTN in-band signalling
• Birth of SS7 and standards driven approaches
• Functions properties of an SS7 network
• SS7 entities
• SS7 protocol stack

2
Problems with the original PSTN

• Demand and all telcos could do was add more and
  more wires
• Engineers began to consider how more traffic
  could be driven through the saturated network
• Concept was simple if wires could be 10 more
  efficient then each wire could handle 10 more
  traffic reducing demand for more wires

3
Problems with the original PSTN

• Answer was simple, stop using the wires for
  non-voice traffic
• Recall from PSTN lecture that each analogue
  signal consisted of both voice and signalling
• Just what was the signalling?

4
In-band Signalling

• Signalling carried the data required to establish
  manage connection
• Basically analogue representation of system state
• For example the dial tone that tells a user that
  the phone is connected to the local switch
• When dialling each digit was represented by an
  interruption in the signal between the handset
  and the local switch
• Once dialling completed the switch compared it
  with routing table to route through the network

5
Problems with In-band Signalling

• As technology improved the switch would route the
  call as it was being made (this could lead to the
  busy tone being received before dialling was
  complete as any switch that was congested could
  return busy tone)
• This meant that all of the intervening switches
  had to carry the call until the busy signal was
  returned and the caller replaced the handset
• Frequently, a user receiving a busy signal hangs
  up and redials exacerbating the problem

6
Common Channel Signalling

• With arrival of digitisation (TDM, packet
  switching, etc) thousands of signals could share
  a common channel each packet containing
  signalling data
• Thus for the loss of 1 voice channel, signalling
  could be removed from the voice circuits
• Concept was called Common Channel Signalling (CCS)

7
Common Channel Signalling

• CCS would allow services such
• 0800 / 900 numbers
• Telephone charge cards
• Calling cards
• Paging
• Caller identification
• Adding Intelligence to the network (IN)
• To achieve this there had to be international
  standardisation
• International Telecommunications Union (ITU)
  formed in 1934

8
Birth of SS7

• First CCS standard released 1976 (called Common
  Channel Interoffice Signalling Systems 6)
• Not until 1980 (when second version of standard
  was released) that the international standards
  technology were in-place for the first truly CCS
  based system to be deployed
• The new version had the slightly catchier title
  of Signalling System 7 (SS7)

9
Functions of SS7

• Basic call setup, management and tear down
• Wireless services
• Roaming
• Subscriber Identification
• Local Number Portability (LNP)
• Toll-free services (800) and premium (0907)
• Advanced call features
• Call forwarding
• Number display
• Conference/party calls
• Worldwide standards based telecommunications

10
Properties of SS7

• Interconnected network elements exchange messages
  using standard protocol
• Uses 56 or 64 kbps bi-directional channels called
  signalling links
• All signalling is out-of-band on dedicated
  channels rather than in-band
• Enables
• Faster call setup
• More efficient use of voice channels
• Support for IN services using none-voice related
  network elements such as databases
• Better monitoring security

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Properties of SS7

• Three networks entities or node called Signalling
  Points
• Service Switching Point (SSP)
• Signal Transfer Point (STP)
• Service Control Point (SCP)
• Each point has a unique numeric code (numeric
  point code) used in messages to identify the
  source/destination node
• Each node has a routing table

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SS7 Logical Architecture
SCP
STP
SSP
SCP
STP
SSP
Standard Voice Trunks
SS7 Links
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SS7 Logical Architecture
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SSP / STP / SCP Overview

• SSP
• Used to originate, terminate and tandem calls
• Can request routing information from the SCP
• STP
• Packet switching hub
• Removes need for all Signalling Points to be
  interconnected
• May act as a firewall between network operators
• SCP
• Routing databases
• STP SCP usually deployed in mated pairs
• SSPs and SCPs are also called end points

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A Links

• (Access) link - connects end points
• to an STP
• Only messages starting at,
• or terminating at,
• an end point are
• transmitted on A links

SCP
STP
SCP
STP
SSP
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B Links

• (Bridge) link connects different networks
  STPs (called primary STP or inter-network
  gateway)
• Typically a quad of links used

STP
STP
STP
STP
Network B
Network A
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C Links

• (Cross) link connects STPs performing
  identical functions on a network as a mated pair
• Only used in case of link failure

STP
STP
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D Links

• (Diagonal) link connects local or regional
  STPs (called secondary STP)
• Typically a quad of links is used

STP
STP
STP
STP
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E Links

• (Extended) link connects SSP to alternate STPs
  in case of failure in an A link
• Rarely provisioned due to cost

STP
STP
STP
STP
SSP
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F Links

• (Fully associated) link connects two end points
• Not usually provisioned in networks using STPs
• NOTE SCPs are never interconnected

SSP
SSP
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SS7 Protocol Stack
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The Future?

• SS7 lies at the heart of all modern
  telecommunications
• Advantages
• Secure
• Proven
• Standardised
• Well engineered (has coped with expansion of
  data, land and mobile networks without need for
  modification)
• Disadvantages
• Expensive
• Highly redundant
• SS8 is in development
• Internet Protocol providing competition