1. ENGR 475 – Telecommunications
SS #7
ENGR 475 – Telecommunications
Harding University
October 17, 2006
Jonathan White

2. Outline Overview of SS7 What it replaces
In Band versus out of band signaling
Benefits
Packet switching versus circuit switching
New features
Types of circuits used in SS7
Protocol Stack

3. Overview of SS7 Signaling System #7:
A set of protocols which are used to setup, monitor, and tear down telephone calls very quickly.
Runs over a separate network than the actual voice traffic.
Packet switched
Out of Band
Also adds other features:
Database access, caller id, load balancing, fault detection and correction, tariffs, 911 tracking, calling card features

4. Overview of SS7 Replaces SS6 and SS5 in the United States.
SS6 was introduced in the mid 1960s.
It was the first widely used, high speed (2400 bps), digital, packet switched network.
Many people consider this to be the first packet switched network.
Called CCS7 in the rest of the world.
Common Channel Interoffice Signaling System Number Seven

5. SS7 Overview Not every telephone network uses SS7.
Some telephone networks have not been updated since the mid 1960s.
SS7 requires new hardware along the network, and this isn’t always cost effective.
New telephone installations almost always use SS7 as the management layer, however.

6. SS7 Definition
Set of telephony protocols and devices that are used to manage phone calls quickly.
Tells the network when to start and stop a call, who to bill, who dialed, etc…
Travels on a separate network than the actual voice traffic.
The network is a digital packet switched network as opposed to the circuit switched voice.
The packets have a definite format, but they can be variable length.

7. Pre SS7 Phreaking Control signals were passed in band.
Control signals were passed in band.
The tones were used at the beginning and end of a conversation. These included:
The numbers dialed, ringing, busy tone, no such number, etc…
These tones could be heard, and they passed with the actual voice conversation to each device on the network.

8. Pre SS7
Phreaking wasn’t the primary reason telephone companies went to out of band signaling.
How long does it take you to dial a number ?
Or, what can dial faster, you or a machine?
Since it is traveling with the voice, what you dial must be passed to every device on down the line.
The primary reason SS7 is used is to make call setup/teardown much quicker.

9. Pre SS7
Without SS7, call setup/teardown would take around 12 – 24 seconds.
With SS7, call setup/teardown takes 1 – 4 seconds.
SS7 is used for both the landline and wireless network.
What is the savings to the phone company in a year, ignoring the costs of the SS7 network?
Assume that there are 1 billion people in the world that use phones, making 50 phone calls a month. The use of a phone line costs the telephone company around ¼ of a cent per minute.

10. Pre SS7
In band signaling passes all control tones with the actual voice traffic.
Equipment must continually check for the control tones because it doesn’t know when they will appear on the line.
Each device must do this.
Before SS7, the switches are the intelligent devices. They don’t use or connect to databases.
Out of band signaling allows:
Faster transport of the signaling data.
Signaling at any time during the entire duration of the call.
Allows signaling to other network elements, such as databases.

11. SS7 Architecture
A call must make many “hops” traveling across the network.
Each switch must know where the call is coming from and where it is going to in order to maintain service.
This takes a lot of coordination.
This is what the SS7 architecture addresses.

12. SS7 Architecture 3 important devices:
1. Signal Switching point: SS7 capable telephone switches. They originate, terminate, or switch calls.
SSP
2. Signal Transfer point: SS7 data packet switches. They receive and route incoming signaling messages to the proper location.
STP
3. Signal Control point: Databases distributed along the network. They allow advanced call processing capabilities.
SCP
The intelligence in the network is now distributed among 3 types of equipment.

13. SS7 Architecture
The signal transfer (packet switch) and control points (databases) always come in pairs to provide redundancy.
They are exact copies of each other.
This redundancy was put in place so that the telephone network could have a high level quality of service (QOS).
Since the devices come in pairs, they actually have 4 lines among them.
There aren’t necessarily connections to the SCP (database) at every switch.
They aren’t needed at every switch.

14. SS7 Signaling Links
We are talking about the packet switched network, NOT the voice network.
The voice network is still the digital T1 type interface.
The links among the packet switches (STP) are actually fairly slow by our standards:
56 Kbps, full duplex.
However, this system has been in place since the mid 1990’s, and it suits current needs.

15. SS7 Signaling Links 3 types:
Associated
Nonassociated
Quasi-associated
All have to deal with how many packet switches and voice switches that are used.

16. SS7 Signaling Links Associated:
There is a static 1 to 1 correspondence to the voice lines and the signaling packet line.
This occurs when 23 lines of a T1 are used to carry voice and the 24th is used to carry the SS7 signaling.
The most wasteful method by far as the packet channel can often sit empty.

17. SS7 Signaling Links Nonassociated:
The voice path is completely separate from the signaling path.
Many nodes (STPs) must be passed through before the call routing signal is passed to the correct place.
Very often used.

18. SS7 Signaling Links Quasi-associated:
Reduces the number of STPs that the signaling information must be transported through.
Requires much more cabling, but it can be much faster.
The signal is only allowed to pass through 1 STP between the voice switches.

19. SS7 Signal Links
Each type of link is given a different name in the architecture depending on what it does.
The names are labeled A through F.
For example, a C link is the link that occurs between a mated pair of STPs.
These designations just help us to manage what type of links we are looking at on the network.

21. SS7 Signal Links
The SCP (databases) often have many links going to them. Why?
The SSP (voice switches) often have much less connections. Why?

22. SS7 Addresses Similar to an IP address
Every object has a unique address
Uses 3, 8 bit numbers
Exp:
First byte is the network identifier
Second byte is the cluster number
Third byte is the member number

23. SS7 Protocol Stack Much like the OSI Model Uses only 4 layers:
Physical: UTP, DS0 channel
Data Link: Error checking/Flow Control
Network: Addressing, routing, congestion control.
These 3 layers make up the message transfer part.

24. SS7 Protocol Stack Signal Connection Control Part
The top layer of the SS7 protocol stack.
This layer provides the actual functionality that the messages contain. There are several parts:

25. SCCP Parts GTT – Global Title Translation
Allow specific calls to be routed to certain databases.
Enables credit card processing over the phoneline.
This is typically what businesses use when they scan your card at a restaurant.

26. SCCP Parts ISUP – ISDN User Part:
Allows for the establishment and tear down of calls.
Works for both ISDN and non ISDN networks.

27. SCCP Parts TCAP - Transaction Capabilities Application Part
Allows calling card/ functionality to access databases.

28. SCCP Parts OMAP - Operations, Maintenance, and Administration Part
Allows for maintenance of routing tables
Allows for dynamic, flow controlled routing across intelligent links.

29. SS7 Packet Can be variable length: Contains:
Sender/Receiver phone numbers
What links are used.
What devices are used.
Other routing information
A pointer to the actual voice data.

31. SS7 Applications 911 enhancements Calling card fraud prevention
Credit card approval
Virtual networks.
Call tracing
Call blocking
Much quicker call setup/teardown.