SS7 Telecommunications Overview & Benefits

1. 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. • http://www.dmine.com/phworld/sounds/wawina/ • 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 • http://www.webcrunchers.com/crunch/FAQ.html • http://www.telephonetribute.com/signal_and_circuit_conditions.htm • http://www.dsptutor.freeuk.com/dtmf/ToneGenerator.html • 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: 200.12.148 • 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/ 1-800 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.