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

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

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

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

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.

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.

Pre SS7
• Phreaking
– http://www.webcrunchers.com/crunch/FAQ.html
– http://www.telephonetribute.com/signal_and_circuit_condition s.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.

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.

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.

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.

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.

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.

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.

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.

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

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 24 th is used to carry the
SS7 signaling.
– The most wasteful method by far as the packet channel can often sit empty.

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.

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.

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.

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

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

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.

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:

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.

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

SCCP Parts
• TCAP - Transaction Capabilities
Application Part
– Allows calling card/ 1-800 functionality to access databases.

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

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.

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