Transcript Lecture 8: SS7 - Harding University

SS #7
ENGR 475 – Telecommunications
Harding University
October 17, 2006
Jonathan White
Outline
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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 24th 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:
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Sender/Receiver phone numbers
What links are used.
What devices are used.
Other routing information
A pointer to the actual voice data.
SS7 Applications
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911 enhancements
Calling card fraud prevention
Credit card approval
Virtual networks.
Call tracing
Call blocking
• Much quicker call setup/teardown.