Dragos Vju

1
Signaling System 7 (SS7) - Course 3 -

• Dragos Vâju
• Lothar Krupka

2
Overview
How can the network figure out where to send the
call, get the connection, and ring the phone on
the other end so quickly?

• Answer Signaling Systems
• Everything is happening very fast less than 1
  second.
• What happens behind the scenes constitutes the
  backbone of the signaling systems. The networks
  are now dependent on the capability of handling
  subsecond call set-ups and teardowns.

3
Overview

Signaling systems

• SS7 in North America

• CCITT Common Channel Interoffice Signaling
  System 7 (CCS7 for short)

- the names are different, but the functions and
the purposes of the two systems are the same!
4
Systems Before SS7

• Prior to the implementation of SS7, per-trunk
  signaling (PTS) was used exclusively in the
  networks.
• sets up calls between the telephone companies'
  exchanges.
• sends tones or multiple frequencies (MF), as they
  are called, to identify the digits of the called
  party.
• The trunk also provides all of the intelligence
  for monitoring and supervision (call seizure,
  hang up and answer back) of the call.

5
Systems Before SS7

• used in telephone systems at the customer's
  location (PBXs), which are not Integrated
  Services Digital Networks (ISDN) Primary Rate
  Interface (PRI)-compatible
• Drawbacks of PTS
• On a long distance call when a call set-up is
  necessary, each leg of the call repeats the MF
  call set-up procedure until the last exchange in
  the loop is reached. In essence, the call is
  being built by the signaling as the progress is
  occurring on a link-by-link basis.

6
Systems Before SS7

7
Systems Before SS7

• is an inefficient use of the circuitry in both
  major and minor networks
• several complications could arise, causing
  extensive delay or incomplete calls.
• the call establishment part of the connection
  could take as much as 24 seconds, then time out,
  and never get to its end point.

8
Introduction to SS7

• mid-1960s - Signaling System 6 (SS6) developed by
  International Telecommunications Union -
  Telecommunications Standards
• The success of the signaling standards lies in
• the message structure of the protocol (like X.25)
• the network topologies
• The messages travel from one network element to
  another, independent of the actual voice and data
  in an envelope called a packet.
• Data Link Rates
• 1st development used in USA - 2400 bps
• 2nd development upgrade to the 1st one - 4800
  bps
• Messages were sent in the form of data packets
  and could be used to request connections on voice
  trunks between Central Offices, placing 12 signal
  units' (of 28 bits each) assembled packets into a
  data block. This is similar to the methods used
  today in SS7 architectures.

9
Introduction to SS7

• Although SS6 used a fixed-length signal unit, SS7
  uses signal units with varying lengths.
• The later version of SS7 uses a 56 Kbps data
  link.
• North America - 56 Kbps
• rest of the world - 64 Kbps.
• The difference in the speeds between 56 and 64
  Kbps results in the fact that the local exchange
  carriers (LECs) have not yet fully deployed the
  use of B8ZS on the digital circuits.
  Consequently, the 56 Kbps is an anomaly in the
  SS7 networks.
• SS6 networks are slow, whereas SS7 is much
  faster
• 64 Kbps - full DS-1 (1.544 Mbps) (E1 in Europe
  2,048 Mbps)

10
Purpose of SS7

• access remote databases to look up and translate
  information from between number calls

• carry the various stored program controlled
  systems throughout the network

• enable the introduction of additional features
  and capabilities into the network

• enables the full use of the channel for the talk
  path, because the signaling is done out-of-band
  on its own separate channel

11
Out of Band Signaling Analog vs Digital

• Analog the signals that set up a call between
  one switch and another always took place over the
  same trunk that would eventually carry the call

• Digital separate digital channel for the
  exchange of signaling information signaling
  link

Signaling links are used to carry all the
necessary signaling messages between nodes. Thus,
when a call is placed, the dialed digits, trunk
selected, and other pertinent information are
sent between switches using signaling links,
rather than the trunks that will ultimately carry
the conversation. In ISDN, signaling that must
be conveyed between the user station and the
local switch is carried on a separate digital
channel called the D channel.
12
Out of Band Signaling Advantages
1. It allows for the transport of more data at
higher speeds (56 Kbps can carry data much faster
than MF out-pulsing).
2. It allows for signaling at any time in the
entire duration of the call, not only at the
beginning.
3. It enables signaling to network elements
without a direct trunk connection.
13
SS7 Network Architecture
The simplest design would be to allocate one
of the paths between each interconnected pair of
switches as the signaling link. Subject to
capacity constraints, all signaling traffic
between the two switches could traverse this
link. This type of signaling is known as
associated signaling.
The new architecture includes the connection
from the Signal Switching Point (SSP) to a device
called the Signal Transfer Point (STP) . It
is then the responsibility of the STP to provide
the necessary signaling information through the
network to affect the call set-up.
When necessary, the STP sends information to the
Signal Control Point (SCP) for translation
or database information on the routing of the
call.
14
SS7 Network Architecture

• 1. Paired STPs perform identical functions and
  are redundant. Together they are referred to as a
  mated pair of STPs.
• 2. Each SSP has two links (or sets of links), one
  to each STP of a mated pair. All SS7 signaling to
  the rest of the world is sent out over these
  links. Because the STPs are redundant, messages
  sent over either link (to either STP) will be
  treated equivalently.
• 3. A link (or set of links) joins the STPs of a
  mated pair.

4. Four links (or sets of links) interconnect two
mated pairs of STPs. These links are referred to
as a quad. 5. SCPs are usually (though not
always) deployed in pairs. As with STPs, the SCPs
of a pair are intended to function identically.
Pairs of SCPs are also referred to as mated
pairs of SCPs. Note that a pair of links does not
directly join them. 6. Signaling architectures
such as these that provide indirect signaling
paths between network elements are referred to
as providing quasi-associated signaling.
15
SS7 Interconnection

• actual linkage enables the local exchange offices
  to send the necessary information out of band
  across the signaling links

• uses messages in the form of packets to signal
  across the network through the STPs.
• (This enables the full use of the talk path for
  information exchange and the messaging paths for
  informational dialogue between the switching
  systems and the transfer points.)

• the links are used to pass control and billing
  information, network management information, and
  other control functions as necessary without
  interfering with the conversational path

16
Basic Functions of the SS7 Network
1. Exchange of circuit-related information
between the switching points along the network
2. Exchange of non-circuit-related information
between the databases and the control points
within the network
3. It handles the rerouting of network traffic in
the event of circuit failures by using automatic
protection-switching services, such as those
found in SONET (Synchronous Optical Network) or
Alternate Routing information
4. SS7 network prevents misrouted calls, the
duplication of call requests, and lost packets
(requests for service), because it is a
packet-switching concept
5. It enables the full use of out-of-band
signaling using the ITU Q.931 signaling
arrangements for call set-up and teardown
6. It allows for growth / expansion / improvement
( new features and functions can be introduced to
the network without major disruptions )
17
Signaling Links
What is signaling ? Signaling refers to the
exchange of information between call
components required to provide and maintain
service.

• virtually all links are identical

• 56 Kbps or 64 Kbps bidirectional data links (
  data flows in both directions )

56 Kbps DS0A in North America 64 Kbps DS0C
in every other portion of the world 4.8 Kbps
in Japan

• 3 types of signaling links
• 1. associated
• 2. non-associated
• 3. quasi-associated

18
Signaling Links Associated Signaling

• simplest form of signaling

• the link is directly parallel with the voice
  path for which it is providing the signaling
  information.

• The most associated signaling is deployed at
  the end user location

19
Signaling Links Non-Associated Signaling

• STP signal transfer points
• STPs are the packet switches of the SS7 network.
  Perform specialized routing
• functions.
• SSP signal switching points
• SSPs are telephone switches they originate or
  terminate switch calls

• There is a separate logical path from the actual
  voice path

• Is common in many SS7 networks

• The more nodes used, the more processing and
  delay can occur.

• Non-associated signaling involves the use of
  STPs to reach the remote point.

• To establish a trunk connection between the two
  points, a signaling message will be sent via SS7
  and STPs to the adjacent point.

20
Signaling Links Quasi-Associated Signaling

• STP signal transfer points
• STPs are the packet switches of the SS7 network.
  Perform specialized routing
• functions.
• SSP signal switching points
• SSPs are telephone switches they originate or
  terminate switch calls

• It is the preferred method of setting up and
  using an SS7 backbone because each node
  introduces additional delay in signaling delivery.

• The delay can be minimized by eliminating some
  of the processors on the set-up

• Both nodes are connected to the same STP.

The signaling path is still through the STP to
the adjacent SSP.
21
Link Architecture

• Signaling links are logically organized by link
  type (A through F), according to their use in the
  SS7 signaling network.

• A An access link connects a signaling end point
  (SCP or SSP) to a STP. Only messages originating
  from or destined to reach the signaling end point
  are transmitted on an A link.
• A access
• Examples of A
• links are 2-8, 3-7, and 5-12

• B A bridge link connects one STP to another STP.
  Typically, a quad of B links interconnects peer
  (or primary) STPs (the STPs from one network to
  the STPs of another network). The distinction
  between a B link and a D link is rather
  arbitrary. For this reason, such links may be
  referred to as B/D links.
• B brigde
• 7-11 and 7-12 are
• examples of B links

22
Link Architecture

• C A cross link connects STPs performing identical
  functions into a mated pair. A C link is used
  only when an STP has no other route available to
  a destination signaling point due to link
  failure(s). Note that SCPs can also be deployed
  in pairs to improve reliability. Unlike STPs,
  however, signaling links do not interconnect
  mated SCPs.
• C cross
• 7-8, 9-10, and 11-12
• are C links

• D A diagonal link connects a secondary (local or
  regional) STP pair to a primary (internet work
  gateway) STP pair in a quad-link configuration.
  Secondary STPs within the same network are
  connected via a quad of D links. The distinction
  between a B link and a D link is rather
  arbitrary. For this reason, such links may be
  referred to as B/D links.
• D diagonal
• 8-9 and 7-10 are
• examples of D links

23
Link Architecture

• E An extended link connects an SSP to an
  alternate STP. E links provide an alternate
  signaling path if a SSP's "home" STP cannot be
  reached via an A link. E links are not usually
  provisioned unless the benefit of a marginally
  higher degree of reliability justifies the added
  expense.
• E extended
• 1-11 and 1-12 are E links

• F A fully associated link connects two signaling
  end points (SSPs and SCPs). F links are not
  usually used in networks with STPs. In networks
  without STPs, F links directly connect signaling
  points.
• F fully associated
• 1-2 is an F link

24
Links and Linksets

• linkset is a grouping of links joining the same
  two nodes

• linkset 1 to 16 or 32 links

• normally, SSPs have one or two links connecting
  to their STPs based on normal capacity and
  traffic requirements.

• SCPs have many more links in their linksets to
  handle the large amount of messaging

• STP signal transfer points
• STPs are the packet switches of the SS7 network.
  Perform specialized routingfunctions.
• SSP signal switching points
• SSPs are telephone switches they originate or
  terminate switch calls
• SCP signal control points
• SCPs are databases that provide the information
  necessary for advanced call-processing
  capabilities.

25
Combined Linksets

• routing from a SSP or SCP toward the related
  STP where two linksets share the traffic outward
  to the STP and beyond

• the normal practice is to have equally sized
  groupings of linksets connecting the same end
  node

The load balancing can help you do the
following 1. Be aware when a link fails. 2.
Recognize when congestion is occurring in the
network. 3. Use the links when traffic is not
critical to know when a link is down before it
becomes critical

• STP signal transfer points
• SSP signal switching points
• SCP signal control points

26
Routes and Routesets

• routeset refers to the routing capability of
  addressing a node within the SS7 network.
• point code - unique address of every node within
  the network

• Divided in 3 parts (abc def ghi)
• The first three digits refer to the network
  identifier (abc).
• The next three digits refer to the cluster number
  (def).
• The final three digits refer to the member number
  (ghi).

• In any given network, there can be 256 clusters
  with 256 members.

• The routing of SS7 messages to a destination
  point code can take different paths or routes.
• From the SSP perspective, there are only two
  ways out from the node, one toward each of its
  mated STPs. From that point on, the STPs decide
  which routes are appropriate, based on the time,
  resources, and status of the network.

27
SS7 Protocol Stack
The SS7 uses a four-layer protocol stack that
equates to the seven-layered OSI model
These protocols provide different services,
depending on the use of the signaling network.
The upper portion of the stack performs the data
process function.
The bottom three layers are considered the
communications transmission of the messages
28
SS7 Protocol Stack

• SS7 protocol is layered

• the SS7 protocol stack is specifically designed
  for the reliable data transfer between different
  signaling elements on the network

29
Basic Call Set Up with ISUP

• When a call is placed to an out-of-switch number,
  the originating SSP transmits an ISUP initial
  address message (IAM) to reserve an idle trunk
  circuit from the originating switch to the
  destination switch (1a).

• The IAM includes the originating point code,
  destination point code, circuit identification
  code dialed digits, and, optionally, the calling
  party numbers and name. In the following example,
  the IAM is routed via the home STP of the
  originating switch to the destination switch
  (1b). Note that the same signaling links are used
  for the duration of the call unless a link
  failure condition forces a switch to use an
  alternate signaling link.

• The destination switch examines the dialed
  number, determines that it serves the called
  party, and that the line is available for
  ringing. The destination switch transmits an ISUP
  address complete message (ACM) to the originating
  switch (2a) (via its home STP) to indicate that
  the remote end of the trunk circuit has been
  reserved.

30
Basic Call Set Up with ISUP

• The destination switch rings the called party
  line and sends a ringing tone over the trunk to
  the originating switch. The STP routes the ACM to
  the originating switch (2b), which connects the
  calling party's line to the trunk to complete the
  voice circuit from the calling party to the
  called party. The calling party hears the ringing
  tone on the voice trunk.

• When the called party picks up the phone, the
  destination switch terminates the ringing tone
  and transmits an ISUP answer message (ANM) to the
  originating switch via its home STP (3a).

• The STP routes the ANM to the originating switch
  (3b), which verifies that the calling party's
  line is connected to the reserved trunk and, if
  so, initiates billing.

• The STP routes the REL to the destination switch
  (4b). If the called party hangs up first or if
  the line is busy, the destination switch sends an
  REL to the originating switch indicating the
  release cause (such as a normal release or a busy
  signal).

• If the calling party hangs up first, the
  originating switch sends an ISUP release message
  (REL) to release the trunk circuit between the
  switches (4a).

31
Basic Call Set Up with ISUP

• Upon receiving the REL, the destination switch
  disconnects the trunk from the called party's
  line, sets the trunk state to idle, and transmits
  an ISUP release complete (RLC) message to the
  originating switch (5a) to acknowledge the
  release of the remote end of the trunk circuit.

• When the originating switch receives (or
  generates) the RLC (5b), it terminates the
  billing cycle and sets the trunk state to idle in
  preparation for the next call. ISUP messages can
  also be transmitted during the connection phase
  of the call, such as between the ISUP ANM and REL
  messages.

32
SS7 Applications

• Use of AIN features, ISDN features, and wireless
  capabilities 112 enhancements
• Class features
• Calling card toll fraud prevention
• Credit card approvals and authentication

33
SS7
Thank you for participating