TEL 355: Communication and

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30 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

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TEL 355: Communication and
Information Systems in


Signaling System 7 (SS7)

Professor John F. Clark


Exchange of info between call components
required to provide and maintain service

Users signal network elements and network
elements signal each other

SS7 is the way network elements talk

SS7 is characterized by high
speed packet
data and out
band signaling

Band Signaling

Does not take place over the same path as
the conversation

Establishes a separate


signaling link

Takes place at speeds of 56 or 64 kbps

Somewhat like ISDN except ISDN extends
band signaling all the way to the user
via a D channel and two B channels

Why Out
Band Signaling?

Several advantages over in
band signaling

Transports more data at higher speeds

and 64 kbps are faster than MF outpulsing

Allows signaling at any time during the
duration of the call, not just the beginning

Enables signaling to network elements to
which there is no direct trunk connection

Signaling Network Architecture

Associated Switching

One of the many paths between an inter
connected pair of switches is the signaling path

The rest of the paths are voice trunks

The signaling path handles all the signaling for
all the voice trunks

Works well only between switches directly
connected by trunks

Much more complicated when signaling
between switches without a direct connection

N. Amer. Signaling Architecture

Three essential components, connected by
signaling links:

Signal switching points (SSPs)

switches with SS7 software that originate,
terminate, or switch calls

Signal transfer points (STPs)

packet switches
that receive and route incoming signaling

Signal control points (SCPs)

databases that
provide information for advanced call processing

N. Amer. Signaling Architecture II

Each element is held to exacting standards

Precisely defined protocols manage the
routing of signaling through the network

And an SS7 network is highly redundant to
assure reliability

STPs and SCPs are deployed in redundant pairs

They may not be co
located, but they perform
identical functions

N. Amer. Signaling Architecture III

Associated Switching

Mated pairs of STPs perform identical functions

Each SSP has a link to each STP of a mated pair

The STPs of a mated pair are joined by a link

The two mated pairs of associated networks are
joined by four links

a quad

SCPs are usually (not always) deployed in pairs

SCPs of a mated pair are not joined by links

SS7 Link Types

All SS7 signaling links are identical in that

are bidirectional data links

are 56 kbps or 64 kbps

support the lower three layers of the protocol

However, they are used for different

SS7 Link Types II

“A” links connect an STP to either an SSP or
an SCP

“A” for Access

“C” links connect mated STPs

“C” for Cross

“B,” “D,” and “B/D” links connect two mated
pairs of STPs

“B” for Bridge and “D” for

Optional “E” links connect an SSP to an
additional non
home pair of STPs for added

“E” for Extended

“F” links bypass STPs (and SCPs) and directly
connect two SSPs

“F” for Fully Associated

Layers of the SS7 Protocol

Physical layer

physical and electrical
properties of the medium

always DS

thus 56 or 64 kbps

Message Transfer Part (MTP level 2)

layer functionality

error checking, flow
control, and sequence checking

Message Transfer Part (MTP level 3)

network layer functionality

addressing, routing, and congestion control

Layers of the SS7 Protocol II

Signaling Connection Control Part (SCCP)

Addresses applications within a node such as
800 call processing, calling
card processing, and
advanced intelligent network (AIN)

Uses global title translation (GTT) to perform
incremental routing

minimizes the need for
STPs to maintain information about nodes that
are far removed from them

also allows STP
load sharing among mated SCPs in both normal
and failure modes

Layers of the SS7 Protocol III

ISDN User Part (ISUP)

defines the
messages and protocol used over the PSN

always MTP in North America

Transaction Capabilities Application Part

defines the messages and protocol
used to communicate between applications in
a node

delivered by SCCP

Operations, Maintenance, and Administration
Part (OMAP)

assists system administrators

Signaling Units

SUs are 8
bit bytes (octets) and are
transmitted continuously in both directions
on any link that is in service

There are three types of SU:

Message signal units (MSUs)

contains all the
actual signaling information

Link status signal units (LSSUs)

information about the signaling link

in signal units (FISUs)

occupy the link
when no MSUs or FISUs are being transmitted

Signal Unit Structure

All three types of SUs have 4 common fields:


marks the beginning of an SU and thus
delimits the SUs in a transmission


a calculation for error checking

Length indicator

indicates the number of octets
between itself and the checksum

between types of SUs


confirm receipt of SUs,
ensure they are in the right sequence, and provide
flow control

Signal Unit Structure II

An LSSU has an additional “Status Field”

Comprised of one or two octets

Signals the initiation of link alignment

Signals the quality of received signaling traffic

Signals the status of the processors at either end

Requires no addressing information since they
are always sent between the signaling points at
the end of the link

Signal Unit Structure III

An MSU has a “Service Information Octet”

Four bits indicate the type of information in the
signal information field

Two bits indicate national or international

Two bits identify a message priority

An MSU has a “Signaling Information Field”

8 to 272 octets in length

Contains all the actual signaling information,
beginning with the routing label