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Common Channel Signaling System Number 7 (SS7) 2

7.1 Introduction

Inchannel signaling

Common channel signaling

Network control signaling transition from inchannel to a common channel approach

Signaling System Number 7 (SS7) was first issued by CCITT in 1980 and

revised in 1984, 1988, and 1992.

Designed to be an open-ended common-channel signaling standard for a

variety of digital circuit-switched networks.

Specifically designed to be used in ISDN: provides internal control and

network intelligence essential to an ISDN.

Primary characteristics of SS7 optimized for use in digital telecommunication networks in conjunction with

digital stored program-control exchanges, utilizing 64-kbps digital channels

designed to meet present and future information transfer requirements for call

control, remote control, management, and maintenance

designed to be a reliable means for the transfer of information in the correct

sequence without loss or duplication

suitable for operation over analog channels and at speeds below 64 kbps

suitable for use on point-to-point terrestrial and satellite links

The scope of SS7 is immense, because it must cover all aspects of controlsignaling for complex digital networks, including the reliable routing and

delivery of control messages and application-oriented content of those

messages.

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7.2 SS7 Architecture

Associated

Disassociated

Functional Architecture

Control messages in a common channel signaling system are

short packets routed through the network.

Although the network is a circuit-switched network, the control

signaling is implemented using packet switching technology.

SS7 functions could be implemented as additional functions in

circuit-switching nodes!associated signaling mode

Or, the network can have separate switching points for carrying

control packets only !disassociated signaling mode

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Signaling Network Elements

Three functional entities in SS7

signaling points (SP): any point in signaling network capable of handlingSS7 control messages, e.g. an endpoint (such as a circuit switching node)

signal transfer points (STP): a signaling point capable of routing

control messages, e.g. a pure routing node

signaling links: a data link that connects signaling points

Two planes of operation: control planeand information plane

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Signaling Network Structures

A complex network may have both SPs and STPs in a hierarchical

structure, SPs at lower level and STPs at one or more higher levels. Parameters related to network design and number of levels

STP capacities

number of signaling links that can be handled, signaling message transfer

time, and the message throughput capacity

Network performance

number of SPs and the signaling delays

Availability and reliability

ability of the network to provide service when STP failures

ITU-T suggestions for better reliability

In a signaling network with a single STP level

each SP that is not an STP at the same time connected to at least 2 STPs

the meshing of STPs is as complete as possible (full mesh)

In a signaling network with two STP levels

each SP that is not an STP at the same time is connected to at least two

STPs of the lower level

each STP in the lower level is connected to at least two STPs of theupper level

the STPs in the upper level are fully meshed

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Example of Links Used in an SS7 Network

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Protocol Architecture SS7 has a layered protocol architecture similar to that of OSI model.

Message transfer part (MTP):the lowest three levels providing a reliable butconnectionless (a datagram style) service for routing messages

signaling data link: a full-duplex physical link dedicated to SS7 (OSI L1)

includes control links between STPs, between an STP and an SP, between SPs

signaling link: a data link control protocol, corresponds to OSI layer 2

provides reliable sequenced delivery of data across signaling data link

signaling network: provides routing data across multiple STPs from

control source to control destination

Signaling connection control part (SCCP):added in 1984 version of SS7

SCCP + MTP = NSP (Network Service Part) contains different network-layer services to meet needs of NSP users

NSP is a message delivery system

ISDN user part (ISUP):controls signaling needed in an ISDN to deal with

ISDN subscriber calls and related functions

Transaction capabilities application part (TCAP):provides the mechanisms

for transaction-oriented (not connection-oriented) applications and functions

Operation, maintenance, and administration part (O&MAP):specifies

network management functions and message related to operation&maintenace

Application service elements (ASEs):add. modules to support new applicats

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7.3 Signaling Link Level

corresponds to the data link control layer of OSI model

to turn an unreliable physical link into a reliable data link

reliability implies that

transmitted blocks of data are delivered with no loss or duplication

same order delivery of data blocks as they were transmitted

receiver is capable of exercising flow control over the sender

use of well-known data link control protocol (LAPD and LAPB)

Signal Unit Formats 1. MSU 2. LSSU 3. FISU

FSN

FSN

FSN

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Message signal unit (MSU)

carries user data from level 4

Level status signal unit (LSSU)

carries control information needed at the signaling link level

Fill-in signal unit (FISU)

transmitted when no other signal units are available

Definition of different fields in signal units

flag:delimits the signal unit at both ends (01111111)

bit stuffing may be used as with LAPB and LAPD

Four fields for flow- (sliding-window) and error-control (go-back-N ARQ)

backward sequence number (BSN)

contains the number of last MSU successfully received at the other

side; for piggyback acknowledgement

backward indicator bit (BIB)

negative ack of BSN is indicated by inverting this bit

forward sequence number (FSN)

for numbering MSUs uniquely in modulo 128

forward indicator bit (FIB)

indications of MSU is new or retransmitted (e.g. after negative ack)

length indicator (LI)

specifies the length in octets of the following upper-level fields

cross-check on closing flag, also a signal unit type indicator

FISU has no user data field!LI = 0

LSSU has a single user data field of one octet!LI = 1 or 2

MSU has a data portion that is longer than two octets!LI = 3 to 63

service information octet (SIO)

indicates the nature of the MSU, consists of two subfields:

service indicator: specifies the type of message being carried

subservice field: indicates whether the message is national or internation.

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Signaling information field (SIF)

contains information for signaling network level and SS7 level 4

consists of two subfields:

routing label: a 32-bit (14 bits source and destination address each + 4

bits signaling link selection filed, used in traffic distribution)

data: user data from some SS7 application or network management data

check bits (CK)

contains an error-detecting code (CRC-16 from all except flags)

status field (SF)(only in LSSU)

used to indicate the senders view of the actual status of the link

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Operation

Basic functions of the signaling link protocol

1. flow control 2. error control 3. error monitoring

flow control

Employs a sliding-window technique

Each MSU is given a new FSN, in modulo 128

LSSUs and FISUs not numbered, but carry the last MSUs FSN

All three types of signal unit can have negative acknowledgements and

piggybacked acknowledgements.

In the case of LSSU flow control, if one side is unable to keep up with theflow of data from the other side, busy indication is performed by the

status field.

For long congestion, timer control is used. Rules are:

If a receiver becomes overloaded, it must send a busy signal to stop

transmission from the other side. The receiver withholds ack of the MSUs. If

the overload condition persists, the node must repeatedly send a busy indication

at intervals of T5 time units (80-120 ms). Other side suspends tx of MSUs.

When congestion abates at the receiver, the receiver signals the end of busy

condition by resuming positive ack of incoming MSUs. Even if repeated busy indications are received every T5 time units, a node will

report the network level that the link is out of service every T6 (3-6 sec).

Error-free signal unit exchange

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error control

two forms of error control:

Basic method:applies for signaling links where one-way PD is < 15 ms.

Preventive cyclic retransmission method:applies for signaling links where

one-way PD is "15 ms (include signaling links established via satellite).

Basic method is simply a go-back-N ARQ (figure below)

When the PD is long, the message unit is relatively short and the link is

idle for the most of the time!low efficiency

In this case, it is better not to wait and retransmit all unacknowledged

MSUs whenever a node has no MSUs to send. Only positive acks are sent

by the other side. Forced retransmission procedure:because of only positive acks, there

may be undetected error for a considerable period of time. When a

predetermined number of outstanding unacknowledged signal units exists,

the transmission of new units is interrupted and the retained signal units

are retransmitted cyclically until the number of unacknowledged signal

units is reduced.

Transmission of MSUs

with error correction

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error monitoring

two types of signaling link error-rate monitoring

signaling unit error-rate monitor

alignment error-rate monitor

signaling unit error-rate monitor

is employed while the signaling link is in service

a means to decide a link be taken out of service due to errors

leaky bucket algorithm

using a counter initially set to zero and manipulated based on:

T: threshold above which an error is signaled to level 3

1/D: the lowest error rate(ratio of signal unit errors to signal units) that

will eventually cause an error to be signaled to level 3

counter increments one for each signal unit received in error

counter decrements one for every sequence ofDreceived signal

units, whether in error or not

an unreliable link is that when the counter reaches threshold T

for 64-kbps links: T= 64 andD= 256 (1/D= 0.004)

detects a consistent error rate not occasional surge of errors

alignment error-rate monitor

is employed while the signaling link is being initialized and aligned

alignment:transmitter and receiver are aligned with respect to the

opening flag field of each transmitted frame

provides the criteria for rejecting a signaling link for service due an

excessive error rate

using a counter initially set to zero and

increments by one for each signal unit received in error

If the counter exceeds a threshold before the end of an initial

proving period, the proving period is aborted.

Five successive failures result in the link being declared unreliable.

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