SS7 TDC 364. Introduction SS7 is a continuation of CCIS SS7 actually called CCS7 in North America...

SS7

TDC 364

Introduction

• SS7 is a continuation of CCIS

• SS7 actually called CCS7 in North America

• ITU-T’s version is SS7

• Japan has a slightly different version

Advantages and Disadvantages

• + Flexible – a packet switched network that sends signals – any signals – old and new

• + One language (SS7) for local calls, long distance calls, overseas, 1-800, …

Types of Nodes

• STP (Signal Transfer Point) – Packet switches that route signal messages to their proper point

• STPs always occur in pairs• SSP (Service Switching Point) – The

interface into the SS7 network• SSPs convert ISDN or DTMF signals to

SS7 signals for the STPs

Types of Nodes

• SCP (Service Control Point) – Database of information for credit card authorizations, billing information, 1-800 conversions, etc.

Types of Links

• Cross links – interconnect the STPs in a pair

• Bridge links – connect pairs of STPs (into quads)

• Access links – connect SCPs and SSPs to STPs

• (See Figures 18.3, 18.4, 18.5)

Timothy S. RamtekeNetworks, Second Edition

Copyright ©2001 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.

FIGURE 18-3 An STP quad configuration, showing the three types of SPs and the signaling links that connect them.




FIGURE 18-4 A fully interconnected SS7 network between an LEC and an IXC.




FIGURE 18-5 AIN (Advanced Intelligent Network) architecture.

SS7 Architecture

• Recall: SS7 is a packet switched network

• A virtual connection, called an SCCP (Signaling Connection Control Part) is established between the two endpoints of the connection (the SSPs) before any signals are sent

• The STPs are the switching nodes

The Four Layers of SS7

• Layer 1 (the lowest) – Message Transfer Part – Level 1 (MTP-L1) – the physical layer

• Layer 2 – Signaling link layer, or MTP-L2 – provides error detection and correction on signaling links

The Four Layers of SS7

• Layer 3 – Signaling network layer (MTP-L3) – routes messages

• Layer 4 – fairly complex layer with many possible protocols and services, such as flow control, sequence control, end-to-end call control, converting 1-800 numbers to POTS numbers, and more

• (See Figure 18.6)




FIGURE 18-6 The layered architecture of SS7 and the units of transfer.

Signaling Units

• Frames in SS7 are called Signaling Units (HDLC-like)

• Three forms– MSU (Message SU)– LSSU (Link Status SU)– FISU (Fill-In SU) when a link is idle but you

want to convey there is still a connection




FIGURE 18-7 The three kinds of signal units are shown. Fields which are unique to one kind of a signal unit are shaded.

SS7 TDC 364. Introduction SS7 is a continuation of CCIS SS7 actually called CCS7 in North America...

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