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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.
Timothy S. RamtekeNetworks, Second Edition
Copyright ©2001 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 18-4 A fully interconnected SS7 network between an LEC and an IXC.
Timothy S. RamtekeNetworks, Second Edition
Copyright ©2001 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
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)
Timothy S. RamtekeNetworks, Second Edition
Copyright ©2001 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
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
Timothy S. RamtekeNetworks, Second Edition
Copyright ©2001 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 18-7 The three kinds of signal units are shown. Fields which are unique to one kind of a signal unit are shaded.