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Figure 10.10 CCS modes of operation.
Exchange Trunks
Common Channel
Associated Mode
Nonassociated Mode
Trunks
STP Common Channel
Exchange SP
The limited line speed of CCIS and SS6 4.8 kbps and 2.4 kbps, respectively and the limited size of the SUs has lead to the development of a replacement for
SS6. This is discussed in the next section.
10.3. Signaling System Number 7
CCITT’s Signaling System Number 7 SS7 was designed with the requirements of digital telephone networks in mind. As a common channel signaling standard, it is
suitable for use with a wide range of circuit-switched digital networks. Like its predecessor, SS6, it uses packet switching, but is primarily designed to work with
64 kbps digital channels of modern exchanges. However, it can also operate over analog channels of lower speeds, as well as digital channels of higher speeds.
SS7 is a very large and complex system. It is the signaling system of choice for ISDN discussed in the next chapter. The CCITT Q.700 series of
recommendations which is comprised of numerous parts is entirely devoted to the definition of SS7.
Since SS7 is a data network similar to X.25, but designed for the specific application of signaling, it is useful to compare its protocol architecture to the OSI
reference model. This is illustrated in Figure 10.Error Bookmark not defined.. Each level is separately discussed below.
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Figure 10.11 SS7 protocol architecture. OSI Layer
SS7 Level Purpose
higher layers Operations and
Maintenance Applications Part
Provides network management functions for the operation, administration, and maintenance of the
network. 4
User Parts Deals with signaling data contents.
Network SCCP
Provide additional network services. 3
Signaling Network Functions
Routing of signaling data through a set of signal transfer points.
Data Link 2
Signaling Link Control
Reliable, ordered delivery of signal units over a signaling data link.
Physical 1
Signaling Data Link
Transmission of signaling data bits over the physical channel.
10.3.1. Signaling Data Link
This layer maps to the OSI physical layer and is responsible for the transmission of signaling data bits over a full-duplex physical channel. Although SS7 has been
designed to work with 64 kbps digital channels, it can also operate with higher and lower speed channels as well as analog channels via modems.
10.3.2. Signaling Link Control
This layer maps to the OSI data link layer and is responsible for providing a reliable data link for exchanging signal units. It ensures that transmitted signal units are
delivered in order, without loss or duplication. It also provides flow control capabilities. In its operation, this layer is very similar to HDLC see Chapter 3. The
main difference is in message formats.
Figure 10.Error Bookmark not defined. illustrates the general structure of a
signal unit equivalent to a frame in HDLC. The signal unit is delimited in exactly the same fashion as a HDLC frame. The BSN and FSN correspond, respectively, to
HDLC receive and send sequence numbers. These are used to implement an error control Go-Back-N mechanism for dealing with transmission errors, and a flow
control sliding window mechanism for dealing with congestion situations. The Data Length field specifies the length of the following Data field which contains
information used by levels 3 and 4 in octets. The Checksum field is a 16-bit CRC over the whole unit except for the flags and the CRC field itself.
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Figure 10.12 Signal unit structure. Field
Description
01111110 Start flag: marks the beginning of the signal unit.
BSN Flag Backward Sequence Number and associated flag.
FSN Flag Forward Sequence Number and associated flag.
Data Length Length of the following data field in octets.
Data Contains the data for levels 3 and 4.
Checksum An error checksum field for error detection.
01111110 End flag: marks the end of the signal unit.
There are three possible structures for the Data field, leading to three different signal unit types:
•
Message Signal Unit
MSU. This is used for carrying signaling information from higher levels. The data field consists of a Service Information Octet SIO
and a Signaling Information Field SIF. The SIO denotes the role of the MSU i.e., what type of message it is and what type of network it relates to. The SIF
consists of source and destination message addresses, a Signaling Link Selection SLS field, and user data from a higher level entity.
•
Link Status Signal Unit LSSU. This is used for carrying signaling link control
information. The data field consists of a Status Field SF which is used to communicate the link status between signaling points, and may be used by
network management entities. One major use of LSSUs is for flow control.
•
Fill-in Signal Unit
FISU. This is used for continued transmission in absence of other signals. The data field is empty.
10.3.3. Signaling Network Functions
Figure 10.Error Bookmark not defined. illustrates the SS7 signaling network functions, which consist of two groups: signaling message handling functions and
signaling network management function.
The signaling message handling functions are responsible for communicating
signaling messages between two signaling points. These consist of three main functions:
•
The message discrimination function looks at the SIF field of the MSU to decide if the message has reached its final destination i.e., its destination address
matches this signaling point or it should be routed further. If the latter is the case, the MSU is handed over to the routing functions. Otherwise, it is handed over to
the distribution function for local consumption.
