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Connection Oriented Networks - Perros H.G

Perros H.G Connection Oriented Networks - John Wiley & Sons, 2005. - 359 p.
ISBN 0-470-02163-2
Download (direct link): connectionorientednetworks2005.pdf
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5.6 THE FORMAT OF THE SIGNALING MESSAGE
The format of the signaling message is shown in Figure 5.11. This message format is used by the signaling protocols Q.2931, Q.2971, and PNNI. The protocol discriminator field is used to identify the signaling protocol. Bytes 3 to 5 give the call reference number to which the signaling message pertains. This is simply a number assigned to each call (i.e., connection) by the side that originates the call. It is a unique number that has local significance, and it remains fixed for the lifetime of the call. After the call ends, the call reference value is released and it can be used for another call. The call reference value is used by the signaling protocol to associate messages to a specific call, and it has nothing to do with the VPI/VCI values that will be assigned to the resulting ATM connection. The length of the call reference value is indicated in byte 2. For instance, 0011 indicates a 3-byte length. Since the call reference value is selected by the side that originates the call, two calls originating at the opposite sides of the interface might have the same call reference value. The call reference flag, in byte 3, is used to address this problem. Specifically, the side that originates the call sets the flag to 0 in its message, whereas the destination sets the flag to 1 when it replies to a message sent by the originating side.
byte 1 2
3
4
5
6
7
8 9
>10
Protocol discriminator
0 0 0 0 length of call ref. value
Flag
Call reference value
Call reference value
Call reference value
Message type
Message type
Message length
Message length
variable length information elements
1
2
3
4
5
6
7
8
Figure 5.11 The signaling message format.
THE SIGNALING PROTOCOL Q.2931
127
Figure 5.12 The structure of an information element.
The message type field of the signaling message, bytes 6 and 7, is used to identify the type of the signaling message.
The message length field, bytes 8 and 9, is used to indicate the length of the signaling message, excluding the first nine bytes. Typically, there is a variety of information that has to be provided with each signaling message. This information is organized into different groups, known as information elements (IE). Each signaling message contains a variable number of information elements, which are appended to the signaling message starting at byte 10. The total length of all of the information elements appended to a signaling message is given in the message length field. The structure of an information element is shown in Figure 5.12. The first byte contains the IE identifier, which is used to uniquely identify the information element. The second byte contains various fields, such as the coding standard, i.e., ITU-T, ISO/IEC, national, network specific standard (private or public), and the IE action indicator. Bytes 3 and 4 give the length of the information element, excluding the first four bytes, and the remaining bytes starting at byte 5 contain the information specific to the IE.
5.7 THE SIGNALING PROTOCOL Q.2931
This protocol establishes a point-to-point SVC over the private UNI in real-time. In this section, we first examine the information elements used in the Q.2931 messages, and then we describe the Q.2931 messages and we show how they are used to establish and terminate a call. The terms calling user or party and, conversely, called user or party are used interchangeably. The calling user is a user in the end device that initiates a call, and the called user is the user at the end device that is being called.
5.7.1 Information Elements (IE)
Each signaling message contains a variety of information organized into different groups, known as information elements (IE). The following are some of the information elements used in Q.2931 messages:
AAL parameter IE: Indicates the AAL parameter values used between the end devices.
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