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In addition to the basic rate, the primary rate was defined for users with greater requirements for bandwidth, such an organization with a digital PBX or a local network.
F L B1 L D L F L B2 L D L B1 L D L B2 L D L
Figure 12.6 The frame structure from the N-ISDN user to the network.
F Time Time Time Time
slot 1 slot 2 slot 3 slot 24
(a) Frame structure for the 1.544 Mbps interfcae
Time Time Time Time
slot 0 slot 1 slot 2 slot 31
(a) Frame structure for the 2.048 Mbps interfcae
Figure 12.7 Primary rate frame structures.
In the US, Canada, and Japan, the primary rate is 1.544 Mbps (the same rate as in DS1). In Europe, the primary rate is 2.048 (the same as in E1). The frame structure for the 1.544-Mbps interface and the 2.048-Mbps interface are shown in Figure 12.7.
The frame structure for the 1.544-Mbps interface consists of twenty-four 8-bit time slots and a framing bit (bit F). The frame consists of 193 bits, and it repeats every 125 ^sec. Of the 24 time slots, 23 are used for 64-Kbps B-channels and one time slot is used for a 64-Kbps D-channel. This transmission structure is referred to as 23B + D.
The frame structure for the 2.048-Mbps interface consists of thirty-two 8-bit time slots. The frame consists of 256 bits, and it repeats every 125 ^sec. The first 8-bit time slot is used for framing and synchronization. Of the remaining thirty-one 8-bit time slots, thirty are used for 64-Kbps B-channels, and one 8-bit time slot is used for a 64-Kbps D-channel. This transmission structure is referred to as 30B + D.
It is possible for a customer with a lower bandwidth requirement to employ fewer B-channels than those provided by the primary rate interface.
The interface for the primary rate can also support channels with a transmission rate higher than the B-channel. These higher rate channels are: the 384-Kbps H0-channel, the 1536-Kbps H11-channel, and the 1920-Kbps H12-channel. The primary rate interface H0 channel structure can support the combinations 3H0 + D and 4H0 for the 1.544Mbps interface, and 5H0 + D for the 2.048-Mbps interface. The primary rate interface H1 channel structure can support the H11 channel structure consisting of one 1536-Kbps H11-channel, and the H12 channel structure consisting of one 1920-Kbps H12-channel and one D channel.
Primary rate interface structures have also been defined that permit 0 or 1 D-channel plus any combination of B-channels and H0-channels up to the capacity of the physical interface, such as 3H0 + 5B + D and 3H0 + 6B.
Frame relay is a networking architecture that was defined originally to interconnect N-ISDN users. As it turned out, it is possible to use frame relay as a stand-alone protocol for transporting data and more recently voice over a wide area network. Currently, frame relay is a popular networking solution, and is typically offered by public network operators.
VOICE OVER ATM AND MPLS
Broadband ISDN (B-ISDN)
N-ISDN was the first step towards realizing the ISDN vision of providing a wide range of services for voice and non-voice applications over the same network. The second generation of ISDN that provides for very high speeds was referred to as the broadband ISDN (B-ISDN). The asynchronous transfer mode (ATM) is the preferred network architecture for B-ISDN (see Chapters 3 - 5).
12.1.5 Digital Subscriber Signaling System No. 1 (DSS1)
The Digital Subscriber Signaling System No. 1 (DSS1) is used for signaling between a N-ISDN user and its local exchange. DSS1 is message oriented and many of DSS1 concepts are similar to SS7. DSS1 messages are transported over the D-channel. It is divided into the data link layer and the network layer. The data link layer, also known as the LAP-D link access protocol, is concerned with the reliable transfer of frames between the terminal equipment (TE) and its local exchange.
The network layer protocol is usually referred to by the ITU-T recommendation Q. 931, in which it was specified. The ATM signaling protocol Q. 2931 (described in Chapter 5) is based on Q.931, and is also referred to as the digital subscriber signaling system no. 2 (DSS2). Q. 931 includes the following messages: setup (SETUP), setup acknowledgment (SETACK), call proceeding (CALPRC), progress message (PROG), alerting message (ALERT), connect (CONN), connect acknowledgment (CONACK), disconnect (DISC), release (RLSE), release complete (RLCOM), and information (INFO). The signaling takes place between the TE of an N-ISDN user and the local exchange to which the user is attached. The signaling messages are carried over the D-channel.
The message format consists of a protocol discriminator, call reference value, length, message type, and information elements, such as bearer capability, called party number, calling party number, called subaddress, cause, channel ID, high-layer capability, keypad, low-layer capability, progress indicator signal, transit network selection, and user-to-user information.