B-ISDN User-Network Interface Broadband ISDN

166 Communication Networks Copyright © 2005 PragSoft ATM transport network is divided into two layers, both of which are hierarchically organized. Figure 12.139 Layered structure of B-ISDN. B-ISDN Higher Layer Functions ATM Transport Network ATM Layer Transport Functions Physical Layer Transport Functions Virtual Channel Level Virtual Path Level Transmission Path Level Digital Section Level Regenerator Section Level The ATM layer transport functions are divided into virtual channel level and virtual path level. A Virtual Channel VC denotes the transport of ATM cells which have the same unique identifier, called the Virtual Channel Identifier VCI. This identifier is encoded in the cell header. A virtual channel represents the basic means of communication between two end-points, and is analogous to an X.25 virtual circuit. A Virtual Path VP denotes the transport of ATM cells belonging to virtual channels which share a common identifier, called the Virtual Path Identifier VPI, which is also encoded in the cell header. A virtual path, in other words, is a grouping of virtual channels which connect the same end-points. This two layer approach results in improved network performance. Once a virtual path is setup, the additionremoval of virtual channels is straightforward. The physical layer transport functions are divided into three levels of functionality. The transmission path connects network elements that assemble and disassemble the transmission system payload. This payload may contain user or signalling information to which transmission overheads are added. The digital section connects network elements such as switches that assemble and disassemble continuous bitbyte streams. The regenarator section is simply a portion of a digital section which connects two adjacent repeaters along a transmission path which is otherwise too long to sustain the signal. www.pragsoft.com Chapter 13: Broadband ISDN and ATM 167 Figure 12.140 illustrates the relationship between virtual channel, virtual path, and transmission path. Figure 12.140 Virtual channel, virtual path, and transmission path. Transmission Path VP VP VP VP VP VP VC VC VC VC VC VC Two types of switching are possible: VP switching and VC switching. A VP switch connects a set of incoming VP terminations to a set of outgoing VP terminations. The mapped VPs will have different VPIs, but the VCI will remain unchanged. A VC switch, on the other hand, connects a set of incoming VC terminations to a set of outgoing VC termination. VCIs are not preserved. Because VCs are embedded in VPs, VC switching also involves VP switching.

12.2.2. ATM Cells

An ATM cell consists of 53 consecutive octets, of which 48 octents are used to represent user information and the remaining 5 octets form a header for the private use of the ATM protocols see Figure 12.141. Figure 12.141 ATM cell structure. Field Description GFC Generic Flow Control field for end-to-end flow control VPI Virtual Path Identifier for routing Header VCI Virtual Channel Identifier for routing 5 octets PT Payload Type for the information field CLP Cell Loss Priority for discarding of cells in congestions HEC Header Error Control code Body 48 octets Information User information or inband signaling information The GFC field is intended for the user-network interface only. It does not appear in the header format of cells used for the internal operation of the network. Its 4 bits are instead used by the VPI. It can be used by applications to provide end- to-end flow control. The VPI and VCI fields collectively support the routing of a cell by the network. The PT field denotes the type of information stored in the information field.