Figure 72. 1 also shows the individual subfields of the cell header. The first field, called generic flow control (GFC), is only available at the user-network interface(UNI). Its main purpose is media access control in shared medium configurations(LAN-like configurations) within the customer premises [Goldner and Huber, 1991] The proposed GFC procedures are based either on the distributed queue algorithm or the reset timer control mechanism [Handel and Huber, 1991a]. At the network-node interface(NNI) these bits are part of the virtual The VPI together with the virtual channel identifier(vCI)form the routing label (identifier of the connec- on). The VPI itself marks only the virtual path(VP). The VP concept allows the flexible configuration of individual subnetworks(e.g, signaling network or virtual private network), which can be independent of the underlying transmission network. VP networks are under the control of network management. The bandwidth of a Vp will be allocated according to its requirements within the Vp network the individual connections are established and cleared down dynamically(by signaling The payload type field in the cell header differentiates the information in the cell payload of one connection (e.g, user information, operation and maintenance information for ATM). The value of the cell loss priority bit distinguishes cells that can be discarded under some exceptional network conditions without disturbing the quality significantly from those cells that may not be discarded. The last field of the cell header forms the header error control field. The cell header is protected against errors with a mechanism that allows the correction of a single bit error and the detection of multibit errors. The high transmission speeds for ATM cell transfer require very high-performance switching nodes. There fore, the switching networks(SNs) have to be implemented in fast hardware. Within the SN the self-routing principle will be applied [Schaffer, 1990]. At the inlet of the sn the cell is extended by an SN-internal header It is evident that the SN-internal operational speed has to be increased. When passing the individual switching elements, for the processing of the SN-internal header only simple hard-wired logic is necessary. This reduces the control complexity and provides a better failure behavior. When starting several years ago with the imple mentation of the ATM technology, only the emitter coupled logic(ECL) was available. Nowadays, the comple mentary metal-oxide semiconductor( CMOS) technology with its low power consumption is used [Fischer etal,1991] Transmission of b-isdn Signals Transmission systems at the UNI provide bit rates of around 150 and 622 Mbit/s. In addition to these rates, at the NNI around 2.5 Gbit/s and up to 10 Gbit/s will be used in the future [Baur, 1991]. In addition to the high capacity switching and multiplexing technology, high-speed transmission systems are required. Optical fibers are especially suitable for this purpose; however, for the lower bit rates coaxial cables can be used. Optical transmission uses optical fibers as the transmission medium in low-diameter and low-weight cables to provide large transmission capacities over long distances without the need for repeaters. Optical transmission equipment currently tends to mono-mode fiber and laser diodes with wavelengths of around 1310 nm. For both directions in a transmission system either two separate fibers or one common fiber with wavelength division multiplexing can be used. The second solution may be a good alternative for subscriber lines and short trunk lines[ Bauch, 1991] For ATm cell transmission, two possibilities exist, which are shown in Fig. 72. 2: synchronous pulse frame or continuous cell stream(cell-based). The basis for the pulse frame concept is the existing synchronous digital hierarchy(SDH). In SDH the cells are transported within the SDh payload; the frame overhead includes operation and maintenance(OAM)of the transmission system. In the cell-based system the oAm for the transmission system is transported within cells. The SDH solution is already defined, whereas for cell-based transmission some problems remain to be solved (e. g, OAM is not yet fully defined). ATM Adaptation Layer The ATM adaptation layer(AAL) is between the ATM layer and higher layers. Its basic function is the enhanced adaptation of the services provided by atM to the requirements of the layers above. In order to minimize the number of AaL protocols, the service classification shown in Fig. 72.3 was defined. This classification was made with respect to timing relation, bit rate, and connection mode. e 2000 by CRC Press LLC© 2000 by CRC Press LLC Figure 72.1 also shows the individual subfields of the cell header. The first field, called generic flow control (GFC), is only available at the user-network interface (UNI). Its main purpose is media access control in shared medium configurations (LAN-like configurations) within the customer premises [Göldner and Huber, 1991]. The proposed GFC procedures are based either on the distributed queue algorithm or the reset timer control mechanism [Händel and Huber, 1991a]. At the network-node interface (NNI) these bits are part of the virtual path identifier (VPI). The VPI together with the virtual channel identifier (VCI) form the routing label (identifier of the connec￾tion). The VPI itself marks only the virtual path (VP). The VP concept allows the flexible configuration of individual subnetworks (e.g., signaling network or virtual private network), which can be independent of the underlying transmission network. VP networks are under the control of network management. The bandwidth of a VP will be allocated according to its requirements. Within the VP network the individual connections are established and cleared down dynamically (by signaling). The payload type field in the cell header differentiates the information in the cell payload of one connection (e.g., user information, operation and maintenance information for ATM). The value of the cell loss priority bit distinguishes cells that can be discarded under some exceptional network conditions without disturbing the quality significantly from those cells that may not be discarded. The last field of the cell header forms the header error control field. The cell header is protected against errors with a mechanism that allows the correction of a single bit error and the detection of multibit errors. The high transmission speeds for ATM cell transfer require very high-performance switching nodes. There￾fore, the switching networks (SNs) have to be implemented in fast hardware. Within the SN the self-routing principle will be applied [Schaffer, 1990]. At the inlet of the SN the cell is extended by an SN-internal header. It is evident that the SN-internal operational speed has to be increased. When passing the individual switching elements, for the processing of the SN-internal header only simple hard-wired logic is necessary. This reduces the control complexity and provides a better failure behavior. When starting several years ago with the imple￾mentation of the ATM technology, only the emitter coupled logic (ECL) was available. Nowadays, the comple￾mentary metal-oxide semiconductor (CMOS) technology with its low power consumption is used [Fischer et al., 1991]. Transmission of B-ISDN Signals Transmission systems at the UNI provide bit rates of around 150 and 622 Mbit/s. In addition to these rates, at the NNI around 2.5 Gbit/s and up to 10 Gbit/s will be used in the future [Baur, 1991]. In addition to the high￾capacity switching and multiplexing technology, high-speed transmission systems are required. Optical fibers are especially suitable for this purpose; however, for the lower bit rates coaxial cables can be used. Optical transmission uses optical fibers as the transmission medium in low-diameter and low-weight cables to provide large transmission capacities over long distances without the need for repeaters. Optical transmission equipment currently tends to mono-mode fiber and laser diodes with wavelengths of around 1310 nm. For both directions in a transmission system either two separate fibers or one common fiber with wavelength division multiplexing can be used. The second solution may be a good alternative for subscriber lines and short trunk lines [Bauch, 1991]. For ATM cell transmission, two possibilities exist, which are shown in Fig. 72.2: synchronous pulse frame or continuous cell stream (cell-based). The basis for the pulse frame concept is the existing synchronous digital hierarchy (SDH). In SDH the cells are transported within the SDH payload; the frame overhead includes operation and maintenance (OAM) of the transmission system. In the cell-based system the OAM for the transmission system is transported within cells. The SDH solution is already defined, whereas for cell-based transmission some problems remain to be solved (e.g., OAM is not yet fully defined). ATM Adaptation Layer The ATM adaptation layer (AAL) is between the ATM layer and higher layers. Its basic function is the enhanced adaptation of the services provided by ATM to the requirements of the layers above. In order to minimize the number of AAL protocols, the service classification shown in Fig. 72.3 was defined. This classification was made with respect to timing relation, bit rate, and connection mode