B. It also shows the channel assignment, the type of payload, and the loss tolerance. In essence, the header provides the QoS metric, and the payload makes up the other 48 bytes of that cell. QoS is one of the great strengths of ATM, and ATM defines a series of specific QoS parameters that tailor cells to fit the video, data, voice, and mixed-media traffic. Where ATM Fits in the Network As technologies evolve, they seem to find their first placement in the core network, where there are high traffic volumes, to justify the investments required for the new technologies. Therefore, much of today's ATM equipment is in core net- works-those of ISPs, telcos, or other network operators. ATM then filters into the access network and into the metropolitan area network. Typically, you find it first where there are concentrations of early adopter-type customers. Ultimately, a new technology makes its way into the LAN, and it is then necessary to reengineer the local enterprise to provide QoS, not just high bandwidth (see Figure 7.21). Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Generic Flow Control Virtual Path Identifier Virtual Path Identifier Virtual Channel Identifier Virtual Channel Identifier Cell- Virtual Channel Identifier Payload Type Priority Header Error Control Payload Figure 7.20 ATM cell structure Packet-Switched Networks Access Network ATM Core Network Fiber Local Network Switch Backbone Switch Frame Relay Workgroup Fiber Hub Switch Fiber Access Backbone Switch Switch Fiber Backbone Enterprise Switch Switches Analog LAN Devices Figure 7.21 The ATM infrastructure Advantages and Disadvantages of ATM ATM's benefits can be summarized as follows: Provides hardware switching, which results in high performance Allows dynamic bandwidth for bursty data Provides CoS and QoS support for multimedia Scales in speed and network size Provides a common LAN/WAN architecture Provides opportunities for simplification via its virtual circuit architecture Has strong traffic-engineering and network management capabilities The f