Internetworking with TCP/IP- P41: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | Sec. ATM Adaptation Layers 359 software in host computer host interface board optical fiber Figure The conceptual organization of ATM interface hardware and the flow of data through it. Software on a host interacts with an adaptation layer protocol to send and receive data the adaptation layer converts to and from cells. When establishing a connection a host must specify which adaptation layer protocol to use. Both ends of the connection must agree on the choice and the adaptation layer cannot be changed once the connection has been established. To summarize Although ATM hardware uses small fixed-size cells to transport data a higher layer protocol called an ATM Adaptation Layer provides data transfer services for computers that use ATM. When a virtual circuit is created both ends of the circuit must agree on which adaptation layer protocol will be used. 360 TCP IP Over ATM Networks Chap. 18 ATM Adaptation Layer 5 Computers use ATM Adaptation Layer 5 AAL5 to send data across an ATM network. Interestingly although ATM uses small fixed-size cells at the lowest level AAL5 presents an interface that accepts and delivers large variable-length packets. Thus the interface computers use to send data makes ATM appear much like a connectionless technology. In particular AAL5 allows each packet to contain between 1 and 65 535 octets of data. Figure illustrates the packet format that AAL5 uses. 8-octet trailer Between 1 and 65 535 octets of data a 8-BIT UU 8-BIT CPI 16-BIT LENGTH 32-BIT FRAME CHECKSUM b Figure a The basic packet format that AAL5 accepts and delivers and b the fields in the 8-octet trailer that follows the data. Unlike most network frames that place control information in a header AAL5 places control information in an 8-octet trailer at the end of the packet. The AAL5 trailer contains a 16-bit length field a 32-bit cyclic redundancy check CRC used as a frame checksum and two 8-bit fields labeled UU and CPI that are currently unusedf. .
