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IEEE 802.3 recommends a generic media access control frame format for use in data transmit and receive tasks. All frames are preceded by a preamble. The media access control frame structure is flexible enough to accommodate baseband and broadband implementations. Both baseband and broadband implementations require addition of a preamble to the frame during transmission. In the broadband implementation, the frame is encapsulated with a preamble and postamble. The following various frame fields defined in IEEE 802.3 are shown in Exhibit 1-1-14.
The IEEE 802.3 standard specifies a CMSA/CD media access control operation at the media access control sublayer. On receiving a request for transmission, the transmit data encapsulation component constructs a protocol data unit frame using logical link control data. The transmit media access management samples the communication channel. When the medium becomes clear, transmission begins and the logical link control is informed. In the case of a collision, the transmit media access management transmits a jam signal to ensure that all transmitting stations have detected the collision. The transmission is terminated and retry begins after a back-off interval. When a station enters the data receive mode, the physical layer signaling interface detects an incoming frame and discards the leading synch bits, the preamble, and the start frame delimiter. The physical layer signaling then forwards the data to the receive data decapsulation component. This process continues until the carrier-sense signal goes off. Then, the receive data decapsulation component evaluates the destination address and checks the data for transmission errors. If the address is valid and data is error free, the frame is forwarded to the logical link control; frames with invalid addresses or data transmission errors are discarded or passed to network management for further processing. The transmit and receive media management activities are supported by the physical layers three components: the physical layer signaling (PLS), attachment unit interface (AUI), and physical medium attachment (PMA). The physical layer signaling shields the media access control layer from the intricacies of establishing, maintaining, and tearing down the serial communications pipe. This interface supports transmit-receive bit streams and channel-allocation and contention-resolution tasks. The physical signaling layer uses five primitives. Those can be grouped according to their functions as shown in Exhibit 1-1-15.
The attachment unit interface includes specifications for cable connectors and transmission circuitry used to interconnect the physical layer signaling and media access unit in compliance with the following characteristics:
The MAU is the portion of the physical layer between the medium-dependent interface and the AUI. It links the medium to the connector cable and contains the electronic circuitry that sends, receives, and manages the encoded signals impressed on and recovered from the trunk coaxial cable. The medium-dependent interface is the mechanical and electrical interface between the trunk cable medium and the medium attachment unit. The physical layer options for IEEE 802.3 are listed in Exhibit 1-1-16. The recommendation calls for the media access control layer to use the parameter values shown in Exhibit 1-1-17 and the typical specifications given in Exhibit 1-1-18.
For baseband implementation the IEEE 802.3 standard uses a 10M-bps channel with a minimum frame of 512 bits. The network can use 500-m segments for coaxial cable and 1-km segments for optical fiber. The distance between the cable tap and the station should not exceed 50 m.
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