Curs PC si internet cap 7.2

7.2 Basics of Fiber Distributed Data Interface (FDDI) 7.2.2 FDDI format Instructor Note Again, building upon the generic frame format in Chapter 6 and the Token Ring Frame Format just introduced, present the FDDI frame format. Again a Token flag is present; all the typical aspects of frames are present as well. Emphasize that frame formats are the basic layer 2 PDUs and thus contain a lot of information about how a given layer 2 technology works. The fields of an FDDI frame are as follows: preamble - prepares each station for the upcoming frame start delimiter - indicates the beginning of the frame, and consists of signaling patterns that differentiate it from the rest of the frame frame control - indicates the size of the address fields, whether the frame contains asynchronous or synchronous data, and other control information destination address - contains a unicast (singular), multicast (group), or broadcast (every station) address, destination addresses are 6 bytes (like Ethernet and Token Ring) source address - identifies the single station that sent the frame, source addresses are 6 bytes (like Ethernet and Token Ring) data - control information, or information destined for an upper-layer protocol frame check sequence (FCS) - filled by the source station with a calculated cyclic redundancy check (CRC), value dependent on the frame contents (as with Token Ring and Ethernet). The destination station recalculates the value to determine whether the frame may have been damaged in transit. If it has been, the frame is discarded. end delimiter - contains non-data symbols that indicate the end of the frame frame status - allows the source station to determine if an error occurred and if the frame was recognized and copied by a receiving station 7.2 Basics of Fiber Distributed Data Interface (FDDI) 7.2.3 FDDI MAC Instructor Note The details of FDDI's MAC method are presented. Note that while FDDI relies on Token Passing, with its dual ring there are more variations possible than with typical Token Ring networks. FDDI's MAC method is one of the reasons for its reliability. FDDI uses a token passing strategy similar to Token Ring. Token-passing networks move a small frame, called a token, around the network. Possession of the token grants the right to transmit data. If a node receiving the token has no information to send, it passes the token to the next end-station. Each station can hold the token for a maximum period of time, depending on the specific technology implementation. When a station that is in possession of the token has information to transmit, it seizes the token and alters one of its bits. The token then becomes a start-of-frame sequence. Next, the station appends the information that it transmits to the token, and sends this data to the next station on the ring. There is no token on the network while the information frame is circling the ring, unless the ring supports early token release. Other stations on the ring must wait for the token to become available. FDDI networks have no collisions. If early token release is supported, a new token can be released when the frame transmission has finished.The information frame circulates the ring until it reaches the intended destination station, which then copies the information for processing. The information frame circles the ring until it reaches the sending station and is then removed. The sending station can check the returning frame to see whether the frame was received, and subsequently copied by the destination. Unlike CSMA/CD networks, such as Ethernet, token-passing networks are deterministic. This means, you can calculate the maximum time that will pass before any end station will be able to transmit. FDDIs dual ring assures that not only are stations guaranteed their turn to transmit, but if one part of one ring is damaged or disabled for any reason, the second ring can be used. This makes FDDI very reliable. FDDI supports real-time allocation of network bandwidth, making it ideal for a variety of different application types. FDDI provides this support by defining two types of traffic - synchronous and asynchronous. SynchronousSynchronous traffic can consume a portion of the 100 Mbps total bandwidth of an FDDI network, while asynchronous traffic can consume the rest. Synchronous bandwidth is allocated to those stations requiring continuous transmission capability. This is useful for transmitting voice and video information. The remaining bandwidth is used for asynchronous transmissions. The FDDI SMT specification defines a distributed bidding scheme to allocate FDDI bandwidth. AsynchronousAsynchronous bandwidth is allocated using an eight-level priority scheme. Each station is assigned an asynchronous priority level. FDDI also permits extended dialogues, in which stations may temporarily use all asynchronous bandwidth. The FDDI priority mechanism can lock out stations that cannot use synchronous bandwidth, and that have too low an asynchronous priority. 7.2 Basics of Fiber Distributed Data Interface (FDDI) 7.2.4 FDDI signaling Instructor Note This target indicator describes the layer 1 issue of how FDDI encodes bits. The scheme (4B/5B) is somewhat abstract and presented for background purposes only. It is not crucial that the students at this level deeply understand 4B/5B. For your information, 4B/5B incorporates the desirable features of Manchester Encoding (the clock signal is encoded along with the data, hence making the clock easier for the receiving computer to recover) along with an avoidance of long durations of high or low signals (which can cause loss of clock signal and susceptibility to errors). FDDI uses an encoding scheme called 4B/5B. Every 4 bits of data are sent as a 5 bit code. The signal sources in FDDI transceivers are LEDs or lasers.