Ch. 16 High-Speed LANs

16.1 The Emergence of High- Speed LANs Trends –Computing power of PCs has continued to grow. –MIS organizations recognize the importance of LANs in the transfer of large volumes of data in a transaction oriented environment. Table 16.1 Characteristics of High Speed LANs.

16.2 Ethernet (CSMA/CD) IEEE Medium Access Control –Carrier Sense Multiple Access with Collision Detection (CSMA/CD) –The most commonly used medium access control technique for bus/tree and star topologies. –Original baseband version was developed by Xerox and formed the basis IEEE CSMA/CD standard.

16.2 Ethernet (p.2) ALOHA –Developed for packet radio networks (Abrahamson, 1970). –If a station has something to send, it does so. –Then the station listens for an ACK. –If no ACK is received, then a collision is assumed and the frame is retransmitted. –Simple but maximum efficiency is only 18%.

16.2 Ethernet (p.3) Slotted ALOHA –Stations always wait for the beginning of a slot--this reduces collisions. – Improves the performance --37% maximum efficiency.

16.2 Ethernet (p.4) Carrier Sense Multiple Access (CSMA) –Suppose propagation delays are small relative to frame transmission times--then stations usually know when the line is being used. –With CSMA, a station first listens--if the medium is in use it will wait.

16.2 Ethernet (p.5) 1-persistent CSMA –1. If the medium is idle, transmit; otherwise, go to step 2. –2. If the medium is busy, continue to listen until the channel is sensed idle; then transmit immediately. Other Approaches –Nonpersistent--if busy, wait a random amount of time. –p-persistent--if idle, transmit with probability p.

16.2 Ethernet (p.6) CSMA/CD (Fig. 16.2) –1. If the medium is idle, transmit; otherwise, go to step 2. –2. If the medium is busy, continue to listen until the channel is idle, then transmit immediately. –3. If a collision is detected during transmission, transmit a brief jamming signal to assure that all stations know that there has been a collision and then cease transmission. –4. After transmitting the jamming signal, wait a random amount of time, then attempt to transmit again (repeat from step 1).

16.2 Ethernet (p.7) How long does it take to detect a collision? –Frames should be long enough to allow collision detection prior to the end of transmission (distance and speed are factors also.) How long can segments be? –The signal is attenuated and long segments will cause problems in CD algorithm. –IEEE standards recommend maximum lengths.

16.2 Ethernet (p.8) IEEE MAC Frame--Fig –Preamble--7-octet pattern of alternating 0s and 1s. –Start frame delimiter--pattern –Destination address--48 bit address. –Source address--48 bit address. –Length--length of the LCC data field--2 bytes. –Pad--octets added to ensure that the frame is long enough for proper CD operation. –Frame check sequence--32-bit cyclic redundancy check (does not cover preamble and SFD.)

16.2 Ethernet (p.9) IEEE Mbps Specification (Ethernet) –10BASE5 –10BASE2 –10BASE-T –10BASE-F –Table 16.2 summarizes the 10Mbps alternatives.

16.2 Ethernet (p.10) IEEE Mbps (Fast Ethernet) –Generic designation is 100BASE-T. –All options use IEEE MAC protocol and frame format. –100BASE-X (TX and FX) Set of options that use a signal encoding scheme defined for FDDI--4B/5B NRZI. –100BASE-T4 Uses four twisted pair lines between nodes.

16.2 Ethernet (p.11) Gigabit Ethernet –Uses the IEEE MAC protocol and frame format. –A new medium and transmission specification is defined. –Fig illustrates a typical Gigabit Ethernet application.

16.2 Ethernet (p.12) Gigabit Ethernet--Media Access Layer –Two enhancements for shared medium hub. Carrier extension--appends a set of special symbols to the end of short MAC frames so that the resulting block is as least 4096 bit-time in duration--this makes the transmission time longer than the propagation time. Frame bursting--allows for multiple short frames to be transmitted consecutively--avoids the overhead of carrier extension when a number of small frames are ready for transmission. –The above are not needed in switching hubs.

16.2 Ethernet (p.13) Gigabit Ethernet--Physical Layer –1000BASE-SX--supports duplex links of up to 275 meters and 550 meters depending on multimode fiber diameter--wavelengths are 770 to 860 nm. –1000BASE-LX--supports duplex links from 550 m to 5km depending on fiber diameter--wavelengths are 1270 to 1355 nm. –1000BASE-CX--2 shielded twisted pair (25m.) –1000BASE-T--4 CAT 5 unshielded twisted pair (100m).

16.2 Ethernet (p.14) 10-Gbps Ethernet –Satisfies increased bandwidth demand. –Initially used for backbone connectivity. –Provides for connectivity between ISPs and NSPs co-located facilities. –Allows MANs to be constructed, and begins to compete with ATM.

16.2 Ethernet (p.15) 10-Gbps Ethernet (p.2) –10GBASE-S--(up to 300 meters). –10GBASE-L--(up to 10 kilometers) –10GBASE-E--(up to 40 kilometers) –10GBASE-LX4--(up to 10 kilometers; uses WDM) –Figure 16.7.

16.3 Token Ring IEEE Medium Access Control –A small token packet circulates the ring. –When all stations are idle, the token is "free". –To transmit, a station waits for a "free" token and then changes it into a data frame. –The transmitting station sends a "free token", when the station has completed the transmission and the frame has returned.

16.3 Token Ring (p.2) IEEE Medium Access Control (cont.) –Advantages--traffic can be regulated. –Disadvantages--token management. –Fig illustrates token ring operation.

16.3 Token Ring (p.3) IEEE Medium Access Control (cont.) –IEEE includes a specification for an optional priority mechanism. 8 levels of priority are supported A priority field and reservation field is used. Stations have the responsibility to manage the priority scheme. –An early token release scheme has been added, for efficient use of the ring-- the token is generated immediately after transmission. –Table IEEE medium alternatives.

16.4 Fibre Channel Fibre channel is designed to combine the best features of I/0 channels and network communications. Some features: –Full duplex links with two fibers per link. –Data rates from 100 Mbps to 800 Mbps. –Support for distances of up to 10 Km. Fig. 16,10 shows Fibre Channel applications. Table 16.5 shows the maximum distances.