Ch. 16 Ethernet

16.1 Traditional Ethernet 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.

16.1 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%.

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

16.1 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.

16.1 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.

16.1 Ethernet (p.6) CSMA/CD (Fig. 16.1) –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).

16.1 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.

16.1 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.)

16.1 Ethernet (p.9) Three types of MAC Frames –Basic –Q-tagged frame—supports Q VLAN –Envelope frame—allows additional prefixes and suffixes to the data field (Provider Bridges and MAC Security—IEEE Working group) –also ITU-T, and IETF (MPLS or multiprotocol label switching)

16.1 Ethernet (p.10) IEEE Mbps Specification (Ethernet) –10BROAD36—rarely used today –10BASE5 –10BASE2 –10BASE-T –10BASE-F (3 options) –Table 16.2 summarizes the 10Mbps alternatives.

16.2 High Speed Ethernet 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 High Speed Ethernet (p.2) 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.

16.2High Speed Ethernet (p.3) 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 High Speed Ethernet (p.4) 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 High Speed Ethernet (p.5 ) 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.

16.2High Speed Ethernet (p.6) 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.

16.2 High Speed Ethernet(p.7) 100 – G bps Ethernet –IEEE 802.3ba standard was finalized in –40G bps and 100 G bps –Expected to be deployed in switch uplinks inside the data center as well as inter-building, intercampus, MAN, and WAN connections.

16.3 IEEE Q VLAN Standard (2005) Defines the operation VLAN bridges and switches that permits the definition, operation and administration of VLAN topologies. Fig shows the “tagged” IEEE MAC Frame Format.