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 802.3 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 802.3 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 802.3 MAC Frame--Fig. 16.3– Preamble--7-octet pattern of alternating 0s and 1s.– Start frame delimiter--pattern 10101011.– 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 802.3 10-Mbps Specification (Ethernet)

– 10BASE5

– 10BASE2

– 10BASE-T

– 10BASE-F

– Table 16.2 summarizes the 10Mbps alternatives.

16.2 Ethernet (p.10)

• IEEE 802.3 100-Mbps (Fast Ethernet)– Generic designation is 100BASE-T.– All options use IEEE 802.3 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 802.3 MAC protocol and frame

format.– A new medium and transmission specification

is defined.– Fig. 16.5 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 802.5 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 802.5 Medium Access Control (cont.)– Advantages--traffic can be regulated.– Disadvantages--token management.– Fig. 16.9 illustrates token ring operation.

16.3 Token Ring (p.3)• IEEE 802.5 Medium Access Control (cont.)

– IEEE 802.5 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 16.4--IEEE 802.5 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.