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.