ECS 152A

9. Local Area Networks

LAN Applications (1)• Personal computer LANs

—Low cost—Limited data rate

• Back end networks—Interconnecting large systems (mainframes

and large storage devices)• High data rate• High speed interface• Distributed access• Limited distance• Limited number of devices

LAN Applications (2)• Storage Area Networks

—Separate network handling storage needs—Detaches storage tasks from specific servers—Shared storage facility across high-speed network—Hard disks, tape libraries, CD arrays—Improved client-server storage access—Direct storage to storage communication for backup

• High speed office networks—Desktop image processing—High capacity local storage

• Backbone LANs—Interconnect low speed local LANs—Reliability—Capacity—Cost

Storage Area Networks

LAN Architecture• Topologies• Transmission medium• Layout• Medium access control

Topologies• Tree• Bus

—Special case of tree• One trunk, no branches

• Ring• Star

LAN Topologies

Bus and Tree• Multipoint medium• Transmission propagates throughout medium • Heard by all stations

—Need to identify target station• Each station has unique address

• Full duplex connection between station and tap—Allows for transmission and reception

• Need to regulate transmission—To avoid collisions—To avoid hogging

• Data in small blocks - frames• Terminator absorbs frames at end of medium

Frame Transmissionon Bus LAN

Ring Topology• Repeaters joined by point to point links in

closed loop—Receive data on one link and retransmit on another—Links unidirectional—Stations attach to repeaters

• Data in frames—Circulate past all stations—Destination recognizes address and copies frame—Frame circulates back to source where it is

removed• Media access control determines when station

can insert frame

Frame TransmissionRing LAN

Star Topology• Each station connected directly to central

node—Usually via two point to point links

• Central node can broadcast—Physical star, logical bus—Only one station can transmit at a time

• Central node can act as frame switch

Choice of Topology• Reliability• Expandability• Performance• Needs considering in context of:

—Medium—Wiring layout—Access control

Bus LAN Transmission Media (1)• Twisted pair

—Early LANs used voice grade cable—Didn’t scale for fast LANs—Not used in bus LANs now

• Baseband coaxial cable—Uses digital signalling—Original Ethernet

Bus LAN Transmission Media (2)• Broadband coaxial cable

— As in cable TV systems— Analog signals at radio frequencies— Expensive, hard to install and maintain— No longer used in LANs

• Optical fiber— Expensive taps— Better alternatives available— Not used in bus LANs

• All hard to work with compared with star topology twisted pair

• Coaxial baseband still used but not often in new installations

Ring and Star Usage• Ring

—Very high speed links over long distances—Single link or repeater failure disables network

• Star—Uses natural layout of wiring in building—Best for short distances—High data rates for small number of devices

Choice of Medium• Constrained by LAN topology• Capacity• Reliability• Types of data supported• Environmental scope

Media Available (1)• Voice grade unshielded twisted pair (UTP)

—Cat 3—Cheap—Well understood—Use existing telephone wiring in office building—Low data rates

• Shielded twisted pair and baseband coaxial—More expensive than UTP but higher data rates

• Broadband cable—Still more expensive and higher data rate

Media Available (2)• High performance UTP

—Cat 5 and above—High data rate for small number of devices—Switched star topology for large installations

• Optical fiber—Electromagnetic isolation—High capacity—Small size—High cost of components—High skill needed to install and maintain

• Prices are coming down as demand and product range increases

Protocol Architecture• Lower layers of OSI model• IEEE 802 reference model• Physical• Logical link control (LLC)• Media access control (MAC)

IEEE 802 v OSI

802 Layers - Physical• Encoding/decoding• Preamble generation/removal• Bit transmission/reception• Transmission medium and topology

802 Layers -Logical Link Control• Interface to higher levels• Flow and error control

Logical Link Control • Transmission of link level PDUs between

two stations• Must support multiaccess, shared medium• Relieved of some link access details by

MAC layer• Addressing involves specifying source and

destination LLC users—Referred to as service access points (SAP)—Typically higher level protocol

LLC Services• Based on HDLC• Unacknowledged connectionless service• Connection mode service• Acknowledged connectionless service

LLC Protocol• Modeled after HDLC• Asynchronous balanced mode to support

connection mode LLC service (type 2 operation)

• Unnumbered information PDUs to support Acknowledged connectionless service (type 1)

• Multiplexing using LSAPs

Media Access Control• Assembly of data into frame with address

and error detection fields• Disassembly of frame

—Address recognition—Error detection

• Govern access to transmission medium—Not found in traditional layer 2 data link

control• For the same LLC, several MAC options

may be available

LAN Protocols in Context

Media Access Control• Where

—Central• Greater control• Simple access logic at station• Avoids problems of co-ordination• Single point of failure• Potential bottleneck

—Distributed• How

—Synchronous• Specific capacity dedicated to connection

—Asynchronous• In response to demand

Asynchronous Systems• Round robin

—Good if many stations have data to transmit over extended period

• Reservation—Good for stream traffic

• Contention—Good for bursty traffic—All stations contend for time—Distributed—Simple to implement—Efficient under moderate load—Tend to collapse under heavy load

MAC Frame Format• MAC layer receives data from LLC layer• MAC control• Destination MAC address• Source MAC address• LLS• CRC• MAC layer detects errors and discards frames• LLC optionally retransmits unsuccessful

frames

Generic MAC Frame Format

Bridges• Ability to expand beyond single LAN• Provide interconnection to other

LANs/WANs• Use Bridge or router• Bridge is simpler

—Connects similar LANs—Identical protocols for physical and link layers—Minimal processing

• Router more general purpose—Interconnect various LANs and WANs—see later

Why Bridge?• Reliability• Performance• Security• Geography

Functions of a Bridge• Read all frames transmitted on one LAN

and accept those address to any station on the other LAN

• Using MAC protocol for second LAN, retransmit each frame

• Do the same the other way round

Bridge Operation

Bridge Design Aspects• No modification to content or format of frame• No encapsulation• Exact bitwise copy of frame• Minimal buffering to meet peak demand• Contains routing and address intelligence

—Must be able to tell which frames to pass—May be more than one bridge to cross

• May connect more than two LANs• Bridging is transparent to stations

—Appears to all stations on multiple LANs as if they are on one single LAN

Bridge Protocol Architecture• IEEE 802.1D• MAC level

—Station address is at this level• Bridge does not need LLC layer

—It is relaying MAC frames• Can pass frame over external comms system

—e.g. WAN link—Capture frame—Encapsulate it—Forward it across link—Remove encapsulation and forward over LAN link

Connection of Two LANs

Fixed Routing• Complex large LANs need alternative routes

—Load balancing—Fault tolerance

• Bridge must decide whether to forward frame• Bridge must decide which LAN to forward

frame on• Routing selected for each source-destination

pair of LANs—Done in configuration—Usually least hop route—Only changed when topology changes

Bridges and LANs withAlternativeRoutes

Spanning Tree• Bridge automatically develops routing

table• Automatically update in response to

changes• Frame forwarding• Address learning• Loop resolution

Frame forwarding• Maintain forwarding database for each port

—List station addresses reached through each port• For a frame arriving on port X:

—Search forwarding database to see if MAC address is listed for any port except X

—If address not found, forward to all ports except X—If address listed for port Y, check port Y for blocking

or forwarding state• Blocking prevents port from receiving or transmitting

—If not blocked, transmit frame through port Y

Address Learning• Can preload forwarding database• Can be learned• When frame arrives at port X, it has come

form the LAN attached to port X• Use the source address to update

forwarding database for port X to include that address

• Timer on each entry in database• Each time frame arrives, source address

checked against forwarding database

Spanning Tree Algorithm• Address learning works for tree layout

—i.e. no closed loops• For any connected graph there is a

spanning tree that maintains connectivity but contains no closed loops

• Each bridge assigned unique identifier• Exchange between bridges to establish

spanning tree

Loop of Bridges

Layer 2 and Layer 3 Switches• Now many types of devices for

interconnecting LANs • Beyond bridges and routers • Layer 2 switches • Layer 3 switches

Hubs• Active central element of star layout• Each station connected to hub by two lines

— Transmit and receive• Hub acts as a repeater• When single station transmits, hub repeats signal on

outgoing line to each station• Line consists of two unshielded twisted pairs• Limited to about 100 m

— High data rate and poor transmission qualities of UTP• Optical fiber may be used

— Max about 500 m• Physically star, logically bus• Transmission from any station received by all other stations• If two stations transmit at the same time, collision

Hub Layouts• Multiple levels of hubs cascaded• Each hub may have a mixture of stations and

other hubs attached to from below• Fits well with building wiring practices

—Wiring closet on each floor—Hub can be placed in each one—Each hub services stations on its floor

Two Level Star Topology

Buses and Hubs• Bus configuration

—All stations share capacity of bus (e.g. 10Mbps)—Only one station transmitting at a time

• Hub uses star wiring to attach stations to hub—Transmission from any station received by hub

and retransmitted on all outgoing lines—Only one station can transmit at a time—Total capacity of LAN is 10 Mbps

• Improve performance with layer 2 switch

Shared Medium Bus and Hub

Shared Medium Hub andLayer 2 Switch

Layer 2 Switches• Central hub acts as switch• Incoming frame from particular station

switched to appropriate output line• Unused lines can switch other traffic• More than one station transmitting at a

time• Multiplying capacity of LAN

Layer 2 Switch Benefits• No change to attached devices to convert bus

LAN or hub LAN to switched LAN• For Ethernet LAN, each device uses Ethernet MAC

protocol • Device has dedicated capacity equal to original

LAN—Assuming switch has sufficient capacity to keep up with

all devices—For example if switch can sustain throughput of 20 Mbps,

each device appears to have dedicated capacity for either input or output of 10 Mbps

• Layer 2 switch scales easily—Additional devices attached to switch by increasing

capacity of layer 2

Types of Layer 2 Switch• Store-and-forward switch

—Accepts frame on input line—Buffers it briefly, —Then routes it to appropriate output line—Delay between sender and receiver—Boosts integrity of network

• Cut-through switch—Takes advantage of destination address appearing at

beginning of frame—Switch begins repeating frame onto output line as soon

as it recognizes destination address—Highest possible throughput —Risk of propagating bad frames

• Switch unable to check CRC prior to retransmission

Layer 2 Switch v Bridge• Layer 2 switch can be viewed as full-duplex hub• Can incorporate logic to function as multiport bridge• Bridge frame handling done in software• Switch performs address recognition and frame

forwarding in hardware• Bridge only analyzes and forwards one frame at a time• Switch has multiple parallel data paths

—Can handle multiple frames at a time• Bridge uses store-and-forward operation• Switch can have cut-through operation• Bridge suffered commercially

—New installations typically include layer 2 switches with bridge functionality rather than bridges

Problems with Layer 2 Switches (1)• As number of devices in building grows, layer 2

switches reveal some inadequacies• Broadcast overload• Lack of multiple links• Set of devices and LANs connected by layer 2

switches have flat address space—Allusers share common MAC broadcast address—If any device issues broadcast frame, that frame is

delivered to all devices attached to network connected by layer 2 switches and/or bridges

—In large network, broadcast frames can create big overhead

—Malfunctioning device can create broadcast storm• Numerous broadcast frames clog network

Problems with Layer 2 Switches (2)• Current standards for bridge protocols dictate no

closed loops—Only one path between any two devices—Impossible in standards-based implementation to provide

multiple paths through multiple switches between devices• Limits both performance and reliability.

• Solution: break up network into subnetworks connected by routers

• MAC broadcast frame limited to devices and switches contained in single subnetwork

• IP-based routers employ sophisticated routing algorithms —Allow use of multiple paths between subnetworks going

through different routers

Problems with Routers• Routers do all IP-level processing in software

—High-speed LANs and high-performance layer 2 switches pump millions of packets per second

—Software-based router only able to handle well under a million packets per second

• Solution: layer 3 switches—Implementpacket-forwarding logic of router in

hardware• Two categories

—Packet by packet —Flow based

Packet by Packet or Flow Based• Operates insame way as traditional router• Order of magnitude increase in performance

compared to software-based router• Flow-based switch tries to enhance

performance by identifying flows of IP packets—Same source and destination—Done by observing ongoing traffic or using a

special flow label in packet header (IPv6)—Once flow is identified, predefined route can be

established

Typical Large LAN Organization• Thousands to tens of thousands of devices• Desktop systems links 10 Mbps to 100 Mbps

—Into layer 2 switch• Wireless LAN connectivity available for mobile users• Layer 3 switches at local network's core

—Form local backbone—Interconnected at 1 Gbps—Connect to layer 2 switches at 100 Mbps to 1 Gbps

• Servers connect directly to layer 2 or layer 3 switches at 1 Gbps

• Lower-cost software-based router provides WAN connection

• Circles in diagram identify separate LAN subnetworks• MAC broadcast frame limited to own subnetwork

Typical Large LAN OrganizationDiagram

High Speed LANs• Range of technologies

—Fast and Gigabit Ethernet—Fibre Channel—High Speed Wireless LANs

Why High Speed LANs?• Office LANs used to provide basic connectivity

—Connecting PCs and terminals to mainframes and midrange systems that ran corporate applications

—Providing workgroup connectivity at departmental level—Traffic patterns light

• Emphasis on file transfer and electronic mail• Speed and power of PCs has risen

—Graphics-intensive applications and GUIs• MIS organizations recognize LANs as essential

—Began with client/server computing• Now dominant architecture in business environment• Intranetworks• Frequent transfer of large volumes of data 

Applications Requiring High Speed LANs• Centralized server farms

—User needs to draw huge amounts of data from multiple centralized servers

—E.g. Color publishing• Servers contain tens of gigabytes of image data• Downloaded to imaging workstations

• Power workgroups• Small number of cooperating users

—Draw massive data files across network—E.g. Software development group testing new software version

or computer-aided design (CAD) running simulations• High-speed local backbone

—Processing demand grows—LANs proliferate at site—High-speed interconnection is necessary

Ethernet (CSMA/CD)• Carriers Sense Multiple Access with

Collision Detection• Xerox - Ethernet• IEEE 802.3

IEEE802.3 Medium Access Control• Random Access

— Stations access medium randomly• Contention

—Stations content for time on medium

ALOHA• Packet Radio• When station has frame, it sends• Station listens (for max round trip time)plus small

increment• If ACK, fine. If not, retransmit• If no ACK after repeated transmissions, give up• Frame check sequence (as in HDLC)• If frame OK and address matches receiver, send ACK• Frame may be damaged by noise or by another

station transmitting at the same time (collision)• Any overlap of frames causes collision• Max utilization 18%

Slotted ALOHA• Time in uniform slots equal to frame

transmission time• Need central clock (or other sync

mechanism)• Transmission begins at slot boundary• Frames either miss or overlap totally• Max utilization 37%

CSMA• Propagation time is much less than transmission time• All stations know that a transmission has started

almost immediately• First listen for clear medium (carrier sense)• If medium idle, transmit• If two stations start at the same instant, collision• Wait reasonable time (round trip plus ACK

contention)• No ACK then retransmit• Max utilization depends on propagation time

(medium length) and frame length—Longer frame and shorter propagation gives better utilization

Nonpersistent CSMA1. If medium is idle, transmit; otherwise, go to 22. If medium is busy, wait amount of time drawn from

probability distribution (retransmission delay) and repeat 1

•  Random delays reduces probability of collisions— Consider two stations become ready to transmit at same

time • While another transmission is in progress

— If both stations delay same time before retrying, both will attempt to transmit at same time

• Capacity is wasted because medium will remain idle following end of transmission— Even if one or more stations waiting

• Nonpersistent stations deferential

1-persistent CSMA• To avoid idle channel time, 1-persistent protocol

used• Station wishing to transmit listens and obeys

following: 1. If medium idle, transmit; otherwise, go to step 22. If medium busy, listen until idle; then transmit

immediately• 1-persistent stations selfish• If two or more stations waiting, collision

guaranteed— Gets sorted out after collision

P-persistent CSMA• Compromise that attempts to reduce collisions

— Like nonpersistent• And reduce idle time

— Like1-persistent• Rules:1. If medium idle, transmit with probability p, and delay

one time unit with probability (1 – p)— Time unit typically maximum propagation delay

2. If medium busy, listen until idle and repeat step 13. If transmission is delayed one time unit, repeat step

1• What is an effective value of p?

Value of p?• Avoid instability under heavy load• n stations waiting to send• End of transmission, expected number of stations attempting to

transmit is number of stations ready times probability of transmitting— np

• If np > 1on average there will be a collision• Repeated attempts to transmit almost guaranteeing more

collisions• Retries compete with new transmissions • Eventually, all stations trying to send

— Continuous collisions; zero throughput• So np < 1 for expected peaks of n• If heavy load expected, p small• However, as p made smaller, stations wait longer• At low loads, this gives very long delays

CSMA/CD• With CSMA, collision occupies medium for

duration of transmission• Stations listen whilst transmitting

1. If medium idle, transmit, otherwise, step 22. If busy, listen for idle, then transmit3. If collision detected, jam then cease

transmission4. After jam, wait random time then start

from step 1

CSMA/CDOperation

Which Persistence Algorithm?• IEEE 802.3 uses 1-persistent• Both nonpersistent and p-persistent have

performance problems• 1-persistent (p = 1) seems more unstable

than p-persistent —Greed of the stations—But wasted time due to collisions is short (if

frames long relative to propagation delay—With random backoff, unlikely to collide on next

tries—To ensure backoff maintains stability, IEEE 802.3

and Ethernet use binary exponential backoff

Binary Exponential Backoff• Attempt to transmit repeatedly if repeated collisions• First 10 attempts, mean value of random delay doubled• Value then remains same for 6 further attempts• After 16 unsuccessful attempts, station gives up and

reports error• As congestion increases, stations back off by larger

amounts to reduce the probability of collision.• 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads—Low loads, 1-persistence guarantees station can seize channel

once idle—High loads, at least as stable as other techniques

• Backoff algorithm gives last-in, first-out effect• Stations with few collisions transmit first

Collision Detection• On baseband bus, collision produces much

higher signal voltage than signal• Collision detected if cable signal greater

than single station signal• Signal attenuated over distance• Limit distance to 500m (10Base5) or 200m

(10Base2)• For twisted pair (star-topology) activity on

more than one port is collision• Special collision presence signal

IEEE 802.3 Frame Format

10Mbps Specification (Ethernet)• <data rate><Signaling method><Max segment

length>

• 10Base5 10Base2 10Base-T 10Base-F

• Medium Coaxial Coaxial UTP 850nm fiber

• Signaling Baseband Baseband Baseband Manchester• Manchester Manchester Manchester

On/Off• Topology Bus Bus Star Star• Nodes 100 30 - 33

100Mbps Fast Ethernet• Use IEEE 802.3 MAC protocol and frame format• 100BASE-X use physical medium specifications

from FDDI—Two physical links between nodes

• Transmission and reception—100BASE-TX uses STP or Cat. 5 UTP

• May require new cable—100BASE-FX uses optical fiber—100BASE-T4 can use Cat. 3, voice-grade UTP

• Uses four twisted-pair lines between nodes• Data transmission uses three pairs in one direction at a time

• Star-wire topology—Similar to 10BASE-T

100Mbps (Fast Ethernet)• 100Base-TX 100Base-FX 100Base-

T4

• 2 pair, STP 2 pair, Cat 5 UTP 2 optical fiber 4 pair, cat 3,4,5

• MLT-3 MLT-3 4B5B,NRZI 8B6T,NRZ

100BASE-X Data Rate and Encoding• Unidirectional data rate 100 Mbps over

single link —Single twisted pair, single optical fiber

• Encoding scheme same as FDDI —4B/5B-NRZI—Modified for each option

100BASE-X Media• Two physical medium specifications• 100BASE-TX

—Two pairs of twisted-pair cable—One pair for transmission and one for reception—STP and Category 5 UTP allowed—The MTL-3 signaling scheme is used

• 100BASE-FX—Two optical fiber cables—One for transmission and one for reception—Intensity modulation used to convert 4B/5B-NRZI code

group stream into optical signals—1 represented by pulse of light—0 by either absence of pulse or very low intensity pulse 

100BASE-T4• 100-Mbps over lower-quality Cat 3 UTP

—Taking advantage of large installed base —Cat 5 optional—Does not transmit continuous signal between packets—Useful in battery-powered applications

• Can not get 100 Mbps on single twisted pair—Data stream split into three separate streams

• Each with an effective data rate of 33.33 Mbps—Four twisted pairs used—Data transmitted and received using three pairs—Two pairs configured for bidirectional transmission

• NRZ encoding not used—Would require signaling rate of 33 Mbps on each pair—Does not provide synchronization—Ternary signaling scheme (8B6T)

100BASE-T Options

Full Duplex Operation• Traditional Ethernet half duplex

—Either transmit or receive but not both simultaneously• With full-duplex, station can transmit and receive

simultaneously• 100-Mbps Ethernet in full-duplex mode, theoretical

transfer rate 200 Mbps• Attached stations must have full-duplex adapter cards• Must use switching hub

—Each station constitutes separate collision domain—In fact, no collisions—CSMA/CD algorithm no longer needed—802.3 MAC frame format used—Attached stations can continue CSMA/CD

Mixed Configurations• Fast Ethernet supports mixture of existing 10-

Mbps LANs and newer 100-Mbps LANs• E.g. 100-Mbps backbone LAN to support 10-Mbps

hubs—Stations attach to 10-Mbps hubs using 10BASE-T —Hubs connected to switching hubs using 100BASE-T

• Support 10-Mbps and 100-Mbps—High-capacity workstations and servers attach directly to

10/100 switches—Switches connected to 100-Mbps hubs using 100-Mbps

links—100-Mbps hubs provide building backbone

• Connected to router providing connection to WAN

Gigabit Ethernet Configuration

Gigabit Ethernet - Differences• Carrier extension• At least 4096 bit-times long (512 for

10/100)• Frame bursting

Gigabit Ethernet – Physical• 1000Base-SX

—Short wavelength, multimode fiber• 1000Base-LX

—Long wavelength, Multi or single mode fiber• 1000Base-CX

—Copper jumpers <25m, shielded twisted pair• 1000Base-T

—4 pairs, cat 5 UTP

• Signaling - 8B/10B

Gbit Ethernet Medium Options(log scale)

10Gbps Ethernet - Uses• High-speed, local backbone interconnection between large-

capacity switches• Server farm• Campus wide connectivity• Enables Internet service providers (ISPs) and network

service providers (NSPs) to create very high-speed links at very low cost

• Allows construction of (MANs) and WANs— Connect geographically dispersed LANs between campuses or

points of presence (PoPs)• Ethernet competes with ATM and other WAN technologies• 10-Gbps Ethernet provides substantial value over ATM

10Gbps Ethernet - Advantages• No expensive, bandwidth-consuming conversion

between Ethernet packets and ATM cells • Network is Ethernet, end to end• IP and Ethernet together offers QoS and traffic

policing approach ATM• Advanced traffic engineering technologies

available to users and providers• Variety of standard optical interfaces (wavelengths

and link distances) specified for 10 Gb Ethernet• Optimizing operation and cost for LAN, MAN, or

WAN 

10Gbps Ethernet - Advantages• Maximum link distances cover 300 m to 40 km• Full-duplex mode only• 10GBASE-S (short):

— 850 nm on multimode fiber— Up to 300 m

• 10GBASE-L (long)— 1310 nm on single-mode fiber— Up to 10 km

• 10GBASE-E (extended)— 1550 nm on single-mode fiber— Up to 40 km

• 10GBASE-LX4:— 1310 nm on single-mode or multimode fiber— Up to 10 km— Wavelength-division multiplexing (WDM) bit stream across four light

waves

10Gbps Ethernet Distance Options (log scale)

Token Ring (802.5)• Developed from IBM's commercial token

ring• Because of IBM's presence, token ring has

gained broad acceptance• Never achieved popularity of Ethernet• Currently, large installed base of token

ring products• Market share likely to decline

Ring Operation• Each repeater connects to two others via

unidirectional transmission links• Single closed path• Data transferred bit by bit from one repeater to

the next• Repeater regenerates and retransmits each bit• Repeater performs data insertion, data reception,

data removal• Repeater acts as attachment point• Packet removed by transmitter after one trip

round ring

Listen State Functions• Scan passing bit stream for patterns

—Address of attached station—Token permission to transmit

• Copy incoming bit and send to attached station—Whilst forwarding each bit

• Modify bit as it passes—e.g. to indicate a packet has been copied

(ACK)

Transmit State Functions• Station has data• Repeater has permission• May receive incoming bits

—If ring bit length shorter than packet• Pass back to station for checking (ACK)

—May be more than one packet on ring• Buffer for retransmission later

Bypass State• Signals propagate past repeater with no

delay (other than propagation delay)• Partial solution to reliability problem (see

later)• Improved performance

Ring Repeater States

802.5 MAC Protocol• Small frame (token) circulates when idle• Station waits for token• Changes one bit in token to make it SOF for data

frame• Append rest of data frame• Frame makes round trip and is absorbed by

transmitting station• Station then inserts new token when transmission

has finished and leading edge of returning frame arrives

• Under light loads, some inefficiency• Under heavy loads, round robin

Token RingOperation

Dedicated Token Ring• Central hub• Acts as switch• Full duplex point to point link• Concentrator acts as frame level repeater• No token passing

802.5 Physical Layer• Data Rate 4 16 100• Medium UTP,STP,Fiber• Signaling Differential Manchester• Max Frame 4550 18200 18200• Access Control TP or DTR TP or DTR DTR

• Note: 1Gbit specified in 2001—Uses 802.3 physical layer specification

Fibre Channel - Background• I/O channel

—Direct point to point or multipoint comms link—Hardware based—High Speed—Very short distance—User data moved from source buffer to destiation buffer

• Network connection—Interconnected access points—Software based protocol—Flow control, error detection &recovery—End systems connections

Fibre Channel• Best of both technologies• Channel oriented

—Data type qualifiers for routing frame payload—Link level constructs associated with I/O ops—Protocol interface specifications to support

existing I/O architectures• e.g. SCSI

• Network oriented—Full multiplexing between multiple destinations—Peer to peer connectivity—Internetworking to other connection technologies

Fibre Channel Requirements• Full duplex links with two fibers per link• 100 Mbps to 800 Mbps on single line

— Full duplex 200 Mbps to 1600 Mbps per link• Up to 10 km• Small connectors• High-capacity utilization, distance insensitivity• Greater connectivity than existing multidrop channels• Broad availability

— i.e. standard components• Multiple cost/performance levels

— Small systems to supercomputers• Carry multiple existing interface command sets for existing channel

and network protocols • Uses generic transport mechanism based on point-to-point links and a

switching network• Supports simple encoding and framing scheme• In turn supports a variety of channel and network protocols

Fibre Channel Elements• End systems - Nodes• Switched elements - the network or fabric• Communication across point to point links

Fibre Channel Network

Fibre Channel Protocol Architecture (1)• FC-0 Physical Media

—Optical fiber for long distance—coaxial cable for high speed short distance—STP for lower speed short distance

• FC-1 Transmission Protocol—8B/10B signal encoding

• FC-2 Framing Protocol—Topologies—Framing formats—Flow and error control—Sequences and exchanges (logical grouping of frames)

• FC-3 Common Services—Including multicasting

• FC-4 Mapping—Mapping of channel and network services onto

fibre channel• e.g. IEEE 802, ATM, IP, SCSI

Fibre Channel Protocol Architecture (2)

Fibre Channel Physical Media• Provides range of options for physical

medium, the data rate on medium, and topology of network

• Shielded twisted pair, video coaxial cable, and optical fiber

• Data rates 100 Mbps to 3.2 Gbps• Point-to-point from 33 m to 10 km

Fibre Channel Fabric• General topology called fabric or switched topology• Arbitrary topology includes at least one switch to

interconnect number of end systems• May also consist of switched network

—Some of these switches supporting end nodes• Routing transparent to nodes

—Each port has unique address—When data transmitted into fabric, edge switch to which

node attached uses destination port address to determine location

—Either deliver frame to node attached to same switch or transfers frame to adjacent switch to begin routing to remote destination

Fabric Advantages• Scalability of capacity

—As additional ports added, aggregate capacity of network increases

—Minimizes congestion and contention—Increases throughput

• Protocol independent• Distance insensitive• Switch and transmission link technologies may

change without affecting overall configuration• Burden on nodes minimized

—Fibre Channel node responsible for managing point-to-point connection between itself and fabric

—Fabric responsible for routing and error detection

Alternative Topologies• Point-to-point topology

—Only two ports—Directly connected, with no intervening switches—No routing

• Arbitrated loop topology—Simple, low-cost topology—Up to 126 nodes in loop—Operates roughly equivalent to token ring

• Topologies, transmission media, and data rates may be combined

Five Applications of Fibre Channel

Fibre Channel Prospects• Backed by Fibre Channel Association• Interface cards for different applications available• Most widely accepted as peripheral device

interconnect—To replace such schemes as SCSI

• Technically attractive to general high-speed LAN requirements

• Must compete with Ethernet and ATM LANs• Cost and performance issues should dominate

the consideration of these competing technologies