CSC 335 Data Communications and Networking Lecture 7b: Local Area Networking Dr. Cheer-Sun Yang Fall 2000.

CSC 335 Data Communications

and Networking

Lecture 7b: Local Area Networking

Dr. Cheer-Sun Yang

Fall 2000

Topologies

• Bus: A single communication line, typically a twisted pair, coaxial cable, or optical fiber, represents the primary medium.

• Ring: packets can only be passed from one node to it’s neighbor.

• Star: A hub or a computer is used to connect to all other computers.

• Tree: no loop exists (logical connection).

Token Passing

• Token Ring (802.5) : P. 183, Section 6.3

• Token Bus (802.4) : P. 186, Section 6.4

Token Passing

• The difficulty with many networks is that no central control or authority makes decisions on who sends when.

• Token passing is designed to deal with this issue and hopefully the link utilization can be increased.

Token Passing

• In order to send, a station must obtain an admission pass, called a token.

• In a token ring, the token is passed from one station to another.

• When a station does not need it, it simply passes it on.

• Token ring network must pass the token orderly to it’s neighbor.

• Token bus network can pass a token to any other station directly.

Token Passing

• However, a token bus network cannot be added as simply as with the CSMA/CD bus.

• All stations must know who and where its neighbor is in a token bus.

6.3 Token Ring: IEEE 802.5• 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

Token Ring (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

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 in development

Ring Repeater States

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 Media

• Twisted pair

• Baseband coaxial

• Fiber optic

• Not broadband coaxial– Would have to receive and transmit on multiple

channels, asynchronously

Two observations

1. Ring contention is more orderly than with an Ethernet. No wasted bandwidth.

Two observations

2. The failure of one station can cause network failure. More discussion will be provided in next slide.

Potential Ring Problems

• Break in any link disables network• Repeater failure disables network• Installation of new repeater to attach new station

requires identification of two topologically adjacent repeaters

• Timing jitter• Method of removing circulating packets required

– With backup in case of errors

• Mostly solved with star-ring architecture (the wire center approach).

Network Failure Problem

The failure of one station can cause network failure: This problem can be solved by using a wire center (Fig. 6.11). Instead of connecting neighboring stations directly, they all communicate through a wire center. The wire center contains a bypass relay. If a station fails, the bypass relay will allow a frame to bypass the station.

This architecture is called a Star Ring Architecture.

Star Ring Architecture

• Feed all inter-repeater links to single site– Concentrator– Provides central access to signal on every link– Easier to find faults– Can launch message into ring and see how far it gets– Faulty segment can be disconnected and repaired later– New repeater can be added easily– Bypass relay can be moved to concentrator– Can lead to long cable runs

• Can connect multiple rings using bridges

Timing Jitter• Clocking included with signal

– e.g. differential Manchester encoding– Clock recovered by repeaters

• To know when to sample signal and recover bits• Use clocking for retransmission

– Clock recovery deviates from midbit transmission randomly• Noise• Imperfections in circuitry

• Retransmission without distortion but with timing error• Cumulative effect is that bit length varies• Limits number of repeaters on ring

Solving Timing Jitter Limitations

• Repeater uses phase locked loop– Minimize deviation from one bit to the next

• Use buffer at one or more repeaters– Hold a certain number of bits– Expand and contract to keep bit length of ring

constant

• Significant increase in maximum ring size

Token Ring MAC Frame

Token and Frame Formats

• Start Delimiter (SD), End Delimiter (ED): 1 octet• Access Control (AC) : 1 octet, 3 priority bits, 1

token bit, 1 monitor bit, 3 reserved bits.• Frame Control (FC): used to distinguish control

frame from data frame.• Frame Status(FS): 1 octet (acxxacxx) A: address

recognized bit, C: frame copied bit, X: undefined bit. – A = 0, C=0: dest not present or not power up– A = 1, C = 0: dest present but frame is not accepted– A = 1, C = 1: dest present and frame copied.

Reserving and Claiming Tokens

C

B

D

A token

Reserving and Claiming Tokens

C

B

D

A

Station A requests the token and sends its data to D

Reserving and Claiming Tokens

C

B

D

A

Station C can reserve the next open tokenBy entering its priority code in the AC field.

Reserving and Claiming Tokens

C

B

D

A

Station D copies the frame and sends the data back to the ring.

Reserving and Claiming Tokens

C

B

D

A

Station A receives the frame and releases the token

Reserving and Claiming Tokens

C

B

D

A

Station C can send its data now.

Token RingOperation

Disadvantage of Token Ring

• Token maintenance requires extra work.

• Loss of token prevents further utilization of the ring.

• Duplication token can disrupt the operation.

• A monitor station is required. It becomes a crucial point for a single point failure.

Advantage of Token Ring

• The flexible control over access that it provides.

• The access is fair.

• It is easy to provide priority and guaranteed bandwidth services.

Priority Scheme

1. A station having a higher priority frame to transmit than the current frame can reserve the next token for its priority level as the frame passes by.

2. When the next token is issued at a station A, it will be at the reserved priority level. The station reserving the token can use this token to transmit data frame.

3. The station A is responsible to down-grade the priority of the token later.

Priority Scheme

Priority Scheme

• A sends a frame to B at priority 0.• When the frame passes by D, D makes a

reservation at priority 3.• When the token is sent back to A, A changes the

priority to 3 and issues a new token.• D can use this token to send a frame to any station.• After the data is seized by the destination and the

token is passed back to A, A is responsible for changing the priority back to 0. (Why A?)

Time Limits

• Token holding time: the time duration a station is allowed to hold the token

• Token rotation time: the total time a token is allowed to rotate around the ring.

• TRT >= N * THT

Ring Maintenance

Things can go wrong. For example:

1. A station sends a short frame over a long ring and subsequently crashes. It is not able to drain the token. This frame is called an orphan frame.

2. A station receives a frame or token crashes before it can send it. Now there is no token circulating.

3. Line noise damages a frame.

Ring Maintenance

Some problems can be handled by giving one of the stations a few different responsibilities and designating it a monitor station.

1. When a monitor station receives a frame, it sets the monitor bit to 1. If the frame is received the second time and the monitor bit is still set to 1, the monitor station deletes the frame.

Ring Maintenance

2. The monitor station also detect a lost token using a built-in timer which is determined based on the length of the ring, number of stations, and maximum frame size. Whenever the monitor sends a frame or token, it starts the timer. If the monitor does not receive another frame or token before the timer expires, it assumes that the token is lost. It then creates another one.

Ring Maintenance

Some problems cannot be solved even with a monitor station. For example, what if the malfunction station is the monitor station? What if a break in the ring causes a lack of tokens? Sending new ones does nothing to correct the problem. These problems are handled using control frames.

Ring Maintenance

Some example control frames:• Claim token frame – for submitting bids to elect

a monitor station.• Active monitor present (AMP) frame – to notify

others that a monitor station has been produced.• Standby monitor present (SMP) – frame.• Beacon frame – to inform stations that a

problem has occurred and the token-passing protocol has stopped.

Ring Efficiency

T1 = time to send a frame

T2 = time to send a token

%100TT

TU

21

1

Other Ring Networks: FDDI

• 100Mbps

• LAN and MAN applications

• Token Ring

FDDI MAC Frame Format

FDDI MAC Protocol

• As for 802.5 except:

• Station seizes token by aborting token transmission

• Once token captured, one or more data frames transmitted

• New token released as soon as transmission finished (early token release in 802.5)

FDDI Operation

FDDI Physical Layer

• Medium Optical Fiber Twisted Pair

• Data rate 100 100

• Signaling 4B/5B/NRZI MLT-3

• Max repeaters 100 100

• Between repeaters 2km 100m

LAN Generations

• First– CSMA/CD and token ring– Terminal to host and client server– Moderate data rates

• Second– FDDI– Backbone– High performance workstations

• Third– ATM– Aggregate throughput and real time support for multimedia

applications

Third Generation LANs

• Support for multiple guaranteed classes of service– Live video may need 2Mbps– File transfer can use background class

• Scalable throughput– Both aggregate and per host

• Facilitate LAN/WAN internetworking

ATM LANs

• Asynchronous Transfer Mode• Virtual paths and virtual channels• Preconfigured or switched• Gateway to ATM WAN• Backbone ATM switch

– Single ATM switch or local network of ATM switches

• Workgroup ATM– End systems connected directly to ATM switch

• Mixed system

Example ATM LAN

ATM LAN HUB

Compatibility

• Interaction between end system on ATM and end system on legacy LAN

• Interaction between stations on legacy LANs of same type

• Interaction between stations on legacy LANs of different types

Fiber 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

Fiber 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

Fiber Channel Elements

• End systems - Nodes

• Switched elements - the network or fabric

• Communication across point to point links

Fiber Channel Network

Fiber 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

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

Fiber Channel Protocol Architecture (2)

Wireless LANs

• IEEE 802.11• Basic service set (cell)

– Set of stations using same MAC protocol– Competing to access shared medium– May be isolated– May connect to backbone via access point (bridge)

• Extended service set– Two or more BSS connected by distributed system– Appears as single logic LAN to LLC level

Types of station

• No transition– Stationary or moves within direct communication range

of single BSS

• BSS transition– Moves between BSS within single ESS

• ESS transition– From a BSS in one ESS to a BSS in another ESS

– Disruption of service likely

Wireless LAN - Physical

• Infrared– 1Mbps and 2Mbps– Wavelength 850-950nm

• Direct sequence spread spectrum– 2.4GHz ISM band– Up to 7 channels– Each 1Mbps or 2Mbps

• Frequency hopping spread spectrum– 2.4GHz ISM band– 1Mbps or 2Mbps

• Others under development

Media Access Control

• Distributed wireless foundation MAC (DWFMAC)

• Distributed coordination function (DCF)– CSMA– No collision detection

• Point coordination function (PCF)– Polling of central master

802.11 MAC Timing

Interconnecting LANs

• Layer 1 connection – repeaters

• Layer 2 connection - bridges

Repeaters

• Layer 1 connections

• Used to expand physical length of a cable when it exceeds the distance limit and attenuation can occur.

Bridges

• Ability to expand beyond single LAN• Provide interconnection to other LANs/WANs• Use Bridge or router• Bridge is simpler

– Connects similar but different types of 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

Types of Bridges

• Transparent bridges

• Spanning tree bridges

• Source routing bridges

Transparent Bridges• Complex large LANs need alternative routes

– Load balancing– Fault tolerance

• Bridge(NOT the source) 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

Multiple LANs

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

Source Routing Bridges

• Although source routing bridges can be used with any type of LAN segment, they are used primarily for the interconnection of token ring LAN segments.

• The spanning tree bridges perform the routing in a way that is transparent to the end stations. Conversely, with source routing, the end stations perform the routing function.

• The necessary information must be included in a frame.

Comparison of LAN Bridges

• See Table 6.8

Reading

• Chapter 6: 6.1-6.5