UNIT III

LAN ACCESS TECHNIQUES

Introduction

Network traffic must flow through some form of media, whether it is a cable, or is wireless.

The most common forms of network media are twisted-pair, coaxial, and fiber-optic cable.

Twisted-Pair Cable T-P cable is the most common of all of the media

types in the average local area network (LAN) environment.

Different categories of T-P cable exist. The different categories of cable specify the maximum data bandwidth that the cable can withstand.

T-P comes in two forms, Unshielded (UTP) or Shielded (Plenum/STP).

Twisted-Pair Categories

CategoryMaximum Data Rate

Usual Application

CAT-1 < 1 Mbps POTS & ISDN

CAT-2 4 MbpsIBM Token Ring

CAT-3 16 MbpsVoice/Data -10baseT

Twisted-Pair Categories (cont.)

CAT-4 20 Mbps16Mbps Token Ring Networks

CAT-5 100 Mbps100baseT, 155Mb ATM

CAT-7(in progress)

1000 Mbps1000baseT, Gigabit Ethernet

Twisted-Pair Comparison

Advantages Cheap Easy to implement Easy to manage LOTS of different

applications Easy to terminate

Disadvantages Susceptible to EMF,RF

interference Limited distance – 100

meters

Twisted-Pair (cont.)

Twisted-pair cable (CAT5 and up) consists of 4 separate pairs of wires, all wound separately.

UTP is shown on the right.

Coaxial Cable

Coaxial cable (coax) is almost the same thing that carries your cable TV signal. Data coax is just held to a higher quality.

Historical Tidbit: Coax cable, although not commonly seen nowadays, was how Ethernet was developed!

Coax (cont.)

The physical medium itself consists of an inner wire, surrounded by an insulator, which is also surrounded by a shield.

Coax Applications

Local Area Networks (LANs) Thinnet (10base2) – 200

meters Thicknet (10base5) –

500 meters Baseband transmissions

only

Wide Area Networks (WANs) T3/DS3/E3 Broadband

transmissions

Baseband v. Broadband

Baseband is where the medium only carries one signal on the line.

Broadband carries multiple signals on a single line.

Coax Comparison

Advantages Highly shielded from

EMF,RF interference Signals propagate much

farther than TP cable. Conforms to standards. More channels than TP

cable.

Disadvantages One cable for all computers. To add additional computers,

network must be taken down. MUST properly terminate. Expensive. Low channel count compared

to fiber.

Fiber Optic Cable

Fiber optic cable is where the future of LAN wiring exists.

It is wicked fast. It is WICKED fast!

Fiber Optic Cable (cont.)

Fiber comes in two different types: Multimode – a

channelized fiber-optic circuit. Multiple carrier frequencies.

Singlemode – a “clear channel” circuit. One carrier frequency.

Fiber Comparison

Advantages Wicked fast! Handles lots of

simultaneous B channels.

Very reliable.

Disadvantages Cost to implement.

Splicing kit. Cable costs. Redundancy (FDDI)?

When disaster strikes, it’s a major ordeal.

Point-to-point only

Fiber Applications

High-bandwidth voice transmission. “Backbone” applications. Very fast data transfer between network

devices.

Network Topologies Network topology is the arrangement of

the various elements (links, nodes, etc.) of a computer or biological network.

Physical – actual layout of the computer cables and other network devices

Logical-The way in which the data access the medium and transmits packets is the Logical Topology

Factors

Cost Scalability Bandwidth Capacity Ease of Installation Ease of fault finding and

maintenance

TOPOLOGIES There are three main local area network (LAN)

topologies: Bus Star Ring

Other network topologies include: Mesh &Wireless

Bus Topology

Bus Topology

Bus Topology (2) Network maintained by a single cable Cable segment must end with a

terminator Uses thin coaxial cable (backbones

will be thick coaxial cable) Extra stations can be added in a

daisy chain manner

Bus Topology (3) Standard is IEEE 802.3 Thin Ethernet (10Base2) has a maximum

segment length of 200m Max no. of connections is 30 devices Four repeaters may be used to a total

cable length of 1000m Max no. of nodes is 150

Bus Topology (4) Thick Ethernet (10Base5) used

for backbones Limited to 500m Max of 100 nodes per segment Total of four repeaters , 2500m,

with a total of 488 nodes

Bus Topology Types (5) Linear bus The type of network topology in

which all of the nodes of the network are connected to a common transmission medium which has exactly two endpoints (this is the 'bus', which is also commonly referred to as the backbone, or trunk) 

All data that is transmitted between nodes in the network is transmitted over this common transmission medium and is able to be received by all nodes in the network simultaneously.

Bus Topology Types (6) Distributed bus The type of network topology

in which all of the nodes of the network are connected to a common transmission medium which has more than two endpoints that are created by adding branches to the main section of the transmission medium – the physical distributed bus topology functions in exactly the same fashion as the physical linear bus topology (i.e., all nodes share a common transmission medium)

Bus Topology (7)Advantages Inexpensive to install Easy to add stations Use less cable than

other topologies Works well for small

networks

Disadvantages No longer recommended Backbone breaks, whole

network down Limited no of devices can

be attached Difficult to isolate problems Sharing same cable slows

response rates

Ring Topology

RING TOPOLOGY The ring topology can use twisted pair or fiber optic cabling. A central device (hub) connects hubs and nodes to the

network. Each node connects to its own dedicated port on the

hub. You can connect multiple hubs to form a larger ring.

The ring topology uses the baseband signaling method. Frames are transmitted around the ring from node to hub to

node. Media Access Control (MAC) is used for token passing.

Ring Topology (2) No beginning or end All devices of equality of access to media Single ring – data travels in one direction only. Each device has to wait its turn to transmit Most common type is Token Ring (IEEE 802.5) A token contains the data, reaches the destination,

data extracted, acknowledgement of receipt sent back to transmitting device, removed, empty token passed on for another device to use.

Ring Topology (3)Advantages Data packets travel

at great speed No collisions Easier to fault find No terminators

required

Disadvantages Requires more

cable than a bus A break in the ring

will bring it down Not as common as

the bus – less devices available

Star Topology

STAR TOPOLOGY The star topology can use coaxial, twisted pair, or

fiber optic cable. A central device (hub) connects hubs and nodes to the

network. Each node connects to its own dedicated port on the hub. Hubs broadcast transmitted signals to all connected

devices. You can connect multiple hubs to form a hierarchical star

topology. The star topology uses the baseband signaling method.

Star Topology (2) Like the spokes of a wheel (without the

symmetry) Centre point is a Hub Segments meet at the Hub Each device needs its own cable to the Hub Predominant type of topology Easy to maintain and expand

Star Topology (3) Advantages Easy to add devices as the

network expands One cable failure does not

bring down the entire network (resilience)

Hub provides centralised management

Easy to find device and cable problems

Can be upgraded to faster speeds

Lots of support as it is the most used

Disadvantages A star network requires

more cable than a ring or bus network

Failure of the central hub can bring down the entire network

Costs are higher (installation and equipment) than for most bus networks

Extended Star Topology

A Star Network

which has been

expanded to include an additional

hub or hubs.

Extended Star Topology

Mesh Topology (Web)

Mesh Topology (2) Not common on LANs Most often used in WANs to interconnect

LANS Each node is connected to every other node Allows communication to continue in the

event of a break in any one connection It is “Fault Tolerant”

Mesh Topology (3)Advantages Improves Fault

Tolerance

Disadvantages Expensive Difficult to install Difficult to

manage Difficult to

troubleshoot

Types of Logical Topology Previous slides showed Physical

Topologies Only two Logical Topologies (Bus or

Ring) Physical Bus or Ring easy to

conceptualise

Logical Bus

•Modern Ethernet networks are Star Topologies (physically)

•The Hub is at the centre, and defines a Star Topology

•The Hub itself uses a Logical Bus Topology internally, to transmit data to all segments

Logical BusAdvantages A single node failure

does not bring the network down

Most widely implemented topology

Network can be added to or changed without affecting other stations

Disadvantages Collisions can occur

easily Only one device can

access the network media at a time

Logical Ring Data in a Star Topology can transmit

data in a Ring The MAU (Multistation Access Unit)

looks like an ordinary Hub, but data is passed internally using a logical ring

It is superior to a Logical Bus Hub – see later slide

Logical Ring (2)

Logical Ring (3)Advantages The amount of

data that can be carried in a single message is greater than on a logical bus

There are no collisions

Disadvantages A broken ring will

stop all transmissions

A device must wait for an empty token to be able to transmit

ETHERNET

History The original Ethernet was developed as

an experimental coaxial cable Network to operate with a data rate of 3 Mbps using (CSMA/CD) Protocol.

Success with that project attracted early attention and specification and led to the 1980 joint development of the 10-Mbps Ethernet Version 1.0.

History (cont) The draft standard was approved by the

802.3 working group in 1983 and published as an official standard in 1985.

Since then, a number of supplements to the standard have been defined to take advantage of improvements in the technologies and to support:

1) additional network media 2) higher data rate capabilities

History (cont) Developed by Bob Metcalfe and others at

Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox, DEC, and Intel in 1978 LAN standards define MAC and physical layer

connectivity IEEE 802.3 (CSMA/CD - Ethernet) standard –

originally 2Mbps IEEE 802.3u standard for 100Mbps Ethernet IEEE 802.3z standard for 1,000Mbps Ethernet

What is The Ethernet Ethernet refers to the family of local area

networks (LAN) products covered by the IEEE 802.3 that operates at many speeds.

It defines a number of wiring for the physical layer, through means of Network access at the Media Access Control (MAC)/Data Link Layer, and a Common addressing format.

What is The Ethernet (cont) The combination of the twisted pair versions

of Ethernet with the fiber optic versions largely replacing standards such as coaxial cable Ethernet.

In recent years, Wi-Fi, the wireless LAN standardized by IEEE 802.11, has been used instead of Ethernet for many home and small office networks and in addition to Ethernet in larger installations.

General Description Ethernet was originally based on the idea

of computers communicating over a shared coaxial cable acting as a broadcast transmission medium.

The common cable providing the communication channel was likened to the ether and it was from this reference that the name "Ethernet" was derived.

General Description (cont)

Three data rates are currently Three data rates are currently defined for operation over optical defined for operation over optical fiber and twisted-pair cables:fiber and twisted-pair cables:

10 Mbps—10Base-T Ethernet10 Mbps—10Base-T Ethernet 100 Mbps—Fast Ethernet100 Mbps—Fast Ethernet 1000 Mbps—Gigabit Ethernet1000 Mbps—Gigabit Ethernet

Ethernet Network Elements

Ethernet LANs consist of network nodes and interconnecting media.

The network nodes fall into two major classes: 1) Data terminal equipment (DTE). 2) Data communication equipment (DCE).

The current Ethernet media options include two types of copper cable: (UTP) and (STP), plus several types of optical fiber cable.

Physical Layer Configurations for 802.3 Physical layer configurations are specified in three parts

Data rate (10, 100, 1,000) 10, 100, 1,000Mbps

Signaling method (base, broad) Baseband

Digital signaling Broadband

Analog signaling

Cabling (2, 5, T, F, S, L) 5 - Thick coax (original Ethernet cabling) F – Optical fiber S – Short wave laser over multimode fiber L – Long wave laser over single mode fiber

Ethernet Standard Defines Physical Layer

802.3 standard defines both MAC and physical layer details

Metcalfe’s originalEthernet Sketch

Ethernet Technologies: 10Base2 10: 10Mbps; 2: under 185 (~200) meters cable length Thin coaxial cable in a bus topology

Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!

10BaseT and 100BaseT 10/100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate “star topology”

Distance of any node to hub must be < 100M

Ethernet Overview Most popular packet-switched LAN technology Bandwidths: 10Mbps, 100Mbps, 1Gbps Max bus length: 2500m

500m segments with 4 repeaters

Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or

switch Detects line state and sends/receives signals

Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the

medium Switches break this model

Problem: Distributed algorithm that provides fair access

Ethernet Overview (contd.) Ethernet by definition is a broadcast

protocol Any signal can be received by all hosts Switching enables individual hosts to

communicate Network layer packets are transmitted

over an Ethernet by encapsulating Frame Format

Destaddr

64 48 32

CRCPreamble Srcaddr

Type Body

1648

Data link layer divided into two functionality-oriented sublayers

Taxonomy of multiple-access protocols discussed in this chapter

RANDOM ACCESSRANDOM ACCESS

In In random accessrandom access or or contentioncontention methods, no station is superior to methods, no station is superior to another station and none is assigned the control over another. No another station and none is assigned the control over another. No station permits, or does not permit, another station to send. At station permits, or does not permit, another station to send. At each instance, a station that has data to send uses a procedure each instance, a station that has data to send uses a procedure defined by the protocol to make a decision on whether or not to defined by the protocol to make a decision on whether or not to send. send.

ALOHACarrier Sense Multiple AccessCarrier Sense Multiple Access with Collision DetectionCarrier Sense Multiple Access with Collision Avoidance

Topics discussed in this section:Topics discussed in this section:

ALOHA Protocol ALOHA is developed in the 1970s at the

University of Hawaii. The basic idea is simple:

Let users transmit whenever they have data to be sent.

If two or more users send their packets at the same time, a collision occurs and the packets are destroyed.

ALOHA Protocol

If there is a collision, the sender waits a random amount of time and

sends it again.

The waiting time must be random. Otherwise, the same packets will collide again.

A Sketch of Frame Generation

Note that all packets have the same length because the throughput of ALOHA systems is maximized by having a uniform packet size.

Frames in a pure ALOHA network

Throughput

Throughput: The number of packets successfully transmitted

through the channel per packet time.

What is the throughput of an ALOHA channel?

Assumptions

Infinite population of users New frames are generated according to a

Poisson distribution with mean S packets per packet time. Probability that k packets are generated during a

given packet time:

!]Pr[

k

eSk

Sk

Observation on S

If S > 1, packets are generated at a higher rate than the channel can handle.

Therefore, we expect

0 < S < 1 If the channel can handle all the packets, then

S is the throughput.

Packet Retransmission In addition to the new packets, the stations

also generate retransmissions of packets that previously suffered collisions.

Assume that the packet (new + retransmitted) generated is also Poisson with mean G per packet time.

!]Pr[

k

eGk

Gk

Relation between G and S

Clearly, At low load, few collisions: At high load, many collisions: Under all loads,

where P0 is the probability that a packet does not suffer a collision.

SG SG

SG

0GPS

Vulnerable Period

Vulnerable time for pure ALOHA protocol

Throughput

Vulnerable period: from t0 to t0+2t

Probability of no other packet generated during the vulnerable period is:

Using S = GP0, we get

GeP 20

GGeS 2

Relation between G and S

Max throughput occurs at G=0.5, with S=1/(2e)=0.184.

Hence, max. channel utilization is 18.4%.

Slotted Aloha time is divided into equal size slots (= pkt trans. time) node with new pkt: transmit at beginning of next slot if collision: retransmit pkt in future slots with probability p,

until successful.

Success (S), Collision (C), Empty (E) slots

Slotted ALOHA Divide time up into discrete intervals, each

corresponding to one packet. The vulnerable period is now reduced in half. Probability of no other packet generated during the

vulnerable period is:

Hence,

GeP 0

GGeS

slotted ALOHA

The throughput for slotted ALOHA is S = G × e−G .

The maximum throughput Smax = 0.368 when G = 1.

Note

Vulnerable time for slotted ALOHA protocol

Carrier Sense

In many situations, stations can tell if the channel is in use before trying to use it.

If the channel is sensed as busy, no station will attempt to use it until it goes idle.

This is the basic idea of the Carrier Sense Multiple Access (CSMA) protocol.

CSMA Protocols

There are different variations of the CSMA protocols: 1-persistent CSMA Nonpersistent CSMA p-persistent CSMA

12.85

Behavior of three persistence methods

12.86

Flow diagram for three persistence methods

CSMA: Carrier Sense Multiple Access)CSMA: listen before transmit: If channel sensed idle: transmit entire pkt If channel sensed busy, defer transmission

Persistent CSMA: retry immediately with probability p when channel becomes idle (may cause instability)

Non-persistent CSMA: retry after random interval human analogy: don’t interrupt others!

CARRIER SENSE MULTIPLE ACCESS (CSMA)

•CSMA protocol was developed to overcome the problem found in ALOHA i.e. to minimize the chances of collision, so as to improve the performance.

•CSMA protocol is based on the principle of ‘carrier sense’.

•The chances of collision can be reduce to great extent if a station senses the channel before trying to use it.

• Although CSMA can reduce the possibility of collision, but it cannot eliminate it completely.

•The chances of collision still exist because of propagation delay.

1-Persistent CSMA•In this method, station that wants to transmit data continuously sense the Channel to check whether the channel is idle or busy.

•If the channel is busy , the station waits until it becomes idle.

•When the station detects an idle channel, it immediately transmits the frame with probability 1. Hence it is called 1-persistent CSMA.

•This method has the highest chance of collision because two or more station may find channel to be idle at the same time and transmit their frames.

•When the collision occurs, the stations wait a random amount of time and start all over again.

Drawback of 1-persistent•The propagation delay time greatly affects this protocol. Let us suppose, just after the station 1 begins its transmission, station 2 also become ready to send its data and sense the channel. If the station 1 signal has not yet reached station 2, station 2 will sense the channel to be idle and will begin its transmission. This will result in collision.

Non –persistent CSMA

•A station that has a frame to send senses the channel.

•If the channel is idle, it sense immediately.

•If the channel is busy, it waits a random amount of time and then senses the channel again.

•In non-persistent CSMA the station does not continuously sense the channel for purpose of capturing it when it defects the end of precious transmission .

Advantages of non-persistent•It reduces the chances of collision because the stations wait a random amount of time. It is unlikely that two or more stations Will wait for same amount of time and will retransmit at the same time.

Disadvantages of non-persistent•It reduces the efficiency of network because the channel remains idle when there may be station with frames to send. This is due to the fact that the stations wait a random amount of time after the collision.

p-persistent CSMA

•This method is used when channel has time slots such that the time slot duration is equal to or greater than the maximum propagation delay time.

•Whenever a station becomes ready to send the channel.

•If channel is busy, station waits until next slot.

•If the channel is idle, it transmits with a probability p.

•With the probability q=1-p, the station then waits for the beginning of the next time slot.

•If the next slot is also idle, it either transmits or wait again with probabilities p and q.

•This process is repeated till either frame has been transmitted or another station has begun transmitting.

•In case of the transmission by another station, the station act as though a collision has occurred and it waits a random amount of time and starts again.

Advantages of p-persistent•it reduce the chances of collision and improve the efficiency of the network.

12.94

Space/time model of the collision in CSMA

12.95

Vulnerable time in CSMA

A Comparison

CSMA/CD Protocol Carrier sense multiple access with collision detection (CSMA/CD)

is a Media Access Control method. a carrier sensing scheme is used. a transmitting data station that detects another signal while transmitting

a frame, stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to resend the frame.

The jam signal is a signal that carries a 32-bit binary pattern sent by a data station to inform the other stations that they must not transmit.

CSMA/CD

CSMA/CD is a modification of pure carrier sense multiple access (CSMA). CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, thus shortening the time required before a retry can be attempted.

CSMA/CD collision detection

Main procedure1.Is my frame ready for transmission? If yes, it goes on to the next point.2.Is medium idle? If not, wait until it becomes ready3.Start transmitting.4.Did a collision occur? If so, go to collision detected procedure.5.Reset retransmission counters and end frame transmission.

Collision detected procedure1. Continue transmission until minimum packet time is

reached to ensure that all receivers detect the collision.2. Increment retransmission counter.3. Was the maximum number of transmission attempts

reached? If so, abort transmission.4. Calculate and wait random backoff period based on

number of collisions.5. Re-enter main procedure at stage 1.

APPLICATIONS OF CSMA/CD1.CSMA/CD was used in now obsolete shared media Ethernet variants (10BASE5, 10BASE2) and in the early versions of  twisted-pair Ethernet which used repeater hubs. Modern Ethernet networks built with switches and full-duplex connections no longer utilize CSMA/CD though it is still supported for backwards compatibility. 2.Variations of the concept are used in radio frequency systems that rely on frequency sharing, including Automatic Packet Reporting System.

CSMA/CA Carrier sense multiple access with collision

avoidance (CSMA/CA) in computer networking, is a network multiple access methodin which carrier sensing is used, but nodes attempt to avoid collisions by transmitting only when the channel is sensed to be "idle".

It is particularly important for wireless networks.

Collision avoidance is used to improve the performance of the CSMA method by attempting to divide the channel somewhat equally among all transmitting nodes within the collision domain.

Although CSMA/CA has been used in a variety of wired communication systems, it is particularly useful in wireless LANs where it is not possible to listen while sending,and therefore collision detection is not possible. The popular 802.11 based schemes use CSMA/CA.

IEEE 802.11 MAC Protocol: 802.11 CSMA: sender

- if sense channel idle for DIFS sec.

then transmit entire frame (no collision detection)

-if sense channel busy then binary backoff

802.11 CSMA receiver:

if received OK

return ACK after SIFS

CSMA/CA RTS-CTS

CSMA/CA can optionally be supplemented by the exchange of a Request to Send (RTS) packet sent by the sender S, and a Clear to Send (CTS) packet sent by the intended receiver R. Thus alerting all nodes within range of the sender, receiver or both, to not transmit for the duration of the main transmission. This is known as the IEEE 802.11 RTS/CTS exchange.

Implementation of RTS/CTS helps to partially solve the hidden node problem that is often found in wireless networking.

Hidden Terminal effect

hidden terminals: A, C cannot hear each other obstacles, signal attenuation

collisions at B

goal: avoid collisions at B CSMA/CA: CSMA with Collision

Avoidance

Collision Avoidance: RTS-CTS exchange CSMA/CA: explicit

channel reservation sender: send short RTS: request

to send receiver: reply with short CTS:

clear to send

CTS reserves channel for sender, notifying (possibly hidden) stations

avoid hidden station collisions

Collision Avoidance: RTS-CTS exchange

RTS and CTS short: collisions less likely, of

shorter duration end result similar to collision

detection

IEEE 802.11 alows: CSMA CSMA/CA: reservations polling from AP

IEEE 802 Subgroups and their Responsibilities

802.1 Internetworking

802.2 Logical Link Control (LLC)

802.3 CSMA/CD

802.4 Token Bus LAN

IEEE 802 Subgroups and their Responsibilities (Cont.)

802.5 Token Ring LAN

802.6 Metropolitan Area Network

802.7 Broadband Technical Advisory Group

802.8 Fiber-Optic Technical Advisory Group

Token Passing Standards

IEEE 802.5 For the token-ring LANs

IEEE 802.4 For the token-bus LANs

A FDDI protocol is used on large fiber-optic ring backbones

IEEE 802 Subgroups and their Responsibilities (Cont.)

802.9 Integrated Voice/Data Networks

802.10 Network Security

802.11 Wireless Networks

802.12 Demand Priority Access LANs Ex: 100BaseVG-AnyLAN

INTRODUCTION Token Ring defines a method for sending and

receiving data between two network-connected devices

To communicate in a token-passing environment, any client must wait until it receives an electronic token

The token is a special frame that is transmitted from one device to the next

A Token Bus Network

Token Passing in a Token Bus Network

Token Passing in a Token Bus Network

TOKEN RING

Token ring local area network (LAN) technology is a protocol which resides at the data link layer (DLL) of the OSI model.

It uses a special three-byte frame called a token that travels around the ring.

The Token

Token Data packet that could carry data Circulates around the ring Offers an opportunity for each workstation and

server to transmit data

TOKEN RING

Token ring LAN are logically organized in a ring topology with data being transmitted sequentially from one ring station to the next with a control token circulating around the ring controlling access.

This token passing mechanism is shared by ARCNET, token bus, and FDDI, and has theoretical advantages over the stochastic CSMA/CD of Ethernet.

Token Bus

Server

Client Client Client

Token

A token is distributed to each client in turn.

TOKEN FRAME

When no station is transmitting a data frame, a special token frame circles the loop.

This special token frame is repeated from station to station until arriving at a station that needs to transmit data.

TOKEN FRAME

When a station needs to transmit data, it converts the token frame into a data frame for transmission. Once the sending station receives its own data frame, it converts the frame back into a token.

TOKEN FRAME PRIORITY Token ring specifies an optional medium access scheme allowing

a station with a high-priority transmission to request priority access to the token.

8 priority levels, 0–7, are used. When the station wishing to transmit receives a token or data

frame with a priority less than or equal to the station's requested priority, it sets the priority bits to its desired priority. The station does not immediately transmit; the token circulates around the medium until it returns to the station.

FRAME FORMAT

A data token ring frame is an expanded version of the token frame that is used by stations to transmit media access control (MAC) management frames or data frames from upper layer protocols and applications.

Figure 12-6

Token Bus Frame

Frame

Local Area Network Technology There are two types of token-passing

architectures: Token Bus is similar to Ethernet because all

clients are on a common bus and can pick up transmissions from all other stations

Token Ring is different from Token Bus in that the clients are set up in a true physical ring structure

Token Bus Data Pickup A token is sent from one node to the other The client wanting to transmit grabs an empty

token Data is attached Token leaves for the next node and its travel

on the bus until it reaches the address to which the data is destined

Token Bus Data Delivery Token delivers the data to the addressee Acknowledgement is returned to the sender Token is passed to the next node The process continues If there is an error in delivering the information, a

request for retransmission attached to the token and it is sent to the sender

Token Bus Standard and Applications

IEEE 802.4 It can be used in both broadband and

baseband transmission

Token Passing Protocol in Operation

D

A

B

C

Circulating

Token

Server Workstation

Workstation•No collisions

Comparison with CSMA/CD Absence of collision Offers a systematic method of transmitting

information In theory, it is superior to CSMA/CD More sophisticated to implement Protocols used in the newer and most popular

networks are, however, based on CSMA/CD

The Transmitting Workstation

Waits for a free token in order to be able to attach the data to be transmitted to the token

On finding a free token, attach the following: Sender’s address Receiver’s address Data block to be transmitted Error checking details etc.

At the Receiving End

Data is received and checked for errors Outcomes at the receiving end

Data received without errors Date received with errors

Error-free Delivery of Data An acknowledgment is attached to the token Acknowledgment is passed to the sender Token is set free for other nodes to transmit

information At this time, the next workstation on the ring

will receive an opportunity

Correcting Errors in Delivery

A request for retransmission is attached to the token

Token carries the message for retransmission to the sender

The data is thus retransmitted

Token Regeneration

The token is regenerated at regular intervals to sustain the timing of circulation of the token

Usage of Token Passing Used extensively in ring LANs

Especially in the IBM token-ring LAN A version of this protocol is also used on

certain types of bus LANs Token-bus networks

Used in large fiber-optics backbones Used for the construction of very large networks

Usage in Practice

Used in backbones Uses in a number of IBM shops Overall, the usage of Ethernet surpasses the

usage of Token-Ring networks that are based on the Token-Passing protocol