CHAPTER 6ACCESS METHODS

TYPES OF LINKS

Data communication may involve single transmitter with 1 or more receivers, or multiple receivers with 1 or more receivers.

Communication channel that connects adjacent computers along the communication path is called a LINK.

2 types of network links: Point-to-point link Broadcast link

POINT TO POINT LINK

Involves only ONE transmitter and ONE receiver or a link which directly connects transmitter and receiver.

Data transmission on this kind of link is called point-to-point transmission.

Sender only transmits data that is intended to be used by specific receiver

BROADCAST LINK Involves single transmitter with multiple receivers

or multiple transmitters with multiple receivers. Data transmission between this link is called

broadcast transmission. Sends signal to all direction, all receivers whether

they use it or not. They are simple & quick to implement

Ex: TV stations transmitting signal from its tower to thousand of homes.

LAN MEDIA ACCESS METHODS Data on network media must follow certain rules to

avoid collisions with other data. Multiple devices & computers in a LAN cannot talk

simultaneously over same media at same time as it would result in media contention(conflict)

Access methods ensure that the sending and receiving of network data is an orderly process(regulate the flow).

Access methods; Carrier Sense Multiple Access with Collision Detection

(CSMA/CD) Carrier Sense Multiple Access with Collision Avoidance

(CSMA/CA) Token Passing Demand Priority

CARRIER SENSE MULTIPLE ACCESS WITH COLLISION DETECTION (CSMA/CD)

When 2 computers send data over the media at the same time, it will cause collision & the data become unusable.

To avoid it use CSMA/CD (IEEE 802.3) protocol. CSMA/CD forces computers to ‘listen’ to the Ethernet

media to make sure no other computer on the same media is transmitting data before sending their own data.

If a collision is heard, both computers will send a jam signal over the Ethernet which will indicate the jam to all other computers on the same LAN so that no one will send data on the same media.

Then they will wait a random amount of time before repeating the entire process & it helps to avoid the same from happening again.

HOW CSMA/CD WORKS? All computers stay quiet if another computer is

talking Any computer that wants to talk, have to listen to

the media first & wait for a predetermined time If after waiting, the media is silent, then they can

send data If both computer wait & both talk together, a

collision will occur. Other computers recognize the collision & re-

enforce it by talking Then, all computers stop talking Each computer picks a random number & counts

backward to zero. When either one finish first, and begins talking, it

will cause other computers to wait.

CARRIER SENSE MULTIPLE ACCESS WITH COLLISION AVOIDANCE(CSMA/CA)

Specified by IEEE 802.11 standard especially for wireless LANs.

It is not possible for wireless LANs to listen & send data at the same time, therefore collision detection is not possible in wireless LANs, therefore CSMA/CA is used

Other reason is the hidden terminal problem in which one computer may not know whether the other computer is transmitting or not if they are not in range.

Backoff factor = determines the amount of the time the computer must wait until its allowed to transmit data.

Not suitable for radio frequency transmissions.

CONTROL TOKEN

Ina token network (either token bus / token ring) control token is send over the network, & only the computer having the token can send data over the network.

Once its done sending data, it releases the token over the network & other computers will poses it.

2 types of control tokens are: Token Bus Token Ring

CONTROL TOKEN ( TOKEN RING – IEEE 802.5)

Works by passing a single token from one computer to another over the network.

To send data, a computer must wait for the token to reach it, then attach the data to the token & returns both the data & token to the network.

Token passing are Deterministic, which means that its possible to calculate the maximum time that will pass before the any end station capable of transmitting.

Ideal for applications in which delay must be predictable, & robust network operation is important

Ex: factory automation

CONTROL TOKEN ( TOKEN BUS – IEEE 802.4, ISO 8802/4)

Physically the network is a bus, however logically its treated as a ring.

Only the computer with the token has the right to transmit data for a certain time.

Once the time ends, it passes the token to the following station.

No collision & full bandwidth utilization. Workstations also has priority system in which

data is categorized according to its priorities, so when the station has the right, it will transmit the data with highest priority ie, audio, video

The end of the networks never meets!

SLOTTED RING

Basically a ring networks that allows unidirectional data transmission between data stations by transffering data in a predefined slots in the transmission stream over one transmission medium.

HOW SLOTTED RING NETWORK WORKS? Each station contains a shift register to introduce delay. Monitor station initiates the ring by inserting a stream

of continuously circulating bits The bit stream is divided into a fixed number of slots,

each capable of carrying a small frame Initially all slots are marked empty by the monitor When a station wants to transmit, it wait for the empty

slot, fills slot with frame containing source/destination address & data

When frame passes destination station, it is copid into buffer & the response bits at the end of the frame is altered accordingly (received, ignored, selected)

The frame returns to source station which then marks the slot as empty again.

DISADVANTAGES OF SLOTTED RING

Special (vulnerable) monitor node is required to maintain the slot structure

The transmission of a typical link level frame requires several slots, since each slot only contains 16 bit of data.

COMPARISON OF ETHERNET & TOKEN RINGEthernet Token ring

CSMA/CD offers simplest, widely available mechanism that gives lowest average delays for a lightly loaded LAN

Token ring gives each station equal opportunity on heavily loaded LAN while CSMA/CD will fail due to multiple collision

Access to channel on CSMA/CD is probabilistic – it has no priority mechanism

Token passing has average longer delay, but its deterministic under heavy loads. Hence used in MAP & real time applications

CSMA/CD uses a passive transmission medium (regenerators are not required at each station) therefore inherently more reliable

Token ring interface is active – hence require mechanism to cope with stations being switched off / failing

DEMAND PRIORITY (IEEE 802.12)

New access method designed for the 100Mbps Ethernet standard known as 100VG-AnyLAN.

Its based on the fact that repeaters & end notes are the two components that make up all 100VG-AnyLAN networks.

Repeaters manages network access by doing round-robin searches for requests to send from all nodes on the network.

Repeater/hub is responsible for noting all addresses, links & end-nodes and verifying that they are all functioning.

COMPARISON OF DIFFERENT MEDIUM CONTROL ACCESS METHODS

Feature/function

CSMA/CD CSMA/CA TokenPassing

Demand Priority

Communication

Broadcast based

Broadcast based

Token based

Hub based

Access method

Contention

Contention

Non-Contention

Contention

Network Ethernet Local TalkWireless

Token RingArcNet

100VG – AnyLAN

MULTIPLEXING

3 types : TDMA, CDMA & FDMA

Time Division Multiple Access (TDMA)Method for allocating access to communication

channels.Allow large number of users to access a single-

radio-frequency channel without interference.The frequency band is divided into a number of

channels, which are stacked onto short time units, so several users can share single channel without interfering.

Used in Global System for Mobile Communications(GSM) & Personal Digital Cellular (PDC) systems.

Also used in satellite systems, LANs, physical security systems & other combat-net radio systems.

MULTIPLEXING

Frequency Division Multiple Access (FDMA)Oldest & most important of the 3 main ways for

multiple radio transmitters to share the radio spectrum.

Other methods are TDMA & Code Division Multiple Access(CDMA)

Given radio frequency is divided into smaller frequency

A control mechanism is used to ensure 2 or more earth stations do not transmit in the same subdivision at the same time, so phones use different allocated frequency at the same time.

MULTIPLEXING

Code Division Multiple Access (CDMA)Also called spread spectrumTransmitter encodes the signal using a pseudo-

random sequence.The sequence is known by the receiver to decode

the received signal.Each different random sequence corresponds to

different communication channel.CDMA is not a modulation scheme like TDMA or

FDMA, so it does not divide the channel by time or frequency.

Used in GPS and satellite system.