C HAPTER 6 ACCESS METHODS

T YPES 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

P OINT 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

B ROADCAST 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.

L AN 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

C ARRIER 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.

H OW 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.

C ARRIER SENSE MULTIPLE ACCESS WITH COLLISION AVOIDANCE ( CSMA / CA ) Specified by IEEE 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.

C ONTROL 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

C ONTROL TOKEN ( T OKEN R ING – 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

C ONTROL 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!

S LOTTED 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.

H OW 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.

D ISADVANTAGES 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.

C OMPARISON OF E THERNET & T OKEN R ING EthernetToken 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

D EMAND PRIORITY ( IEEE ) 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.

C OMPARISON OF DIFFERENT MEDIUM CONTROL ACCESS METHODS Feature/ function CSMA/CDCSMA/CAToken Passing Demand Priority Communicati on Broadcast based Broadcast based Token based Hub based Access method Contention Non- Contention NetworkEthernetLocal Talk Wireless Token Ring ArcNet 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 spectrum Transmitter 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.