Presentation on theme: "Channel Access Methods"— Presentation transcript:
1 Channel Access Methods When several devices are connected to a single channel, there must be some rules to govern these devices when they access, transmit, and release the channel .There are three basic channel access methods to provide for an orderly and efficient use of that capacity:ContentionPollingToken passingDifferent access methods have different overhead effects on network traffic.
2 Contention With contention systems, network devices may transmit whenever they want.With this technique, no control is exercised to determine whoseturn it is, all stations contend for time. No referee mandateswhen a device may or may not use the channel.This scheme is simple to design.The scheme provides equal access rights to all stations.Stations simply transmit whenever they are ready, withoutconsidering what other stations are doing.Unfortunately, the "transmit whenever ready" strategy has oneimportant shortcoming.Stations can transmit at the same time.When this happens, the resulting co-mingling of signals usuallydamages both to the point that a frame's information is lost.This unhappy event is called a "collision."
3 CollisionNewer contention protocols were developed that called for stationsto listen to the channel first before transmitting.If the listening station detects a signal, it will not starttransmitting and try again later.These protocols are called CSMA (Carrier Sense, Multiple Accesswith collision detection) protocols.These protocols will reduce collisions.However, collisions may still occur when two stations sense thecable, detect nothing, and subsequently transmit.In order to reduce collisions, CSMA/CD protocols compute arandom backoff time before retransmitting the frame (as shown inthe flow diagram). Examples of CSMA/CD protocols :IEEE (Ethernet )
5 Frame format of a CSMA/CD bus Networks PreambleSFDDestination addressSource addressLength indicatorDataPad (optional)Frame Check Sequence7 octets1 octet2 or 6 octets2 or 6 octets2 octets2- octet field<= 15004 octets
6 Operational parameters of a CSMA/CD bus Networks Bit rate10 Mbps (Manchester encode)Slot time512 bit timesInterframe gap9.6 msAttempt limit16Backoff limit10Jam size32 bitsMaximum frame size1518 octetsMinimum frame size512 bits
7 Frame format & operational parameters A medium access control (MAC) unit is responsible for the encapsulation and de-encapsulation of frames for transmission and reception on the cable, error detection, and implementation of MAC algorithm.Each frame consists of 8 fields. All the fields are of fixed length except the data and padding fields.Preamble=> Sent at the head of all frames. ?used to achieve bit synchronization before the actual frame contents are received. It is a sequence of seven octets. (each equal to the binary pattern ).SFD (start of frame delimiter)=> Single octet , signals the start of a valid frame t the receiver, immediately follows the preamble.
8 Frame format & operational parameters Destination & Source addresses => Specify the intended destination station & originating station. Each address can be either 16 or 48 bits. If the first bit in the address field is 0, it specifies the address is an individual address and the transmitted frame is intended for a single destination. If the bit is 1, it specifies a group address and the frame is intended either for a logically related group or for all others stations. In this case, the address field is set to all binary 1s.Length indicator => Specifies the number of octets in the data field.Pad => If the value of length indicator is less than the minimum frame size, a sequence of octets is added, known as padding.FCS => Contains CRC value that is used fir error detection.
11 Advantages CSMA/CD control software is relatively simple and produces little overhead.CSMA/CD network works best on a bustopology with bursty transmission. Burstytraffic is characterized by short, sporadictransmissions. Example: interactiveterminal-host traffic.This technique is efficient for light tomoderate load.
12 Disadvantages CSMA/CD protocols are probabilistic and depends on the network (cable) loading. Performance tends to collapseunder heavy load.Considered unsuitable for channels controlling automatedequipment that must have certain control over channelaccess.We cannot set priorities to give faster access to somedevices.
13 Polling access methodPolling is an access method that designates one device(called a "controller", "primary", or "master") as a channelaccess administrator.This device (Master) queries each of the other devices(“secondaries”) in some predetermined order to seewhether they have information to transmit.If so, they transmit (usually through the master).
14 Polling access methodSecondaries may be linked to the master in many differentconfigurations.One of the most common polling topologies is a star, where thepoints of the star are secondaries and the master is the hub.To get data from a secondary, the master addresses a request fordata to the secondary, and then receives the data from thesecondary sends (if secondary sends any).The primary then polls another secondary and receives the datafrom the secondary, and so forth.System limits how long each secondary can transmit oneach poll.
15 Advantages Polling centralizes channel access control. Maximum and minimum access times and data rates on thechannel are predictable and fixed.Priorities can be assigned to ensure faster access from somesecondaries.When many stations have data to transmit over an extendedperiod of time, round-robin techniques can be very efficient.If only a few stations have data to transmit over an extendedperiod of time, then there is a considerable overhead inpassing the turn from station to station, because most of thestations will not transmit but simply pass their turns.Polling is deterministic and is considered suitable forchannels controlling some kinds of automated equipment.
16 DisadvantagesPolling systems often use a lot of bandwidth sending noticesand acknowledgments or listening for messages.Line turnaround time on a half- duplex line further increasestime overhead.This overhead reduces both the channel's data rate under lowloads and its throughput.
17 Token passing SystemIn token-passing systems, a small frame (the token) ispassed in an orderly fashion from one device to another.A token is a special authorizing message that temporarilygives control of the channel to the device holding the token.Passing the token around distributes access control amongthe channel's devices.Each device knows from which device it receives the tokenand to which device it passes the token.(see fig.)Each device periodically gets control of the token, performsits duties, and then retransmits the token for the next deviceto use.System rules limit how long each device can control thetoken.
19 Token RingToken ring networks are primarily used in technical and office environments.Whenever a station wishes to send a frame, it first waits for the token. When the station gets the token, it start sending frame.The intended recipient retains a copy of the frame and indicates by setting the response bits at the tail of the frame.A station releases the token in one of the two ways depending on the bit rate of the ring.In slower ring (4 Mbps), the token is released only after the response bits have been received.In higher speed rings (16 Mbps), the token is released after transmitting the last bit of a frame (early token).
21 Token format & Frame format in Token Ring 111octetsSDACEDFrame Format:octets<50001112/62/6411SDACFCDASAINFOFCSEDFSStart of frameFCS coverageEnd of frame
22 Field Descriptions of a Token Ring Start delimiter (SD)J K O J K O O OEnd delimiter (ED)J K 1 J K 1 I EP P P T M R R RAccess control (AC)FF Z Z Z Z Z ZFrame control (FC)Source and destinationAddress (SA/DA)I/G /47 bit addressAC xx ACxxFrame status (FS)
26 AdvantagesEven though there is more overhead using tokens than using CSMA/CD, performance differences are not noticeable with light traffic and are considerably better with heavy loads because CSMA/CD will spend a lot of time resolving collisions.A deterministic access method such as Token Ring guarantees that every node will get access to the network within a given length of time. In probabilistic access method (such as CSMA/CD) nodes have to check for network activity when they want to access the network.
27 DisadvantagesComponents are more expensive than for Ethernet or ARCnet.Token Ring architecture is not very easy to extend to wide-area networks (WANs).Token Ring network is much more expensive than Ethernet. This is due to the complex token passing protocol.
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