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: • Contention • Polling • Token passing Different access methods have different overhead effects on network traffic. Contention • With contention systems, network devices may transmit whenever they want. • With this technique, no control is exercised to determine whose turn it is, all stations contend for time. No referee mandates when 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, without considering what other stations are doing. • Unfortunately, the "transmit whenever ready" strategy has one important shortcoming. • Stations can transmit at the same time. • When this happens, the resulting co-mingling of signals usually damages both to the point that a frame's information is lost. • This unhappy event is called a "collision." Collision • Newer contention protocols were developed that called for stations to listen to the channel first before transmitting. • If the listening station detects a signal, it will not start transmitting and try again later. • These protocols are called CSMA (Carrier Sense, Multiple Access with collision detection) protocols. • These protocols will reduce collisions. • However, collisions may still occur when two stations sense the cable, detect nothing, and subsequently transmit. • In order to reduce collisions, CSMA/CD protocols compute a random backoff time before retransmitting the frame (as shown in the flow diagram). Examples of CSMA/CD protocols : IEEE 802.3 (Ethernet ) CSMA / CD collision schematic Frame format of a CSMA/CD bus Networks Preamble 7 octets SFD 1 octet Destination address 2 or 6 octets Source address 2 or 6 octets Length indicator 2 octets Data 2- octet field <= 1500 Pad (optional) Frame Check 4 octets Sequence Operational parameters of a CSMA/CD bus Networks Bit rate 10 Mbps (Manchester encode) Slot time 512 bit times Interframe gap 9.6 ms Attempt limit 16 Backoff limit 10 Jam size 32 bits Maximum frame size 1518 octets Minimum frame size 512 bits 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 10101010). • SFD (start of frame delimiter)=> Single octet 10101011, signals the start of a valid frame t the receiver, immediately follows the preamble. 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. CSMA/CD operation: a) transmit CSMA / CD operation: b) receive Advantages • CSMA/CD control software is relatively simple and produces little overhead. • CSMA/CD network works best on a bus topology with bursty transmission. Bursty traffic is characterized by short, sporadic transmissions. Example: interactive terminal-host traffic. • This technique is efficient for light to moderate load. Disadvantages • CSMA/CD protocols are probabilistic and depends on the network (cable) loading. Performance tends to collapse under heavy load. • Considered unsuitable for channels controlling automated equipment that must have certain control over channel access. • We cannot set priorities to give faster access to some devices. Polling access method • Polling is an access method that designates one device (called a "controller", "primary", or "master") as a channel access administrator. • This device (Master) queries each of the other devices (“secondaries”) in some predetermined order to see whether they have information to transmit. • If so, they transmit (usually through the master). Polling access method • Secondaries may be linked to the master in many different configurations. • One of the most common polling topologies is a star, where the points of the star are secondaries and the master is the hub. • To get data from a secondary, the master addresses a request for data to the secondary, and then receives the data from the secondary sends (if secondary sends any). • The primary then polls another secondary and receives the data from the secondary, and so forth. • System limits how long each secondary can transmit on each poll. Advantages • Polling centralizes channel access control. • Maximum and minimum access times and data rates on the channel are predictable and fixed. • Priorities can be assigned to ensure faster access from some secondaries. • When many stations have data to transmit over an extended period of time, round-robin techniques can be very efficient. • If only a few stations have data to transmit over an extended period of time, then there is a considerable overhead in passing the turn from station to station, because most of the stations will not transmit but simply pass their turns. • Polling is deterministic and is considered suitable for channels controlling some kinds of automated equipment. Disadvantages • Polling systems often use a lot of bandwidth sending notices and acknowledgments or listening for messages. • Line turnaround time on a half- duplex line further increases time overhead. • This overhead reduces both the channel's data rate under low loads and its throughput. Token passing System • In token-passing systems, a small frame (the token) is passed in an orderly fashion from one device to another. • A token is a special authorizing message that temporarily gives control of the channel to the device holding the token. • Passing the token around distributes access control among the channel's devices. • Each device knows from which device it receives the token and to which device it passes the token.(see fig.) • Each device periodically gets control of the token, performs its duties, and then retransmits the token for the next device to use. • System rules limit how long each device can control the token. Control token MAC: Token ring Token Ring • Token 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). Token Ring Token format & Frame format in Token Ring Token Format: 1 1 1 octets SD AC ED Frame Format: octets 1 1 1 2/6 2/6 <5000 4 1 1 SD AC FC DA SA INFO FCS ED FS Start of frame FCS coverage End of frame Field Descriptions of a Token Ring JKOJKOOO Start delimiter (SD) JK1JK1 I E End delimiter (ED) PPPT M RR R Access control (AC) FF ZZZZZZ Frame control (FC) Source and destination I/G 15/47 bit address Address (SA/DA) AC xx ACxx Frame status (FS) Control token MAC: Token bus Slotted ring principles: bit definitions of each slot Slotted ring principles:Outline topology Advantages • Even 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. Disadvantages • Components 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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