Local Area Network Technology PDF

Chapter Five Local Area Networking Technology  Introduction A local area network (LAN) is a data communication network that serves users in a confined geographic area and uses high transmission speeds (typically 10 mbps to few gbps) Designed and developed for communi...

Chapter Five Local Area Networking Technology  Introduction A local area network (LAN) is a data communication network that serves users in a confined geographic area and uses high transmission speeds (typically 10 mbps to few gbps) Designed and developed for communications and resource sharing in a local work environment (room, campus, building). A single shared medium, usually a cable, to which computers can attach. Because sharing occurs: – Cost decreases – Computers have to coordinate the use of the network LANs operate at the physical and data link layers 2  In OSI terms, higher layer protocols (layer 3 or 4 and above) are independent of network architecture and are applicable to LANs, MANs, and WANs LAN protocols is concerned principally with lower layers (Layer 1 and 2) of the OSI model IEEE 802 reference model 3  the lowest layer of the IEEE 802 reference model corresponds to the physical layer Includes functions: – Encoding/decoding of signals – Preamble generation/removal (for synchronization) – Bit transmission/reception 4  The functions associated with providing service to LAN users include: – On transmission, assemble data into a frame with address and error-detection fields. – On reception, disassemble frame, perform address recognition and error detection. – Govern access to the LAN transmission medium. – Provide an interface to higher layers and perform flow and error control 5  Access Methods broadcast channels are sometimes referred to as multi- access channels or random access channels. The methods that can be used to determine how the shared media is accessed are called Access methods. The protocols used to determine who goes next on a multi- access channel. – It belongs to the sub-layer of the data link layer called MAC (Medium Access Control) sub-layer The MAC sub-layer is especially important in LANs, – many of LANs use a multi-access channel as the basis for communication. – WANs, in contrast, use point-to-point links, except for satellite networks 6  Pure ALOHA – The earliest of access methods – was developed for packet radio networks – is also applicable to any shared transmission medium – is a true free-for-all – The station waits for an amount of time (The maximum round-trip propagation delay on the network) – twice the time it takes to send a frame between the two most widely separated stations plus a small fixed time increment – If the station fails to receive an acknowledgment after repeated transmissions, it gives up. – Collision happens if two stations send frame at the same time. – As the number of collisions rises rapidly with increased load, the maximum utilization of the channel is only about 18% 7  Slotted ALOHA – time on the channel is organized into uniform slots whose size equals the frame transmission time. – Some central clock or other technique is needed to synchronize all stations. – Transmission is permitted to begin only at a slot boundary. – still collision is possible; – collided packets are retransmitted after a random delay 8  CSMA - Carrier Sense MA - polite version of ALOHA – to minimize the chance of collision, each station first listens to the medium before sending; “listen before talk” – if the channel is busy, it waits until it is idle. Otherwise it transmits. – if a collision occurs, it waits a random amount of time and starts listening again. – the chance of collision is minimized, but may still occur because of the propagation delay (a station doesn’t know if another one has just started transmitting); or if two or more stations start transmitting at the same time 9 t1 < t2 < t3 < t4 < t5 10 – Two sub-strategies have been defined 1. Non-persistent: sense a line and send if it is idle; otherwise wait a random amount of time (hence less greedy than continuously listening); reduces the chance of collision, but also reduces the efficiency of the network and has longer delays 11 2. persistent: sense a line and “send” if it is idle; otherwise listen; two variations 1-persistent: if the line is idle, send immediately (with probability 1). p-persistent: if the line is idle, send with probability p and refrain from sending with probability 1-p; if p = 0.2, then a station sends 20% of the time that a line is idle, and refrains from sending 80% of the time – it reduces the chance of collision and improves efficiency; depends on the value of p 12  CSMA/CD - CSMA with Collision Detection ₋ adds a procedure to handle a collision ₋ if a collision is detected and to reduce the probability of collision the second time, the sender waits; it has to back off ₋ it waits a little the first time, more if a collision occurs again, much more if it happens a third time, and so on; finally gives up ₋ in the exponential back off method, it waits an amount of time between 0 and 2N X maximum_propagation_time, where N is the number of attempted transmissions ₋ line sensing is done using one of the persistent strategies 13 ₋ sending a jam alerts to the other stations and also to discard the part of the frame received ₋ used in traditional Ethernet; CSMA was never implemented 14  CSMA/CA - CSMA with Collision Avoidance ₋ avoids collision ₋ uses one of the persistence strategies; ₋ after it finds the line idle, it waits an IFG (interframe gap) amount of time; it then waits another random amount of time; after that it sends the frame and sets a timer; if it receives an ack before the timer expires, the transmission is successful; otherwise something is wrong (the frame or the ack is lost); waits for a backoff amount of time and re-senses the line ₋ used in wireless LANs 15  16  the frame or the ack is lost  Token Passing ₋ a station is authorized to send data when it receives a special frame called a token. ₋ the stations are arranged around a ring (each station has a predecessor and a successor) ₋ a token circulates around the ring when no data is transmitted ₋ token: a bit sequence free token: 01111110 busy token: 01111111 ₋ when a node wants to transmit wait for free token remove token from ring (replace with busy token) transmit message when done transmitting, replace free token on ring 17 18 ₋ token failures: tokens can be created or destroyed by noise ₋ distributed solution nodes are allowed to recognize the loss of a token and create a new token collision occurs when two or more nodes create a new token at the same time => need collision resolution algorithms ₋ node failures: since each node must relay all incoming data, the failure of a single node will disrupt the operation of the ring 19  Ethernet 1. Traditional Ethernet (IEEE 802.3) ₋ the most popular LAN physical network architecture in use today ₋ originally created in 1976 at Xerox’s Palo Alta Research Center (not a commercial success for itself) to operate at 10 Mbps usually called traditional Ethernet. ₋ uses 1-persistent CSMA/CD ₋ an Ethernet frame contains 7 fields 20  to alert the recieving system to the coming frame Data and Padding PDU - Protocol Data Unit ₋ minimum and maximum frame lengths are defined without the preamble and the SFD. ₋ padding is used if the packet size is less than the minimum packet size. 21 ₋ Addressing a NIC provides a 6-byte physical address or MAC (Media Access Control address) in hexadecimal; ₋ there are 248 possible LAN addresses; a NIC’s address is permanent - a LAN address is burned into its ROM during manufacturing Unicast, Multicast, and Broadcast addresses: ₋ a source address is always a unicast address; ₋ the destination address can be unicast (only one destination), multicast (multiple destinations), or broadcast (all the stations on the network - 48 1s) 22 ₋ four most common kinds of 10 Mbps Ethernet cabling ₋ Where 10 - 10 Mbps Base - Baseband (against broadband with more bandwidth than standard telephone service) 5 (2) - maximum segment length; rounded to units of 100 meters (for coax) T - Twisted Pair, F - Fiber 23 ₋ a hub is used in 10Base-T and 10Base-F to which each station is connected by a dedicated cable. ₋ 10Base5 is also called Thick Ethernet and 10Base2 is Thin Ethernet ₋ 10Base5 and 10Base2 use bus topology; ₋ 10Base-T and 10Base-F use star topology ₋ Segmentation (of a network not a frame) performance depends on the number of stations. the more stations we have the less will be the performance. when a lot of stations have data to transmit, the network gets congested, and many collisions occur in a network with severe congestion, there may actually be more collisions occurring on the network than data being transmitted 24  Solution: Segmentation ₋ the process of splitting a large Ethernet network into two or more segments linked by routers ₋ the resulting segments have fewer stations to contend with for access to the network ₋ the router transfers data from one segment to the other only when the destination for the data is on the other segment ₋ the rest of the network traffic stays within the segment where it belongs 25 2. Fast Ethernet (IEEE 802.3u) ₋ the need for a higher data rate resulted in the design of the Fast Ethernet protocol (100 Mbps) ₋ basic idea: keep all the old frame formats, interfaces, and procedural rules, but reduce the bit time from 100 ns (10 Mbps) to 10 ns (100 Mbps) ₋ a new feature, called auto-negotiation, is added to allow incompatible devices to communicate with one another, e.g. one with 10 Mbps and one with 100 Mbps - backward compatibility) one device to have multiple capabilities a station to check a hub’s capabilities  the three most common kinds of Fast Ethernet cabling 26 3. Gigabit Ethernet (IEEE 802.3z) ₋ recent need for an even higher data rate resulted in the design of the Gigabit Ethernet protocol (1000 Mbps) ₋ basic idea: make Ethernet go 10 times faster yet remain backward compatible with all existing Ethernet standards ₋ the four most common kinds of Gigabit Ethernet cabling 27 28  Token Bus (IEEE 802.4) proposed by General Motors the stations on the bus form a logical ring, ₋ i.e., the stations are assigned logical positions in an ordered sequence, with the last member followed by the first the physical ordering of the stations on the bus is irrelevant and independent of the logical ordering; it has ring logical topology and bus physical topology it uses token passing medium access control protocol 29  10 20 30 40 50 60 in the example, stations 60, 50, 30, and 10, in that order, are part of the logical ring ₋ station 60 passes the token to 50, which in turn passes it to 30, then to 10, then back to 60; ₋ stations 20 and 40 are not part of the logical ring each participating station knows the address of its predecessor and successor the logical ring is created and maintained dynamically in such a way that the stations are logically ordered in numerically descending order of MAC address, except that the station with the lowest address is followed by the station with the highest address 30  the token bus system requires considerable maintenance; one or more stations must perform the following functions, at minimum: ₋ addition to the ring: ₋ periodically non-participating stations must be granted the opportunity to join the logical ring ₋ deletion from the ring: ₋ a station can remove itself from the logical ring ₋ ring initialization: ₋ when the network is started, some procedure is needed to sort out who goes first, who goes second, and so on ₋ token recovery: ₋ if the token is lost due to a transmission error or station failure, some means of recovery is needed nobody uses it  31  Token Ring (IEEE 802.5) introduced by IBM in early 1980’s has ring logical topology; the physical topology can be ring or star it uses token passing medium access control protocol Only a host that holds a token can send data, and tokens are released when receipt of the data is confirmed. Token ring networks prevent data packets from colliding on a network segment. The IEEE standard version provides for data transfer rates of 4, 16 or 100 Mbps. 32  was once widely used on LANs, but has been nearly entirely displaced by Ethernet thanks to pricing. – token ring products tended to be more expensive than Ethernet at similar speeds still in use at some IBM sites; but virtually nowhere else 33  Ethernet Vs. Token Ring Ethernet Token Ring Access is non-deterministic Access is deterministic (contention-based CSMA/CD) Supports a direct-cable connection Doesn’t support direct cable between two NICs connection. Requires additional software and hardware Alleviates collision by CSMA and Eliminates collision by the use of a by the use of an intelligent switch single-use token and early token release to alleviate the down time Less expensive More expensive 34  References 1. https://searchnetworking.techtarget.com/definition/Token- Ring 2. https://searchnetworking.techtarget.com/definition/Ethernet 3. https://en.wikipedia.org/wiki/Token_ring 35