UNIT 2 Medium Access sub layer Medium Access

UNIT 2 Medium Access sub layer: Medium Access sub layer Channel Allocations, LAN protocols - ALOHA protocols(PURE AND SLOTTED ALOHA), CSMA/CA, CSMA/CD - Overview of IEEE standards - FDDI. Data Link Layer Elementary Data Link Protocols, Sliding Window protocols, Error Handling.

Medium Access sub layer (Introduction) A network of computers based on multi-access medium requires a protocol for effective sharing of the media. As only one node can send or transmit signal at a time using the broadcast mode, the main problem here is how different nodes get control of the medium to send data, that is “who goes next? ”. The protocols used for this purpose are known as Medium Access Control (MAC) techniques. The key issues involved here are - Where and How the control is exercised. A centralized scheme has a number of advantages as mentioned below: • Greater control to provide features like priority, overrides, and guaranteed bandwidth. • Simpler logic at each node. • Easy co-ordination.

Goals of MACs Medium Access Control techniques are designed with the following goals in mind 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Initialisation Fairness Priority Limitations to one station Receipt Error Limitation Recovery Reconfigurability Compatibility Reliability

Medium Access sub layer

MAC(Medium Access Control) Function: a. To perform the control of access to media b. It performs the unique addressing to stations directly connected to LAN. c. Detection of errors LLC(Logical Link Control) Function: a. Error recovery b. Flow control operation c. User addressing

MAC Techniques

Round Robin Techniques Each and every node is given the chance to send or transmit by rotation 1. Polling • The message contains the address of the node being selected for granting access. • No data =“poll reject” message send back

Round Robin Techniques 2. Token Passing • All stations are logically connected in the form of a ring and control of the access to the medium is performed using a token. • Token Ring • Problems like lost token, duplicate token, and insertion of a node, removal of a node. Fig: A token ring network

The Channel Allocation Problem • Static Channel Allocation in LANs and MANs • Dynamic Channel Allocation in LANs and WANs

Static Channel Allocation in LANs and MANs • FDM • TDM • Simple queuing theory - Processing Delay - Queuing Delay - Transmission Delay - Propagation Delay - Retransmission Delay Disadv. When the number of users is large and continuously varying, or the traffic is busy, FDM presents some problem

Static Channel Allocation in LANs and MANs A simple queuing theory calculation For a channel of capacity C bps, with an arrival rate of l frames/sec, each frame having a length drawn from an exponential probablity density function with mean 1/m bits/frame, the mean time delay Now let us divide the single channel up into N independent subchannels, each with capacity C/N bps. The mean input rate on each of the subchannel will now be l/N. Recomputing T, we get

Dynamic Channel Allocation Parameters • Station Model. – N independent stations, each acting as a Poisson Process for the purpose protocol analysis • Single Channel Assumption. – A single channel is available for all communication. • Collision Assumption. – If transmitted frames overlap in time, the resulting signal is garbled. • Transmission Discipline: – Continuous time • Frames can be transmitted at any time – Slotted time • Frames can be transmitted at particular time points • Sensing capability: – Station cannot sense the channel before trying to use it. – Stations can tell if the channel is in use before trying to use it

Multiple Access Protocols • ALOHA - Pure ALOHA - Slotted ALOHA • Carrier Sense Multiple Access Protocols - CSMA/CD - CSMA/CA • Collision-Free Protocols - Bitmap Protocol - Binary Countdown • Limited-Contention Protocols • Wavelength Division Multiple Access Protocols • Wireless LAN Protocols

ALOHA PROTOCOL Transmission technology can be categorized into two categories : ØPoint-to point networks ØBroadcast networks

Point-to-point networks are those in which when a message is sent from one computer to another, it usually has to be sent via other computers in the network. A point-to-point network consists of many connections between individual pairs of computers.

Broadcast networks have a single communication channel that is shared by all the machines on the network. A packet sent by one computer is received by all the other computers on the network. The packets that are sent contain the address of the receiving computer; each computer checks this field to see if it matches its own address. If it does not then it is usually ignored; if it does then it is read. Broadcast channels are sometimes known as multi-access channel. Shared channel

Poisson Process • The Poisson Process is a celebrated model used in Queuing Theory for “random arrivals”. Assumptions leading to this model include: – The probability of an arrival during a short time interval Δt is proportional to the length of the interval, and does not depend on the origin of the time interval (memory-less property) – The probability of having multiple arrivals during a short time interval Δt approaches zero.

Poisson Distribution The probability of having k arrivals during a time interval of length t is given by: where λ is the arrival rate. Note that this is a singleparameter model; all we have to know is λ.

Slotted ALOHA Assumptions • all frames same size • time is divided into equal size slots, time to transmit 1 frame • nodes start to transmit frames only at beginning of slots • nodes are synchronized • if 2 or more nodes transmit in slot, all nodes detect collision Operation • when node obtains fresh frame, it transmits in next slot • no collision, node can send new frame in next slot • if collision, node retransmits frame in each subsequent slot with prob. p until success 5: Data. Link Layer 5 -19

Slotted ALOHA Pros • single active node can continuously transmit at full rate of channel • highly decentralized: only slots in nodes need to be in sync • simple Cons • collisions, wasting slots • idle slots • nodes may be able to detect collision in less than time to transmit packet • clock synchronization 5: Data. Link Layer 5 -20

Slotted Aloha efficiency Efficiency is the long-run fraction of successful slots when there are many nodes, each with many frames to send • Suppose N nodes with many frames to send, each transmits in slot with probability p • prob that node 1 has success in a slot = p(1 p)N-1 • prob that any node has a success = Np(1 -p)N-1 5: Data. Link Layer • For max efficiency with N nodes, find p* that maximizes Np(1 -p)N-1 • For many nodes, take limit of Np*(1 -p*)N-1 as N goes to infinity, gives 1/e =. 37 At best: channel used for useful transmissions 37% of time! 5 -21

Pure (unslotted) ALOHA • unslotted Aloha: simpler, no synchronization • when frame first arrives – transmit immediately • collision probability increases: – frame sent at t 0 collides with other frames sent in [t 0 -1, t 0+1] 5: Data. Link Layer 5 -22

Pure Aloha efficiency P(success by given node) = P(node transmits). P(no other node transmits in [p 0 -1, p 0] = p. (1 -p)N-1 = p. (1 -p)2(N-1) … choosing optimum p and then letting n -> infty. . . = 1/(2 e) =. 18 Even worse ! 5: Data. Link Layer 5 -23

CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: transmit entire frame • If channel sensed busy, defer transmission • Human analogy: don’t interrupt others! 5: Data. Link Layer 5 -24

Carrier Sense Multiple Access (CSMA) • Additional assumption: – Each station is capable of sensing the medium to determine if another transmission is underway

1 -persistent CSMA Station can transmit

1 -persistent CSMA While there is a new frame A to send do 1. Check the medium 2. If the medium is busy, go to 1. 3. (medium idle) Send frame A and wait for ACK 4. If after some time ACK is not received (timer times out), wait a random amount of time and go to 1. End.

Non-persistent CSMA Station can transmit

Non-persistent CSMA While there is a new frame A to send DO 1. Check the medium 2. If the medium is busy, wait some time, and go to 1. 3. (medium idle) Send frame A and wait for ACK 4. If after some time ACK is not received (timer times out), wait a random amount of time and go to 1. End

p-persistent CSMA

p-persistent CSMA While there is a new frame A to send do 1. Check the medium 2. If the medium is busy, go to 1. 3. (medium idle) With probability p send frame A and the go to 4, and probability (1 - p) delay one time slot and go to 1. 4. If after some time ACK is not received (timer times out), wait a random amount of time and go to 1. End.

CSMA Summary § Nonpersistent § 1 -persistent § p-persistent Constant or variable Delay Channel busy CSMA persistence and backoff Non-persistent: Transmit if idle Otherwise, delay, try again Time Ready 1 -persistent: Transmit as soon as channel goes idle If collision, back off and try again p-persistent: Transmit as soon as channel goes idle with probability p Otherwise, delay one slot, repeat process 32

Carrier Sense Multiple Access Protocols CSMA with collision detection (CSMA/CD) Abort a transmission as soon as they detect a collision. Quickly terminating damaged frames saves time and bandwidth. After a station detects a collision, it aborts its transmission, waits a random period of time, and then tries again, assuming that no other station has started transmitting in the meantime.

Contt. A conceptual model for CSMA/CD (How long should each slot be? )

CSMA collisions spatial layout of nodes collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted note: role of distance & propagation delay in determining collision probability 5: Data. Link Layer 5 -35

CSMA/CA • Identical to CSMA/CD but used when listening is not possible while transmitting • Idle channel reservation is done by sending a short request message asking other nodes to defer transmission • If collison is detected then, then random wait is used • Wireless IEEE 802. 11 uses CSMA/CA with an RTS/CTS mechanism

Chapter 4 The Medium Access Sublayer 421 Multiple Access Protocols 4. 2. 2 Carrier Sense Multiple Access Protocols For this reason we will model the contention interval as a slotted ALOHA system with slot width 2 t (t is the end to end delay). On a 1 -km long coaxial cable, t 5 msec. It is important to realize that collision detection is an analog process. The station’s hardware must listen to the cable while it is transmitting. The signal encoding must allow collisions to be detected (e. g. , a collision of two 0 -volt signals may well be impossible to detect). For this reason, special encoding is commonly used.