LANs Local Area Networks LANs provide an efficient network solution : To support a large number of stations Over moderately high speed

Transcription

1 Local Area Networks LANs provide an efficient network solution : To support a large number of stations Over moderately high speed With relatively small bit errors

2 Multiaccess Protocols Communication among stations is provided by means of a unique channel Only a single station can transmit a message at any given time without interference Shared access requires multiaccess control (MAC) protocol to regulate access to the shared medium The access regulation requires mutual agreement among stations, which requires exchange of coordinating information, which requires usage of the channel The recursive aspect of the access problem makes it difficult to solve

3 MAC Classification Control Mechanism Fixed Dynamic TDM FDM Random Reservation Aloha CSMA Polling Token Pure Slotted Pure CSMA/CD

4 FDM Bandwidth is divided into channels with private frequency bands Each user is assigned a channel, i.e., no interference Simple and efficient to implement for a fixed number of constantly active users

5 FDM The scheme may result in poor performance when users are quiescent For a channel of capacity C bps, and a rate of arrival λ frames/sec (Exponential), and a frame length 1 bits/frame (Exponential), the µ response T is given by: T = 1 µc λ Assuming the channel is divided into N independent subchannels: T FDM = 1 = NT µ C λ N N

6 Time Division Multiple Access Frame i-1 Frame i Frame m Time Ctrl OH Data OH Data OH Data Guard Time Station 1 Station 2 Station N

7 Time Division Multiple Access Basic Algorithm Data Packet Ready No Yes Wait For Assigned Slot Transmit Data Packet

8 Roll Call Polling Polling with centralized control is particularly suited for tree topologies and multipoint line Central station sends polling messages to other stations, one by one, in sequential order Polling messages represent permissions to transmit Pulled station transmits data, if any, and indicates that it has completed transmission by sending a go-ahead message Central station continues the polling process by sending a polling message to the next station Once all stations are polled, a cycle is completed and the central station begins a new cycle

9 Hub Polling Hub polling is a modified version of roll call polling In hub polling, the control is transferred from station to station sequentially The next station in the transmitting sequence listens to the transmissions of the stations immediately before it Each transmitting station affixes a next address to their go-ahead message A station that recognizes its address in the go-ahead message can begin transmission without waiting to be pulled by the central station This scheme requires additional sensors at each station to listen to ongoing transmission of preceding station

10 Static Allocation Fixed allocation of resources performs well for stream traffic types No overhead required for contention resolution Fixed allocation does not perform well for non bursty traffic When traffic sources are idle, channel capacity is wasted Dynamic schemes allocate bandwidth on demand Dynamic schemes are better suited for bursty traffic

11 Random Access Techniques Completely decentralized Three techniques are frequently used Pure ALOHA Slotted ALOHA Carrier Sense Multiple Access Carrier Sense Multiple Access with Collision Detection

12 Pure Aloha Stations transmit data, whenever ready, and wait for an acknowledgement from the receiver Waiting time is the time it takes a packet to travel from the sender to the receiver and back If acknowledgment is received, the transmitting station declares success Absence of acknowledgment infers collision among multiple packets Collision occurs if stations attempt to transmit within a packet transmission time t p Colliding stations stagger their attempts to retransmit the message randomly, using a Backoff algorithm In most implementation, colliding stations continue transmitting until success is achieved

13 Pure Aloha Vulnerability Period Packet B Packet C t p t p Packet A time t 0 t 0 +t p t 0 +2t p Vulnerability period for Pure Aloha Vulnerability period is 2 t p

14 Pure Aloha Performance Let N be the number of contending stations Each station generates messages at a rate λ packets/sec on average All messages transmitted are of the same fixed length, m, in units of time Let S denote the traffic intensity S =ρ=n λm What is the limit on S?

15 Pure Aloha Performance Model Assumptions Packet arrivals at each station are Poisson Packet retransmission is also Poisson This assumption is clearly not correct Simulation studies indicate that the assumption is valid if the random retransmission delay time is relatively long The total rate of packets attempting transmission, i.e., newly generated packets and retransmitted packets, λ >λ Consequently, actual traffic intensity or channel utilization is: G = N λ m

16 Pure Aloha Performance Model Assumptions Based on the Poission assumption, we can write: Prob [ n packets in T sec ] = (GT ) n e GT n! Furthermore, the rate of successful transmission S can be expressed as: S = G p Where p is the likelyhood that the transmission of an arbitrary packet is successful

17 Pure Aloha Performance Model Solution Vulnerability periods is 2m, where m is the length of the message in time units A collision will not take place with the currently transmitted packet if no arrivals occur during an interval of 2m sec Prob ( no arrivals in 2 m sec ) = e 2N λ m = e 2G Fraction of messages that get transmitted successfully, represents the probability of success, i.e., probability of no collision Based on the above, we can write: S = e 2G, or S = Ge 2G, G

18 Pure Aloha Performance Model Solution 0.3 Pure Aloha S is maximum, i.e., d (S ) = 0, atg =0.5 dg Maximum throughput S max = 0.5e 1 (0.18)

19 Pure Aloha Performance Pure aloha is only efficient for lightly loaded system Pure aloha has poor performance for heavily loaded systems Random delays upon collision increase total delay

20 Slotted Aloha Slotted aloha is an attempt to improve the throughput limitation of pure aloha The channel is divided into time slots which equal one packet transmission time, (assume equal-length packets) All users are synchronized to the time slots Packets begin transmission at beginning of slot Collision occurs between stations attempting to transmit at the beginning of the same time slot

21 Slotted Aloha Vulnerability Period Transmission time Random Delay Station 1 time Success Collision Station 2 time Vulnerability Period Since the vulnerability period is m, the throughput can be expressed as : S = Ge G This leads to S max = 0.368

22 Carrier Sense Multiple Access Before transmitting, a station listens to determine if another transmission is in progress (carrier sense) If the medium is in use, the station must wait If the medium is idle, the station may transmit Collision can occur only when more than one station begins transmission within the period of propagation delay

23 Carrier Sense Multiple Access is small com- Useful when propagation delay pared to packet transmission time All stations are aware of a new transmission within a fraction of the packet transmission time vulnerability period = 2 propagation delay

24 Carrier Sense Multiple Access Busy Channel Policy If channel is busy, a station can take one of three possible approaches nonpersistent CSMA 1-persistent CSMA p-persistent CSMA

25 Basic CSMA Protocols CSMA Protocol Characteristics Non Persistent If the medium is idle, transmit If the medium is busy, wait a random ammount of time and sense the channel again 1-Persistent If the meduim is idle, transmit If the meduim is busy, continue to listen uintil the channel is sensed idle, then try again p-persistent If medium is idle, transmit with probability p or delay one time unit with probability (1-p) The time unit is typically equal to the max propagation delay If the medium is busy, continue to listen until the medium becomes idle, and try to transmit p-persistently After transmission is delayed 1 unit time, try to transmit p-persistently

26 Slotted Non-Persistent CSMA Variation of nonpersistent CSMA The time axis is slotted into intervals of length τ (τ << transmission time) All stations are synchronized and are required to start transmitting only at the beginning of a slot When a packet arrives, the station senses the state of the channel at the beginning of a slot Station transmits if the channel is idle Defers to a later time slot if there is traffic present Vulnerability period τ

27 CSMA/CD Protocol CSMA/CD attempts to improve performance by forcing the transmitting station to listen while transmitting, in order to to detect collision Rules for the CSMA/CD protocol: If the medium is idle, transmit If the medium is busy, continue listening until the channel is idle, then transmit immediately and continue listening If collision is detected during transmission then 1. Cease transmission 2. Wait a random amount of time, then attempt transmission again

28 Token Based MAC Protocols Token passing provides distributed control of the sequence in which stations transmit The topology could be either a physical or a logical ring A circulating token provides exclusive access to the shared medium Lack of centralization raises several issues to be addressed Who starts the cycle? What happens if the token is lost? What if multiple tokens are generated simultaneously?

29 IEEE Standard LANs LAN Security (SILS) Higher Level and Internetworking (HILI) Logical Link CONTROL CSMA/CD Voice/Data Wireless Token Bus Token Ring MAN LAN LAN Demand Priority Cable TV Based Broad Broadband TAG Fiber OP. TAG

30 IEEE Standard LANs LAN Model Application Presentation Session Transport Network Internetworking Data Link Physical Logical Link Cntrl Media Access Cntrl Physical

31 Logical Link Control Layer (LLC) The LLC layer provides a uniform protocol interface between higher layers and the actual underlying network The LLC layer makes the MAC and physical LAN/MAN implementation transparent to the higher layers It allows an application to run on different types of network

32 LLC-MAC Interface The IEEE 802 LLC is intended to operate with any of the Medium Access Control (MAC) protocols (CSMA/CD, Token Bus, Token Ring, FDDI) A single logical interface to any of the MAC layers is defined The 802 standard does not define an explicit interface, but provides a model of interaction based on the following primitives: MA-Unit-Data.request MA-Unit-Data.indication MA-Unit-Data.response MA-Unit-Data.confirm

33 LLC Service Primitives Confirmed Service LLC Service User LLC Service Provider LLC Service User Request Indication Response Confirm

34 LLC Service Primitives Non-Confirmed Service LLC Service User LLC Service Provider LLC Service User Request Indication

35 LLC Types of Class LLc protocols, also referred to as types of operations, offer three possible services Type 1 operation, to support unacknowledged connectionless service Type 2 operation, to support connection-mode service Type 3 operation, to support acknowledged connectionless service

36 LLC Type 1 Operation Type 1 service is connectionless The protocol does not support acknowledgment, flow control or error control Only error detection and discard at the MAC level is supported Unnumbered Information PDU is used to exchange user data XID PDU is used to exchange type of operation and window size TEST PDU is used for loopback testing

37 LLC Type 2 Operation This service provides logical connection between service access points (SAPs) The service supports flow control, sequencing, and error contrl The protocol uses all three types of PDU formats to establish and manage a connection Information frames, I-frame, to carry user data and sequence numbers, N(S) and N(R) Supervisory frames, S-frames, to enforce flow control Unnumbered frames, U-frames, for connection management (SABME, and DISC), connection rejection (DM), acknowledge unnumbered commands (UA), and report on unacceptable frames (FRMR)

38 LLC Type 3 Operation This type of operation requires that each PDU transmitted is acknowledged A new type of frame is used to carry this service, Acknowledged Connectionless (AC) Information PDU User data is sent in AC command PDU, and must acknowledged using AC response PDU

39 Conclusion Discussed multiaccess protocols Contention-based schemes Token-based schemes Discussed logical link control sublayer design issues