Data and Computer Communications. Chapter 11 Local Area Network

Transcription

1 Data and Computer Communications Chapter 11 Local Area Network

2 LAN Topologies Refers to the way in which the stations attached to the network are interconnected

3 Bus Topology Used with multipoint medium All stations share capacity of bus Heard by all stations Only one station can transmit at a time Need to regulate transmission to avoid collisions Terminator absorbs frames at end of medium

4 Star Topology Each station connects to central node Usually via two point to point links One for transmission and one for reception Central node can act as hub or frame switch Central Hub, switch, or repeater Figure 11.2 Star Topology

5 Hubs Each station connected to hub by two lines Physically a star, logically a bus Operate in broadcast fashion Hub acts as a repeater Transmission from any station received by hub and retransmitted on all outgoing lines Only one station can transmit at a time (hub) If two stations transmit at the same time, there will be a collision Two cables (twisted pair or optical fiber) Transmit IHUB Receive HHUB IHUB Station Station Station Station Figure Two-Level Star Topology Station

6 Hub vs Switch Hub Switch Frame handling done in software Performs frame forwarding in hardware Analyzes and forwards one frame at a time Can handle multiple frames at a time Uses store-andforward operation Can be viewed as full-duplex hub

7 IEEE 802 Standards Working Groups The important ones are marked with *. The ones marked with are hibernating. The one marked with gave up.

8 IEEE 802 Standards OSI Reference Model Application Presentation Session IEEE 802 Reference Model Transport Network Data Link Physical Medium Upper Layer Protocols ( ) ( ) ( ) Logical Link Control Medium Access Control Physical Medium LLC Service Access Point (LSAP) Scope of IEEE 802 Standards Figure 11.3 IEEE 802 Protocol Layers Compared to OSI Model

9 LAN Protocol Architecture

10 Physical Layer Encoding/decoding of signals Preamble generation/removal Bit transmission/reception

11 Media Access Control Govern access to transmission medium Encapsulation & Decapsulation On transmit, assemble data into frame On reception, disassemble frame PDU is referred to as a MAC frame Perform address recognition Physical address (MAC address) Detects errors and discards frames LLC optionally retransmits unsuccessful frames

12 Logical Link Control Reliable transmission of link level PDUs between stations Flow and error control

13 Types of MAC Round robin Each station given turn to transmit data Reservation Divide medium into slots Good for stream traffic Contention All stations contend for time Good for bursty traffic Simple to implement Tends to collapse under heavy load

14 Aloha When station has frame, it immediately transmits its frame completely Then listens for a bit over max round trip time If receive ACK, then fine If not, retransmit after a random backoff time Frame may be damaged by noise or by another station transmitting at the same time (collision) If no ACK after repeated transmissions, give up Maximum utilization of channel about 18% ACK ACK ACK ACK

15 Aloha Send immediately DATA Frame generation ACK Send immediately DATA Backoff DATA Frame generation No ACK

16 Example Backoff

17 Slotted Aloha Time divided into discrete intervals (slot) 1 slot 1 frame transmission time Transmission begins at slot boundary The sending station waits until the beginning of the next slot Need central clock (or other sync mechanism) Frames either miss or overlap totally Increased utilization to about 37% Send at slot beginning DATA Frame generation DATA ACK Frame generation No ACK Backoff DATA

18 CSMA (Carrier Sense Multiple Access) First listen for channel to determine if there is another transmission in progress If idle, transmit and wait for ACK If no ACK in a reasonable time, then collision is assumed and retransmit If two stations start at the same instant, collision Utilization far exceeds ALOHA

19 Collision channel propagation delay

20 Example Backoff

21 Types of CSMA

22 Nonpersistent CSMA 1. If channel idle, transmit immediately 2. If channel busy, random backoff & retry Random backoff reduces probability of collisions Capacity is wasted because the medium will generally remain idle following the end of a transmission even if there are one or more stations waiting to transmit If idle, send immediately DATA Frame generation DATA Sense channel again Defer Backoff

23 1-persistent CSMA 1. If channel idle, transmit immediately; 2. If channel busy, listen until idle; then transmit immediately If idle, send immediately DATA Avoids idle channel time If two or more stations waiting, a collision is guaranteed 1-persistent stations are selfish 1-persistent seems more unstable because of greed of the stations Frame generation DATA Defer Send immediately

24 P-persistent CSMA 1. If channel idle, transmit with probability p, and delay one time slot with probability (1 p) 2. If channel busy, listen until idle and repeat step 1 3. If transmission is delayed one time slot, repeat step 1 A compromise to try and reduce collisions and idle time Delay Delay Delay DATA Frame generation slot DATA Delay Delay DATA

25 Value of p? Have n stations waiting to send, at end of transmission, expected number of sending stations is np If np > 1 on average there will be a collision To avoid catastrophe np < 1 If heavy load expected, p must be small Smaller p means stations wait longer

26 Which Persistence Algorithm? IEEE uses 1-persistent Both nonpersistent and p-persistent have performance problems 1-persistent seems more unstable than p-persistent Because of greed of the stations Wasted time due to collisions is short With random backoff unlikely to collide on next attempt to send

27 CSMA/CD Carrier sense multiple access with collision detection (CSMA/CD) Simple rules for a polite human conversation Listen before talking If someone else begins talking at the same time as you, stop talking : CD (collision detection) Most widely used LAN standard Developed by Xerox - original Ethernet IEEE 802.3

28 CSMA/CD If the medium is idle, transmit; otherwise, go to step 2 If the medium is busy, continue to listen until the channel is idle, then transmit immediately (1-persistent ) If a collision is detected, transmit a brief jamming signal to assure that all stations know that there has been a collision and cease transmission After transmitting the jamming signal, wait a random amount of time, referred to as the backoff, then attempt to transmit again

29 CSMA/CD If idle, send immediately DATA Frame generation DATA Defer DATA Send immediately as channel goes idle If collision, stop & jam & backoff Contention window Jam Backoff Retry slot

30 CSMA/CD

31 Example At time t0, station A begins transmission At t1, both B and C are ready to transmit. B senses a transmission and so defers. However, C begins its own transmission since C is still unaware of A s transmission When A s transmission reaches C, at t2, C detects the collision and ceases transmission The effect of the collision propagates back to A, where it is detected by A at t3, at which time A ceases transmission.

32 CSMA/CD Operation CSMA/CD can be in one of the three states: contention, transmission, or idle

33 Collision Worst Case t t A B A begins transmission t t p t A B B begins transmission just before the frame sent from A arrives at B t t p A B B is aware of collision immediately after it starts transmission t 2t p A B A is aware of collision after 2T p Propagation delay : t p Collisions can occur only during 2T p after transmission After 2T p, the sender is guaranteed successful transmission without collision

34 Slot time Slot_Time = 2T p (round trip propagation delay) + safety margin Time domain during which there is a possibility of collision For 10Mbps Ethernet (d=2.5km), 2T p = 2*(d/V)=25us Slot_Time = 51.2us Slot_Size = Slot_Time * Data_Rate = 512 bits (64byets) The sender should be able to cease its transmission before completing its frame when it is aware of collision. So, Frame Transmission Time (T X = L/R) > Slot_Time Frame Length (L) > Slot_Size

35 Comparison When channel idle Nonpersistent Transmit immediately P-persistent Transmit with probability p or delay one slot with probability (1-p) When channel busy Backoff & retry Listen until idle and transmit with p or delay one slot with (1-p) When collision N/A N/A 1-persistent N/A CSMA/CD Transmit immediately Listen until idle, and then transmit Stop transmission and backoff & retry

36 Binary Exponential Backoff Backoff _ Time UNIFORM (0, CW 1) Slot _ Time CW = 2 k, k = min[n, 10] for n th retransmission On first 10 attempts, contention window (CW) is doubled Mean backoff time doubled Value then remains the same for 6 further attempts After 16 unsuccessful attempts, station gives up and reports error 1-persistent algorithm with binary exponential backoff is efficient over wide range of loads But, backoff algorithm has last-in, first-out effect

37 Example Data rate = 10Mbps, Propagation delay = 12.5us Slot_Time = 2* Propagation delay + safety margin =51.2us Slot_Size = 51.2us*10Mbps = 512bit = 64Byte 1. Assuming that station A and B collide, they independently choose random backoff delays by Backoff_Time = UNIFORM{0,1} * Slot_Time 2. If they collide again at the 2 nd trial, they again choose random backoff delays by Backoff_Time = UNIFORM{0,1,2,3} * Slot_Time

38 Example A 1 t=0 B 1 t=13 C Frame generation time t=10 34 D 1 t=39 B and C collide and backoff A1 C1 0 B1 1 1 A2 C2 C1 D1 1 B1 C2 A, C, and D collide and backoff A and D collide and backoff A2 D1 1 2 B2 A2 4 6 D1 D1 A2 A1 C1 B1 A2 C2 D1 B2 A and D again collide and backoff Backoff time D1 s Waiting time = 31

39 Collision Detection

40 Summary LAN Topologies Hubs and Layer 2 switches LAN protocol architecture IEEE 802 reference model Logical link control Medium access control CSMA CSMA/CD