Chapter 1 5 & 16 LAN (Local Area Network)

1. Chapter 15 & 16 LAN (Local Area Network)

2. Topologies • Tree • Bus • Special case of tree • One trunk, no branches • Ring • Star

3. LAN Topologies

4. Bus and Tree • Multipoint medium • Heard by all stations • Need to identify target station • Each station has unique address • Full duplex connection between station and tap • Allows for transmission and reception • Terminator absorbs frames at end of medium

5. Frame Transmissionon Bus LAN

6. Ring Topology • Repeaters joined by point to point links in closed loop • Receive data on one link and retransmit on another • Links unidirectional • Stations attach to repeaters • Data in frames • Circulate past all stations • Destination recognizes address and copies frame • Frame circulates back to source where it is removed • Media access control determines when station can insert frame

7. Frame TransmissionRing LAN

8. Star Topology • Each station connected directly to central node • Usually via two point to point links • Central node can broadcast • Physical star, logical bus • Only one station can transmit at a time • Central node can act as frame switch

9. LAN Protocol Architecture • Lower layers of OSI model • IEEE 802 reference model • Physical • Data link • Logical link control (LLC) • Media access control (MAC)

10. Logical Link Control • Flow and error control • Based on HDLC • Three types • Unacknowledged connectionless service • Connection mode service • Acknowledged connectionless service • Addressing (De/Multiplexing) • Specifying source and destination LLC users • Referred to as service access points (SAP) • Typically higher level protocol

11. Media Access Control • Govern access to transmission medium • Resource sharing methods • Centralized & Distributed • Centralized • Greater control • Simple access logic at station • Distributed • Synchronous & Asynchronous • Synchronous • Specific capacity dedicated to connection • Asynchronous • In response to demand

12. MAC Methods • Round robin • Order stations and allocate the right to use resources in turn • Reservation • First get exclusive use of resources before access • Contention • Access without explicit co-ordination • Retransmit in case of collision • Good for bursty traffic

13. LAN Protocols

14. Generic MAC Frame Format

15. Ethernet • Contention • Aloha • Slotted Aloha • CSMA(Carrier Sense Multiple Access) • CSMA/CD(Collision Detection) • Splitting

16. t t-1 t+1 Time ALOHA • When station has frame, it sends • Station listens (for max round trip time)plus small increment • Frame may be damaged by noise or by another station transmitting at the same time (collision) • Any overlap of frames causes collision • If frame OK and address matches receiver, send ACK • No Ack in TO, retransmit • Max utilization 18%

17. Simple Analysis of ALOHA • Assumptions • Packet transmission attempt process - Poisson process with rate G • Fixed packet size = 1 • : Packet transmission time of i-th packet • Condition for success • i-th packet is success if • No attempt in && • No attempt in • Prob. of Success ? • Throughput ?

18. Time Collision Idle Success Collision Slotted ALOHA • Time in uniform slots equal to frame transmission time • Need central clock (or other sync mechanism) • Transmission begins at slot boundary • Frames either miss or overlap totally • Max utilization 37%

19. Analysis of Slotted ALOHA • Prob. of success ? • Throughput ? Throughput Slotted ALOHA ALOHA Attempt Rate, G

20. CSMA • Problems of slotted ALOHA • Carrier Sense • Probe if the carrier is busy before transmit • If idle, transmit with some probability • Effects ? • Collision ? • If Propagation time << Transmission time, CSMA improves performance • Three CSMA schemes • Nonpersistent CSMA • 1-persistent CSMA • p-persistent CSMA

21. Nonpersistent CSMA • If medium is idle, transmit; otherwise, go to 2 • If medium is busy, wait amount of time drawn from probability distribution (retransmission delay) and repeat 1 • Random delays reduces probability of collisions • Consider two stations become ready to transmit at same time • If both stations delay same time before retrying, then collision • Capacity is wasted because medium will remain idle following end of transmission • Even if one or more stations waiting

22. 1-persistent CSMA • If medium idle, transmit; otherwise, go to step 2 • If medium busy, listen until idle; then transmit immediately • If two or more stations waiting, collision guaranteed

23. P-persistent CSMA • Compromise that attempts to reduce collisions • Like nonpersistent • And reduce idle time • Like1-persistent • Rules: • If medium idle, transmit with probability p, and delay one time unit with probability (1 – p) • Time unit typically maximum propagation delay • If medium busy, listen until idle and repeat step 1 • If transmission is delayed one time unit, repeat step 1 • What is an effective value of p?

24. Value of p • Avoid instability under heavy load • n stations waiting to send (backlogged station) • End of transmission, expected number of stations attempting to transmit is number of stations ready times probability of transmitting • np • If np > 1 on average there will be a collision • Repeated attempts to transmit almost guaranteeing more collisions • Retries compete with new transmissions • Eventually, all stations trying to send • Continuous collisions; zero throughput • So np < 1 for expected peaks of n • If heavy load expected, p small • However, as p made smaller, stations wait longer • At low loads, this gives very long delays

25. Splitting Algorithms • Total attempt rate in contention methods • Too small => Idle, Resource waste • Too large => Too many collisions • In case of collision • Should reduce attempt rate • Serve contention involved stations first • Contention resolution • Then restart with new arrivals CRP New arrivals Time Contention Resolution Restart Collision

26. Tree Splitting Collision => Split into two subsets, Try the first subset Idle / Success => Try next subset Time All R L LR LRR LRL LL LR R LRL R LL

27. Tree Splitting Slot Xmit set Waiting sets Result Success Success 1 S - C 2 L R C 3 LL LR, R S 4 LR R C 5 LRL LRR, R I 6 LRR R C 7 LRRL LRRR, R S 8 LRRR R S 9 R - I LRRL LRRR Idle Collision LRL LRR Success Collision LL LR Idle Collision L R How to maintain the stack for the waiting sets ? Collision S

28. Improvements of Tree Algorithm • Arrival estimation • Spilt into k subsets • Collision -> Idle sequence • Collision -> Collision sequence

29. CSMA/CD • With CSMA, collision occupies medium for duration of transmission • Stations listen whilst transmitting (Listen while talk) to detect collision • On baseband bus, collision produces much higher signal voltage than non-collided signal • Since signal attenuated over distance, Limit the bus length to 500m (10Base5) or 200m (10Base2) • If medium idle, transmit, otherwise, step 2 • If busy, listen for idle, then transmit • If collision detected, jam then cease transmission • After jam, wait random time then start from step 1

30. t0 • CSMA/CD - For baseband CSMA/CD, worst-case “wasted-time” due to a collision = 2xTprop • Packet length should be at least twice the propagation delay (a  0.5) t1 t2 t3

31. Ethernet MAC Frame Format • Preamble: A 7-octet pattern of alternating 0s and 1s used by the receiver to establish bit synchronization (establishes the rate at which bit are sampled) • Start frame delimiter (SFD): Special pattern 10101011 indicating the start of a frame • Length: Length of the LLC data field • LLC data • Pad: Octets added to ensure that the frame is long enough for proper CD operation • FCS: Error checking using 32-bit CRC

32. IEEE 802.3 BEB • IEEE 802.3 and Ethernet use BEB(Binary Exponential Backoff) • If the medium is idle, send immediately, • 1-persistent can yield higher utilization than non/p-persistent • If the medium is busy, pause for a random interval after the end of busy period • Backoff delay • Because the offered load fluctuates dynamically, it is best to adjust the attempt rate dynamically to optimize the system utilization • How do you guess the offered load? • How do you adjust the total attempt rate?

33. BEB • Guessing the number of backlogged stations • More backlogged station => More collisions • Adjustment of attempt rate • Change the backoff delay based on guessed offered load • BEB(Binary Exponential Backoff) • After each successive collision, double the backoff delay • Randomly select backoff delay (0, 2^n –1) where n is number of trials • 1-persistent algorithm with BEB is efficient • At low loads, 1-persistence guarantees station can seize channel once idle • At high loads, at least as stable as other techniques • Backoff algorithm gives last-in, first-out effect • Stations with few collisions transmit first

34. 10Mbps Specification (Ethernet) • <data rate><Signaling method><Max segment length> • 10Base5 10Base2 10Base-T 10Base-F • Medium Coaxial Coaxial UTP 850nm fiber • Signaling Baseband Baseband Baseband Manchester • Manchester Manchester Manchester On/Off • Topology Bus Bus Star Star • Nodes 100 30 - 33

35. Repeater • A repeater connects two or more bus segments to build a longer bus • A signal received from one bus segment is repeated to all other segments

36. Bus and Star Configurations • Use hub for star wiring • Transmission from any station received by hub and retransmitted on all outgoing lines

37. Hubs • Active central element of star layout • Each station connected to hub by two lines • Transmit and receive • Hub acts as a repeater • When single station transmits, hub repeats signal on outgoing line to each station • Line consists of two unshielded twisted pairs • Limited to about 100 m • High data rate and poor transmission qualities of UTP • Optical fiber may be used • Max about 500 m • Physically star, logically bus • Transmission from any station received by all other stations • If two stations transmit at the same time, collision

38. Hub Layouts • Multiple levels of hubs cascaded • Each hub may have a mixture of stations and other hubs attached to from below • Fits well with building wiring practices • Wiring closet on each floor • Hub can be placed in each one • Each hub services stations on its floor

39. Two Level Star Topology

40. Hub and Layer 2 Switch

41. 100Mbps (Fast Ethernet) • 100Base-TX 100Base-FX 100Base-T4 • 2 pair, STP 2 pair, Cat 5UTP 2 optical fiber 4 pair, cat 3,4,5 • MLT-3 MLT-3 4B5B,NRZI 8B6T,NRZ

42. 100BASE-X • Bidirectional data rate 100 Mbps over two links • Encoding scheme same as FDDI • 4B/5B-NRZI • Modified for each option • 100BASE-TX • Two pairs of twisted-pair cable • STP and Category 5 UTP allowed • The MTL-3 signaling scheme is used • 100BASE-FX • Two optical fiber cables • Intensity modulation used to convert 4B/5B-NRZI code group stream into optical signals • 1 represented by pulse of light • 0 by either absence of pulse or very low intensity pulse

43. 100BASE-T4 • 100-Mbps over lower-quality Cat 3 UTP • Taking advantage of large installed base • Cat 5 optional • Does not transmit continuous signal between packets • Useful in battery-powered applications • Can not get 100 Mbps on single twisted pair • Data stream split into three separate streams • Each with an effective data rate of 33.33 Mbps • Four twisted pairs used • Data transmitted and received using three pairs • Two pairs configured for bidirectional transmission • NRZ encoding not used • Would require signaling rate of 33 Mbps on each pair • Does not provide synchronization • Ternary signaling scheme (8B6T)

44. Full Duplex Operation • Traditional Ethernet half duplex • Either transmit or receive but not both simultaneously • With full-duplex, station can transmit and receive simultaneously • 100-Mbps Ethernet in full-duplex mode, theoretical transfer rate 200 Mbps • Attached stations must have full-duplex adapter cards • Must use switching hub • Each station constitutes separate collision domain • In fact, no collisions • CSMA/CD algorithm no longer needed • Ethernet MAC frame format used • Attached stations can continue CSMA/CD

45. Gigabit Ethernet - Differences • Carrier extension • At least 4096 bit-times long (512 for 10/100) • Frame bursting

46. Gigabit Ethernet – Physical • 1000Base-SX • Short wavelength, multimode fiber • 1000Base-LX • Long wavelength, Multi or single mode fiber • 1000Base-CX • Copper jumpers <25m, shielded twisted pair • 1000Base-T • 4 pairs, cat 5 UTP • Signaling - 8B/10B

47. Gbit Ethernet Medium Options(log scale)

48. Gigabit Ethernet Configuration

49. 10Gbps Ethernet - Uses • High-speed, local backbone interconnection between large-capacity switches • Server farm • Campus wide connectivity • Enables Internet service providers (ISPs) and network service providers (NSPs) to create very high-speed links at very low cost • Allows construction of (MANs) and WANs • Connect geographically dispersed LANs between campuses or points of presence (PoPs) • Ethernet competes with ATM and other WAN technologies • 10-Gbps Ethernet provides substantial value over ATM

50. 10Gbps Ethernet - Advantages • No expensive, bandwidth-consuming conversion between Ethernet packets and ATM cells • Network is Ethernet, end to end • IP and Ethernet together offers QoS and traffic policing approach ATM • Advanced traffic engineering technologies available to users and providers • Variety of standard optical interfaces (wavelengths and link distances) specified for 10 Gb Ethernet • Optimizing operation and cost for LAN, MAN, or WAN