1. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 1 LAN Systems

2. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 2 LAN Systems Ethernet and Fast Ethernet (CSMA/CD, 802.3), and Gigabit Ethernet Token Ring (802.5) & FDDI ATM LANs Wireless LANs (802.11) LAN Switching and VLANs

3. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 3 LAN Generations First CSMA/CD and token ring Terminal to host connectivity and client server architecture Moderate data rates Second FDDI The need of backbone LANs High performance workstations Third ATM, Gigabit Ethernet Aggregate throughput and real time support for multimedia applications

4. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 4 1. Ethernet (CSMA/CD) Carriers Sense Multiple Access with Collision Detection Xerox - Ethernet IEEE 802.3

5. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 5 IEEE802.3 Medium Access Control Random Access – Stations access medium randomly Contention –Stations content for time on medium

6. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 6 ALOHA Packet Radio When station has frame, it sends Station listens (for max round trip time)plus small increment If ACK, fine. If not, retransmit If no ACK after repeated transmissions, give up Frame check sequence (as in HDLC) If frame OK and address matches receiver, send ACK Frame may be damaged by noise or by another station transmitting at the same time (collision) Any overlap of frames causes collision Max utilization 18%

7. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 7 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%

8. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 8 CSMA Carrier Sense Multiple Access Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If medium idle, transmit If two stations start at the same instant, collision Wait reasonable time (round trip plus ACK contention) No ACK then retransmit Max utilization depends on propagation time (medium length) and frame length –Longer frame and shorter propagation gives better utilization

9. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 9 If Busy? If medium is idle, transmit If busy, listen for idle then transmit immediately If two or more stations are waiting, collision will occur

10. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 10 CSMA/CD With CSMA, collision occupies medium for duration of transmission Stations listen whilst transmitting (an improvement) If medium idle, transmit If busy, listen for idle, then transmit If collision detected, jam then cease transmission After jam, wait random time then start again –Binary exponential back off

11. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 11 CSMA/CD Operation

12. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 12 Collision Detection On baseband bus, collision produces much higher signal voltage than signal Collision detected if cable signal greater than single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m (10Base2) For twisted pair (star-topology) activity on more than one port is collision Special collision presence signal

13. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 13 IEEE 802.3 Frame Format Preamble: alternating 0s and 1s for bit synchronization at the receiver Start of Frame Delimiter: the sequence 10101011 Destination and Source Addresses: Ethernet (or MAC) addresses Frame Check Sequence: 32-bit CRC

14. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 14 10Mbps Specification (Ethernet) 10Base510Base210Base-T10Base-FP Nodes10030-33 MediumCoaxialCoaxialUTP850nm fiber SignalingBasebandBasebandBasebandManchester ManchesterManchesterManchesterOn/Off TopologyBusBusStarStar Diameter(mm) 1050.4~0.662.5/125um Max segment(m) 500185100500

15. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 15 10Base5 10Base2 transceivers

16. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 16 10BaseT Hub

17. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 17 Fast Ethernet

18. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 18 IEEE802.3(100Mbps) All 100BASE-X schemes use two physical links between nodes, one transmission and one reception 100BASE-TX use STP or high-quality Cat5 UTP 100BASE-FX uses optical fiber 100BASE-T4 use 4 twisted pairs. Data transmission use 3 pairs in one direction at a time. i.e. two pairs are bidirectional. Data stream split into 3 each with 33.3Mbps (lost cost alternative, may be prewired in office bldgs)

19. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 19 Gigabit Ethernet Configuration

20. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 20 Gigabit Ethernet - Differences Retains the CSMA/CD protocol and 10-Mbps and 100-Mbps Ethernet format Two enhancements: Carrier extension At least 4096 bit-times long (512 for 10/100) so that T f > T p Frame bursting (allows multiple short frames to be tx consecutively)

21. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 21 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

22. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 22 2. Token Ring (802.5)

23. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 23 MAC protocol –Small frame (token) circulates when idle –Station waits for token –Changes one bit in token to make it SOF for data frame –Append rest of data frame –Frame makes round trip and is absorbed by transmitting station –Station then inserts new token when transmission has finished and leading edge of returning frame arrives –Under light loads, some inefficiency –Under heavy loads, round robin Token Ring MAC

24. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 24 Token Ring Operation

25. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 25 Priority Scheme

26. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 26 Fiber Distributed Data Interface 100Mbps Similar MAC protocol as Token Ring, but with two key differences: –A token is completely received before a frame is transmitted. It is impractical to flip a bit to convert a token to a start-of-frame. –After transmitting a frame, a token is released immediately without waiting for the frame to return to the station.

27. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 27 Dual Rings A E D C B

28. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 28 LAN Performance Issues

29. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 29 Throughput Equations “Perfect” protocol: Token Ring:

30. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 30 Throughput Equations CSMA/CD:

31. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 31

32. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 32

33. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 33 3. ATM 3rd Generation LANs: –Support for multiple guaranteed classes of service Live video may need 2Mbps VoIP File transfer can use background class –Scalable throughput Both aggregate and per host –Facilitate LAN/WAN internetworking

34. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 34 ATM LANs Asynchronous Transfer Mode Possible types: –Gateway to ATM WAN –Backbone ATM switch –Workgroup ATM

35. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 35 Example ATM LAN

36. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 36 ATM LAN HUB

37. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 37 4. Wireless LANs The standard is IEEE 802.11 Basic service set (cell) –Set of stations using the same MAC protocol –Competing to access a shared medium –May be used to form an ad hoc network –May be isolated –May be connected to backbone via access point (bridge) Extended service set –Two or more BSS connected by distributed system –Each BSS has an access point (AP) which provides access to the distribution system –Appears as a single logic LAN to LLC level

38. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 38 B D C A Figure 6.65 Ad Hoc Network

39. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 39 A2 B2 B1 A1 AP1 AP2 Distribution System Server Gateway to Internet portal BSS A BSS B Figure 6.66 Infrastructure network and extended service set

40. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 40 Types of stations No transition –Stationary or moves within direct communication range of single BSS BSS transition –Moves between BSS within single ESS ESS transition –From a BSS in one ESS to a BSS in another ESS –Disruption of service likely

41. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 41 Wireless LAN - Physical Infrared –1Mbps and 2Mbps –Wavelength 850-950nm Direct sequence spread spectrum –2.4GHz Industrial, Scientific, and Medical (ISM) band –11 channels defined in US, 9 defined in Europe –Each channel:1Mbps, 2Mbps, 5.5Mbps, or 11Mbps (using different encoding techniques, and providing different ranges of service) Frequency hopping spread spectrum –2.4GHz ISM band –1Mbps or 2Mbps Others under development

42. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 42 Media Access Control The IEEE 802.11 MAC protocol is specified in terms of coordination functions that determine when a station in a BSS is allowed to transmit and when it may be able to receive PDUs over the wireless medium Distributed coordination function (DCF) –Under the DCF, the transmission medium operates in the contention mode, requiring all stations to contend for the channel for each packet transmitted Point coordination function (PCF) –Under PCF the medium can alternate between the contention period and a contention-free period (during which the usage of the medium is controlled by the Access ) In the following, only the operations of DCF are discussed

43. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 43 IEEE 802.11 MAC Architecture Physical Distribution coordination function (CSMA-CA) PCF Contention-free service Contention service MAC

44. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 44 Distributed Coordination Function Based on Carrier Sensing Multiple Access with Collision Avoidance (CSMA-CA) protocol: –If the channel is sensed idle: If the channel is sensed idle for an amount of time equal to or greater than the Distributed Inter Frame Space (DIFS), a station is then allowed to transmit When the receiving station correctly receives the frame, it waits a short period of time (known as the Short IFS, or SIFS) and then sends back an acknowledgement frame –If the channel is sensed busy: The station defers its access until the channel is later sensed idle The station then computes an additional random backoff time and counts down this time as the channel is sensed idle The station transmits its frame when the timers reaches zero

45. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 45 Basic CSMA-CA Opertions Busy Medium DIFS PIFS SIFS Contention Window Next Frame Defer Access Wait for Reattempt Time Time

46. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 46 Data DIFS SIFS Defer access Wait for reattempt time ACK DIFS NAV Source Destination Other NAV: Network Allocation Vector, indicates the amount of time that must be elapse until the current transmission is complete and the channel can be sampled again for idle status.

47. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 47 The Hidden-Station Problem Data Frame A transmits data frame B A B C C transmits data frame and collides with A at B (a) (b) C senses medium, station A is hidden from C Data Frame C A

48. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 48 Why Hidden? Physical obstructions in the environment may prevent station A and station C from hearing each other’s transmission Stations A and C are placed such that their signal strengths are not strong enough for them to detect each other’s transmissions, and yet their transmissions are strong enough to have interfered with each other at station B.

49. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 49 Solution to the Problem Use short Request to Send (RTS) and Clear To Send (CTS) frame to reserve access to the channel. A sender first send a RTS frame to the receiver, indicating the duration of the data packet and the ACK packet. The receiver then responds with a CTS frame.

50. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 50 RTS CTS Data frame A requests to send B C A A sends B B C C remains quiet B announces A ok to send (a) (b) (c)

51. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks LAN Systems Slide 51 Collision Avoidance IEEE 802.11 frame contains a duration field in which the sending station explicitly indicates the length of time that its frame will be transmitting on the channel. This value allows other stations to determine the minimum amount of time (or the NAV) for which they should defer their access. The use of the RTS and CTS frames also helps avoid collisions in 2 important ways: –CTS frames help avoid the hidden station problem. –RTS and CTS frames are short (20 & 14 bytes respectively), a collision involving an RTS or CTS frame will only last for a short period of time. Note that when the RTS and CTS frames are correctly transmitted, there should be no collisions involving the subsequent DATA frame (which can be 2300 bytes) and the ACK frame.