CSC 335 Data Communications and Networking

1. CSC 335Data CommunicationsandNetworking Lecture 7: Local Area Networking Dr. Cheer-Sun Yang Fall 2000

2. Motivation • Up to this point, we’ve talked about point-to-point communication. • We many need to connect many computers together. • Local Area Network(LAN): if they are located in a relatively close geographic area. • Metropolitan Area Network (MAN) : extends over entire city • Wide Area Network (WAN) : extends across public switching network.

3. LAN Applications (1) • Personal computer LANs • Low cost • Limited data rate • Back end networks and storage area networks • Interconnecting large systems (mainframes and large storage devices) • High data rate • High speed interface • Distributed access • Limited distance • Limited number of devices

4. LAN Applications (2) • High speed office networks • Desktop image processing • High capacity local storage • Backbone LANs • Interconnect low speed local LANs • Reliability • Capacity • Cost

5. LAN Architecture • Topologies • Protocol architecture • Physical Layer • Media access control • Logical Link Control

6. Topologies • Bus: A single communication line, typically a twisted pair, coaxial cable, or optical fiber, represents the primary medium. • Ring: packets can only be passed from one node to it’s neighbor. • Star: A hub or a computer is used to connect to all other computers. • Tree: no loop exists (logical connection).

7. LAN Topologies

8. Frame Transmission - Bus LAN

9. 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

10. Frame TransmissionRing LAN

11. 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

12. Bus and Tree • Multipoint medium • Transmission propagates throughout 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 • Need to regulate transmission • To avoid collisions • To avoid hogging • Data in small blocks - frames • Terminator absorbs frames at end of medium

13. Protocol Architecture • Protocol layering (IEEE 802.X) • Physical Layer • Media access control (MAC) Sublayer • Logical link control (LLC)

14. IEEE 802.X • IEEE 802.3 : Ethernet LAN • IEEE 802.4 : Token Bus LAN • IEEE 802.5 : Token Ring LAN • Other Ring Networks: FDDI, Slotted Rings. • IEEE 802.6 : Distributed Queue Dual Bus (DQDB) MAN standard.

15. IEEE 802 vs. OSI Fig 6.4

16. LAN Protocols in Context

17. 802 Physical Layer Design Issues • Encoding/decoding • Preamble generation/removal • Bit transmission/reception • Transmission medium and topology

18. 802 Physical Layer • Required hardware for connecting a PC to Ethernet directly: • Transceiver • Attachment Unit Interface (AUI) cable • Network Interface Card (NIC) also known as Network Adapter • Required hardware for connecting a PC to a remote computer: modem (with the help of PPP)

19. Bus LANs • Signal balancing • Signal must be strong enough to meet receiver’s minimum signal strength requirements • Give adequate signal to noise ration • Not so strong that it overloads transmitter • Must satisfy these for all combinations of sending and receiving station on bus • Usual to divide network into small segments • Link segments with amplifies or repeaters

20. Transmission Media • Twisted pair • Not practical in shared bus at higher data rates • Baseband coaxial cable • Used by Ethernet • Broadband coaxial cable • Included in 802.3 specification but no longer made • Optical fiber • Expensive • Difficulty with availability • Not used • Few new installations • Replaced by star based twisted pair and optical fiber

21. Baseband Coaxial Cable • Uses digital signaling • Manchester or Differential Manchester encoding • Entire frequency spectrum of cable used • Single channel on cable • Bi-directional • Few kilometer range • Ethernet (basis for 802.3) at 10Mbps • 50 ohm cable

22. 10Base5 • Ethernet and 802.3 originally used 0.4 inch diameter cable at 10Mbps • Max cable length 500m • Distance between taps a multiple of 2.5m • Ensures that reflections from taps do not add in phase • Max 100 taps • 10Base5

23. 10Base2 • Cheapernet • 0.25 inch cable • More flexible • Easier to bring to workstation • Cheaper electronics • Greater attenuation • Lower noise resistance • Fewer taps (30) • Shorter distance (200m)

24. Cable Specifications for 802.3 • 10BaseT: 10 Mbps, baseband, unshield twisted • 10Base2: 10Mbps, Cat. 2 coaxial • 10Base5: 10 Mbps, Cat. 5, Cat. 5e coaxial • 100BaseTX: 100 Mbps, twisted cable (Fast Ethernet) • 10Broad36: maximum segment length 3600 meters

25. Gigabit Ethernet • 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

26. Connectors • T-connector: used to form a bus topology • RJ-45 connectors: for connecting a PC to another PC, Ethernet, or hub. • Cross-over: a direct connection to another PC • Straight-through: connection with the Ethernet jack or hub.

27. Repeaters • Transmits in both directions • Joins two segments of cable • No buffering • No logical isolation of segments • If two stations on different segments send at the same time, packets will collide • Only one path of segments and repeaters between any two stations

28. Media Access Control Sublayer • Assembly of data into frame with address and error detection fields • Disassembly of frame • Address recognition • Error detection • Govern access to transmission medium • Not found in traditional layer 2 data link control • Also known as Contention protocols (section 3.4)

29. Collision vs. Contention • When the communication link is used by one station to transmit a frame, another station connecting to the same link tries to send a packet– collision • Contention: accessing the medium with the consideration that a collision may occur. • Contention Protocols: the protocol is designed to deal with collision using contention. • Collision-free Protocols: the protocol is designed so that collision will not occur.

30. Contention Protocols • Pure ALOHA • Slotted ALOHA • Carrier Sense Multiple Access (CSMA) • Persistent and non-persistent CSMA • CSMA with Collision Detection (CSMA/CD)

31. Collision-Free Protocols • A Bit-Map Protocol: reservation protocol • Binary Countdown

32. Pure 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)

33. Pure Aloha(cont’d) • 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% (WHY?)

34. The Efficiency of Pure Aloha G = the traffic measured as the average number of frames generated per slot S = the success rate, success frame / slot Pr[k frames are generated] = G ke–G / k ! This is called a probability distribution function(pdf) for Poisson distribution. (e = 2.7818…) S = Pr[no frame is generated]= e-G = G e–2G (pure Aloha) S = G e–G (slotted Aloha)

35. The Efficiency of Pure Aloha If there is no negative acknowledgement frame received after sending out one frame, the transmission is successful. So P0 = Pr[no frames are generated in 2 time slots] = e -G * e–G = e–2G S = G * P 0 = G e–2G (pure Aloha)

36. The Efficiency of Pure Aloha S = G * P 0= G e–2G (pure Aloha) We need to find the value of G such that S is maximized. S’ = G (-2) e–2G + e–2G = (1 – 2G) * e–2G Let S’ = 0 => G = ½ When G = ½, S = 1/ 2e = 0.184 = 18%

37. Slotted ALOHA • A computer is not allowed to send until the beginning of the next slot. • Time in uniform slots equal to frame transmission time • When a frame is allowed to be transmitted, there is no collision. • Need central clock (or other sync mechanism) • Transmission begins at slot boundary • Max utilization 37% (WHY?)

38. The Efficiency of Slotted Aloha If there is no other frame received after sending out one frame, the transmission is successful. So P0 = Pr[no frames are generated in one time slots] = e-G S = G * P 0 = G e–G (slotted Aloha)

39. The Efficiency of Slotted Aloha S = G * P 0= G e–G (slotted Aloha) We need to find the value of G such that S is maximized. S’ = G (-1) e–G + e–G = (1 –G) * e–G Let S’ = 0 => G = 1 When G = 1, S = 1/ e = 0.368 = 37%

40. Carrier Sense Multiple Access (CSMA) Protocols • Protocols in which stations listen for a carrier (i.e., a transmission) and act accordingly are called carrier sense protocols. • 1-persistent CSMA • Non-persistent CSMA • p-persistent CSMA

41. CSMA • 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)

42. If Busy? • If medium is idle, transmit • If busy, listen for idle then transmit immediately • No ACK then retransmit • If two stations are waiting, it is called a collision.

43. 1-persistent CSMA • When a station has data to send, it first listens to the channel to see if anyone else is transmitting at that moment. • If the channel is busy, the station waits until it becomes idle. • The station retransmits with a probability of 1 when it finds that the channel is idle.

44. Non-persistent CSMA • When a station has data to send, it first listens to the channel to see if anyone else is transmitting at that moment. • If the channel is busy, the station waits until it becomes idle. • The station does not keep trying. It waits for a random number of time and retries.

45. Non-persistent CSMA • This applies to slotted channels. When a station has data to send, it first listens to the channel to see if anyone else is transmitting at that moment. • If the channel is idle, it transmits with a probability p. With a probability of 1-p, it defers until the next slot. • If the next slot is also idle, it transmits or defers again with probability p and q.

46. CSMA • Max utilization depends on propagation time (medium length) and frame length. Longer frame and shorter propagation gives better utilization. • Collisions still can be a problem, especially with p-persistent CSMA. • One way to reduce the frequency of collision with CSMA is to lower the probability that a station will send when a previous is done. (Fig. 3.26) • Smaller values of p => fewer collision.

47. Any Other Way? • Is there another way to improve the successful rate? • Yes if there is a way to detect collision prior to transmission. • Why is this faster?

48. Collisions with and without Detection • Without collision detection, a station must send and then wait for 2 time slots before another attempt to send. • With collision detection, a station can stop transmission if collision detection requires less time than sending a frame.

49. 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

50. CSMA/CD • With CSMA, collision occupies medium for duration of transmission • Stations listen while transmitting • If medium idle, transmit • If busy, listen for idle, then transmit • If collision detected, jam then ease transmission • After jam, wait random time then start again • Binary exponential back off