5.2.3 Collision Free Protocols 5 6.

Transcription

1 5.2.3 Collision Free Protocols

2 5 7. 2

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4 5.2.4 Limited contention Protocols 4

5 Limited Contention protocols 5

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10 The Adaptive Tree Walk protocol (Method for testing soldiers for syphilis)

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12 i i 2 i q22 i i i 2 q 12

13 5.2.6 Wireless LAN Protocols 13

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20 5.3 Ethernet Ethernet Cabling Manchester Encoding TheEthernet MACSublayer Protocol The Binary Exponential Backoff Algorithm Ethernet Performance Switched Ethernet Fast Ethernet Gigabit Ethernet IEEE 802.2: Logical Link Control Retrospective on Ethernet 20

21 Ethernet Cabling The most common kinds of Ethernet cabling. 21

22 Ethernet Cabling (2) Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base T. Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base T 22

23 Ethernet Cabling (3) Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented. 23

24 Ethernet Cabling (4) (a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding. 24

25 Ethernet MAC Sublayer Protocol 7 1 SOF: Start of frame Frame formats. (a) DIX Ethernet, (b) IEEE DIX (DEC, Intel, Xerox) 25

26 Ethernet MAC Sublayer Protocol (2) Collision detection can take as long as 2ττ. 26

27 5.3 Ethernet performance Assumptions a. Heavy and constant load, that is, stations always ready to transmit b. Each station transmits during a contention slot with probability p The probability A that some station acquires the channel in that slot is k 1 A = kp(1 p) A is maximized when p =, with as A 1 e p 1 k k k 27

28 The probability that the contention interval has exactly slots in j 1 it is A (1 A ), so the mean number of slots per contention is given by j = 0 ja (1 A) j 1 1 = A Since each slot has a duration, the mean contention interval 2τ p p 1 A Assuming optimal, i.e. = A = k e as 1 k e = w = 2τe 5. 4τ A If the mean frame takes seconds p to transmit then channel efficiency = where B: bandwidth F: frame length η = L: cable length C: propagation speed p + p 2τ A = 1 j w F B 1 = F + 2 ( L ) e 1+ 2BLe B C CF = 2 τ A 28

29 Ethernet Performance Efficiency of Ethernet at 10 Mbps with 512 bit slot times. 29

30 Switched Ethernet A simple example of switched Ethernet. 30

31 Fast Ethernet The reasons for fast Ethernet 1. The need to be backwardcompatible compatible with existing Ethernet LANs 2. The fear that a new protocol might have unforeseen problems 3. The desire to get the job done before the technology changed All fast Ethernet systems use hubs and switches 100 Base T4 uses 8B/6T coding and 100 Base TX uses 4B/5B coding The original fast Ethernet cabling. 31

32 Gigabit Ethernet All configurations of gigabit Ethernet are point to point Gigabit Ethernet supports full duplex mode (with switch) and half duplex mode (with hub) CSMA/CD protocol is required for half duplex mode operation (maximum distance is 25 meters) When carrier extension (512 bytes frame) and frame bursting are used the distance can be 200meters (a) A two station Ethernet. (b) A multistation Ethernet. 32

33 Gigabit Ethernet (2) Gigabit Ethernet supports both copper and fiber cabling Two wavelengths are permitted = 0.85μm and 1.3μm Three fiber core diameters are permitted = 10, 50, and 62.5μm Gigabit Ethernet cabling. 33

34 IEEE 802.2: 2: Logical Link Control (a)position of LLC. (b) Protocol formats. 34

35 Retrospective on Ethernet 1. Ethernet is simple and flexible reliable, cheap, easy to maintain, easy to install 2. Ethernet interworks easily with TCP/IP 3. Ethernet has been able to evolve in certain crucial ways speeds gone up hub and switches introduced 35

36 Wireless LANs The Protocol Stack The Physical Layer The MAC Sublayer Protocol The Frame Structure Services 36

37 The Protocol Stack FHSS: Frequency Hopping Spread Spectrum (dwell time < 400ms) DSSS: Direct Sequence Spread Spectrum (up to 2 Mb/s) OFDM: Orthogonal Frequency Division Multiplexing ( up to 54 Mb/s) HR DSSS: High Rate Direct Sequence Spread ds Spectrum (11Mb/s) Part of the protocol stack. 37

38 The MAC Sublayer Protocol (a) The hidden station problem. (b) The exposed station problem. 38

39 The MAC Sublayer Protocol (2) NAV: Network Allocation Vector Theuse of virtual channel sensing using CSMA/CA. 39

40 The MAC Sublayer Protocol supports two modes of operation: DCF and PCF A. Distributed Coordination Function (DCF) uses CSMA/CA (CSMA / with hcollision i Avoidance) (a) The first mode supported by CSMA/CA (1) When a station wants to transmit, it senses the channel (2) If it idle, it just starts transmitting (The sender does not sense the channel while transmitting) i (3) If the channel is busy, the sender defers until it goes idle and then starts transmitting (4) If a collision occurs, it wait a random amount of time (exponential back off) and then try again later 40

41 (b) The second mode of CSMA/CA is based on MACAW (Multiple Access withcollisionavoidance for Wireless) (1) When a station wants to transmit, it senses the channel (2) If the channel is idle longer than SIFS interval it transmits an RTS (Request to Send, 30 bytes) which contains the length of the data frame (3) After received the RTS frame, the receiving station replies with a CTS (Clear to Send) d)frame which h contains the data length (copied from the RTS frame) (4) Upon receipt of the CTS, the sender transmits the frame (5) All stations heard the RTS frame should remain silent for a period of time (an estimation based on the information of RTS) (6) All stations heard the CTS frame should remain silent for a period of time (an estimation based on the information of CTS) (7) If the channel is busy, the sender goes to step1 41

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43 B. Point Coordination i Function (PCF) (contention free) (1) When a station wants to gain control of the medium, it sends out a beacon at the end of PIFS. The beacon frame contains system parameters such as hopping sequences, and dwell time (for FHSS), clock synchronization, length of the contention free period, etc. (2) All other stations heard the beacon will keep silent and wait for polling sign up frame (3) After gained control, it invites new stations to sign up for polling service (4) At the end of the contention free period, all station return to DCF mode 43

44 The MAC Sublayer Protocol (3) To deal with the problem of noise channels, allows frames to be fragmental into smallerpieces, each with its own checksum and ack. (using stop and wait protocol) A fragment burst. 44

45 The MAC Sublayer Protocol (4) PCF and DCF can coexist within one cell Interframe spacing in

46 The Frame Structure The data frame. 46

47 Version: Protocol version Type: data, control, or management Subtype: RTS, CTS, ack, To DS and from DS: to or from inter cell distribution system (e.g. Ethernet) MF: more fragments Retry: retransmission Power management: put the receiver into sleep state or take it out More: additional frames coming W: wired equivalent privacy O: processed strictly in order Duration: time length of the frame and ack Addresses1.2.3 and 4: Source, destination, the source and destination base stations for intercell traffic Sequence: Sequence No. 47

48 Services Distribution Services (managing cell membership, and interacting with station outside the cell) Association: To connect to a base station Disassociation: To disconnect from a base station Reassociation: To change its preferred base station Distribution: ib i How to route frames (local or intercell) Integration: Translation from to other protocol frame format 48

49 Services Intracell Services Authentication Deauthentication i (leave the network) Privacy: managing the encryption Data Delivery and decryption 49

50 Data Link Layer Switching Bridges from 802.x to 802.y Local Internetworking Spanning Tree Bridges Remote Bridges Repeaters, Hubs, Bridges, Bid Switches, Routers, Gateways Virtual LANs 50

51 Data Link Layer Switching Multiple LANs connected by a backbone to handle a total load higher than the capacity of a single LAN. 51

52 Bridges from 802.x to 802.y Operation of a LAN bridge from to

53 Bridges from 802.x to 802.y (2) The IEEE 802frame formats. The drawing is not to scale. 53

54 Local Internetworking When the bridges are first plugged in, they use the flooding algorithm and the backward learning algorithm to establish the routing table A configuration with four LANs and two bridges. 54

55 The routing procedure of bridges 1. If destination and source LANs are the same, discard the frame 2. If the destination and source LANs are different, forward the frame 3. If the destination LAN is unknown, use flooding 55

56 Spanning Tree Bridges To prevent looping Two parallel transparent bridges. 56

57 The procedure to establish a spanning tree 1. Take the bridge with lowest serial number as the root 2. Compute the shortest path from the root to ever bridge and LAN 3. Connect these shortest paths to from a tree (no looping) 57

58 Spanning Tree Bridges (2) (a) Interconnected LANs. (b) A spanning tree covering the LANs. The dotted dlines are not part of the spanning tree. 58

59 Remote Bridges To connect two (or more) distance LANs Remote bridges can be used to interconnect distant LANs. 59

60 Repeaters, Hubs, Bridges, Switches, Routers and Gateways (a) Which device is in which layer. (b) Frames, packets, and headers. 60

61 Repeaters, Hubs, Bridges, Switches, Routers and Gateways (2) When two frames arrive simultaneously at a hub, they will collide Bridges and switched will route the frames based on their destination addresses Bridges connect LANs (a) A hub. (b) A bridge. (c) a switch. 61

62 Virtual LANs A building with centralized wiring using hubs and a switch. 62

63 The reasons for virtual LANs (a) Fitting into the organization structure (b) Loading partition (c) Relieving broadcast storm 63

64 Virtual LANs (2) VLANs are based on specially designed VLAN aware switches (bridges) (a) Four physical LANs organized into two VLANs, gray and white, by two bridges. (b) The same 15 machines organized into two VLANs by switches. 64

65 Each VLAN is assigned a dst distinct ctcolor. o Three methods ae are used to distinguish the color of an incoming frame 1. Every port is assigned a VLAN color (when a host moved, the port must be reassigned) 2. Every MAC address is assigned a VLAN color 3. Every layer 3 protocol or IP address is assigned a VLAN color (The payload must be examined by the data link layer, which violates the rule: independence of the layers. When the layer 3 protocol changed, the switch fails.) 65

66 There are some issues for VLAN (a) What is thevlan field format? (b) How to identify VLAN field? (c) Who generates the VLAN field? (d) What happens to frames that are already the maximum size? The 802.1Q will solve these problems 66

67 The IEEE 802.1Q Standard To support VLAN, switches must be VLAN aware. Transition from legacy Ethernet to VLAN aware Ethernet. The shaded symbols are VLAN aware. The empty ones are not. 67

68 The IEEE 802.1Q Standard (2) Priority: This field makes it possible to distinguish hard real time traffic from soft real time traffic from time insensitive traffic. CFI (Canonical Format Indicator): To indicate that the payload contains VLAN Identifier: To indicate which VLAN the frame belong to. The (legacy) and 802.1Q Ethernet frame formats. 68

69 Summary 69 Channel allocation methods and systems for a common channel.