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Data Link Layer

Data Link Layer. Two sublayer: Medium access sublayer (MAC) Logical link control (LLC). Data Link Layer. LANs -- Referred to as: Multiaccess channels Random access channels LANs Characterized by: Data rate of at least several Mbps Low error rates

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Data Link Layer

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  1. Data Link Layer Two sublayer: • Medium access sublayer (MAC) • Logical link control (LLC)

  2. Data Link Layer • LANs -- Referred to as: • Multiaccess channels • Random access channels • LANs Characterized by: • Data rate of at least several Mbps • Low error rates • A diameter of not more than a few kilometers • Complete ownership by a single organization

  3. Networks Two types • Point-to-Point e.g., WANs • Broadcast e.g., Packet radio, Satellite LANs Note: In between LANs and WANs are MANs.

  4. Channel Allocation in LANs and MANs • Static e.g., FDM • Dynamic e.g., Slotted time, Carrier sense Note: MANs use LANs Technology.

  5. The 3 popular types of LCNs

  6. Basic packet radio architecture Central controller Central Resources (a) Centralized (b) Distributed

  7. ALOHA A medium access control technique for multiple access transmission media. • Pure ALOHA -- a station transmits whenever it has data to send. Unacknowledged transmissions are repeated. Notation for analysis: • S: Throughput of the network • G: Total rate of data presented • I: Total Rate of data generated by the stations (input load) • D: Average delay between the time a packet is ready for transmission and the completion of successful transmission.

  8. ALOHA (cont.) Assumptions: 1. All packets are of constant length 2. The channel is noise-free 3. Packets do not queue at individual stations (i.e., I=S) 4. G is Poisson distributed

  9. ALOHANET BroadcastChannel Multiplexing Data packet from user node f1 channel Generate ACK ACK queue 1 f2 channel Data packet queue Data packet to user nodes 2

  10. ALOHA Protocols Collides with the start of the shaded frame Collides with the end of the shaded frame t t0 t0 + t t0 + 2t t0 + 3t Time Vulnerable Vulnerable period for the shaded frame

  11. Pure ALOHA (cont.) G = S + (# of retransmitted packets per unit time) Now, express rate of retransmission as: G ´ Pr(individual packet suffers a collision) For a Poisson process with rate l, The Pr of transmission in a period of time t is 1 - e-lt. Thus the Pr of transmission during the vulnerable period is 1 - e-2G. Therefore G = S + G(1 - e-2G) So ALOHA: S = G e-2G

  12. Pure ALOHA (cont.) Note: If we differentiate S = Ge-2G with respect to G and set it equal to 0, we find the max occurs at G = 0.5 and that S = 1 / 2e = 0.18. So, the maximum thru put is only 18% of capacity. ALOHANET uses a data rate of 9600bps Þ max total throughput (sum of data arriving from all user nodes) is only 0.18 ´ 9600 = 1728bps.

  13. Slotted ALOHA Channel is organized into uniform slots whose size equals the packet transmission time. Transmission is permitted only to begin at a slot boundary. Note: Since the vulnerable period is now reduced in half, the Pr of transmission during this period is 1 - e-G; thus we have S-ALOHA: S = Ge-G Now, differentiating with respect to G, we have the max possible value for S is 1 / e = 0.37 or 37%.

  14. Slotted ALOHA (cont.) Throughput versus offered traffic for ALOHA system

  15. Delay (approx) Time interval from when a user is ready to transmit a packet until when it is successfully received by the central node. Simply the sum of queuing delay, propagation delay, and transmission time. Note: ALOHA has queueing delay = 0. So, we need to view queueing time in the context of above definition for delay.

  16. Delay (approx) (cont.) Expected # of transmissions per packet º G / SÞ Expected # of retransmissions per packet º G / S -1 G / S - 1 = e2G - 1 so D = (e2G - 1) d + a + 1, where d is the average delay for one retransmission ALOHA: D = (e2G - 1)(1 + 2a + w + (K+1)/2) + a + 1 Note: Assume no collision for w

  17. IEEE 802 Standards For LANs Include: ì CSMA/CD í Token bus î Token ring Standards ¸ parts: • 802.1 -- Introduction to set of standards and define the interface primitives • 802.2 -- Describes upper part of data link layer which uses LLC protocol • 802.3 - 802.5 -- Describe the three LAN standards

  18. IEEE Standards For LANs (a) Position of the transiver and interface (b) Connecting two cable segments with a repeater

  19. Cable topology A B C D Tap (a) Linear (a) Spine

  20. Cable topology (cont.) A B C D E F Selective repeater (c) Tree (d) Segmented

  21. Nonpersistent • 1-persistent • P-persistent CSMA persistence and backoff Non-persistent: Transmit if idle Otherwise, delay, try again Constant or variable Delay Time Channel busy Ready 1-persistent: Transmit as soon as channel goes idle If collision, back off and try again P-persistent: Transmit as soon as channel goes idle with probability P Otherwise, delay one slot, repeat process Carrier Sense Multiple Access (CSMA)

  22. CSMA/CD Physical Layer Current Standard Baseband coaxial cable (50W) 500 M segments, 100 Taps/segment Maximum 4 repeaters in path 10 Mbps Similar to Ethernet

  23. For Baseband CSMA/CD, packet length should be at least twice the propagation delay (a£ 0.5)

  24. For Broadband CSMA/CD, packet length should be at least quadruple the propagation delay (a£ 0.5)

  25. Comparison of the channel utilization versus load for various random access protocols.

  26. The 802.3 Frame Format Byte 7 1 2 or 6 2 or 6 2 0 - 1500 0-46 4 • Destination address • High-order bits (bit 47) • 0 Þ ordinary addresses • 1 Þ group addresses (multicast) Dest. address Source address Preamble Data Pad Checksum Start of frame delimiter Length of Data field

  27. The 802.3 Frame Format (cont.) • Destination address • All 1 bits Þ broadcasting Note: Such frame is propagated by all bridges • Bit 46 designated for: • Local address, assigned by network adm. • Global (address, assigned by IEEE) ~ 7 ´ 1013 global addresses. • Data length and data Frame must be at least 64 bits long from the destination address to the checksum. • Pad: Used to fill out the minimum size frame

  28. IEEE STD 802.4: Token Bus • Example: GM (MAP) • Logically, all stations are organized into a ring • Note: 802.4 MAC protocol is very complex, with each station having to maintain 10 different times and more than 2 dozen state variables. More than 200 pages. • Token º A special control frame, and only the token holder is permitted to transmit frames.

  29. IEEE STD 802.4: Token Bus (cont.) A token bus

  30. Token Bus MAC Sublayer Protocol • Stations are inserted into ring in order of station address, from highest to lowest. • Token passing is also done from high to low addresses. • Four priority classes: (0, 2, 4, 6) for traffic, with 0 the lowest and 6 the highest. When the token comes into the station, it passes to priority 6 substation, which may begin transmitting frames, if it has any. When it is done, (or when its timer expires), the token is passed to the priority 4 substations, etc.

  31. Token Bus Priority Scheme

  32. Ring Maintenance Frame control field Name Meaning 00000000 Claim_token Claim token during ring initialization 00000001 Solicit_successor_1 Allows stations to enter the ring 00000010 Solicit_successor_2 Allows stations to enter the ring 00000011 Who_follows Recover from lost token 00000100 Resolve_contention Used when multiple stations want to enter the ring 00001000 Token Pass the token 00001100 Set_successor Allows stations to leave the ring The token bus control frames

  33. Logical Ring Maintenance Adding a station • Each station's interface must maintain address of predecessor and successor stations. • Periodically, the token holder solicits bids from stations not currently in the ring and wish to join. • Resolve contention -- token holder runs an arbitration algorithm when 2 or more stations bid to enter. All station interfaces maintain 2 random bits which are used to delay all bids by 0, 1, 2, or 3 slot times.

  34. Logical Ring Maintenance (cont.) Deleting a station • A station, X, with successor S, and predecessor P, leaves the ring by sending P a set_successor frame. Initialization • Special case of adding new station. When first station comes on line, it notices that there is no traffic for a certain time period. Then it sends a claim_token frame, and later solicit bids from stations to join.

  35. Failure (Stations) If a station tries to pass the token to a failed station, it listens to see if the station either transmits a frame or passes the token. If it does neither, the token is passed a second time. If that also fails, the station transmits a who_follows, specifying the address of its successor. If this fails, the station sends a solicit_successor_2 frame, etc.

  36. Failure (Stations) (cont.) Token failure • Use the ring initialization algorithm. Each station has a timer that is reset whenever a frame appears on the network. When timer hits a threshold value, the station issues a claim_token. Multiple tokens • If a station holding the token notices a transmission from another station, it discards its token.

  37. Sender looks for free token Changes free token to busy token and appends data Receiver copies data addressed to it Sender generates free token upon receipt of physical transmission header (from addressee)

  38. Ring interface Ring interface 1 bit delay (a) A ring network (b) Listen mode (c) Transmission mode

  39. Station Cable Bypass relay Connector Wire center Four stations connected via a wire center

  40. Ring Maintenance (cont.) When the sending station drains the frame from the ring, it examines the A and C bits: 1. A = 0 and C = 0: destination not present or not powered up. 2. A = 1 and C = 0: destination present but frame not accepted. 3. A = 1 and C = 1: destination present and frame copied.

  41. Ring Maintenance • Monitor station oversees the ring • Every station has the capability of becoming the monitor • Monitor station responsibility • Lost token • Ring breaks • Cleaning up ring • Orphan frame • Garbled frame • 802.4 committee interested in fractory issues, 802.5 committee interested in office automation

  42. IEEE token ring priority scheme 1. A is sending to B, D reserves at higher level 2. A generates higher priority token and remembers lower priority 3. D uses higher priority token to send data to C 4. D generates token at higher level 5. A sees the high priority token and captures it. 6. A generates token at the pre-empted, lower priority level 1 4 2 5 3 6

  43. Ring Maintenance (cont.) Frame control field Name Meaning 00000000 00000010 00000011 00000100 00000101 00000110 Duplicate address test Beacon Claim token Purge Active monitor present Stand by monitor present Test if two stations have same address Used to locate breaks in the ring Attempt to become monitor Reinitialize the ring Issued periodically by the monitor Announces the presence of potential monitors Token ring control frames

  44. FDDI (Fiber Distributed Data Interface) • 100 Mbps over distances up to 200km up to 1000 stations. • Distance between 2 successive nodes cannot exceed 2km. • Uses multimode fiber. • Uses LEDs rather than lasers. • Design consists of 2 fiber rings.

  45. An FDDI ring being used as a backbone to connect LANs and computers

  46. (a) FDDI consists of two counterrotating rings. (b) In the event of failure of both rings at one point, the two rings can be joined together to form a single long ring.

  47. FDDI (cont.) • 2 classes of stations, A and B. • Class A stations connect to both rings. • Class B stations only connect to 1 ring. • Traffic (2 types) • Synchronous (e.g., audio, video info) • Asynchronous (e.g., data traffic) • Uses 4 out of 5 encoding schemes to save bandwidth

  48. FDDI token ring operation 1. A seizes token and begins transmitting frame F1 to C 2. A appends token to end of transmission 3. B seizes token transmits F2 to D 4. B emits token. D copies F2. A absorbs F1. 5. A lets F2 and token pass. B absorbs F2. 6. B lets token pass 1 4 2 5 3 6

  49. LAN standard MAC frame formats (a) CSMA/CD Octets 7 1 2, 6 2, 6 2 0 - 1500 4 Preamble SFD DA SA Length Data Pad FCS (a) Token Bus 1 1 1 2, 6 2, 6 >= 0 4 1 Preamble SD FC DA SA Data FCS ED (a) Token Ring 1 1 1 2, 6 2, 6 >= 0 4 1 1 SD AC FC DA SA Data FCS ED FS (a) FDDI 8 1 1 2, 6 2, 6 >= 0 4 1 1 Preamble SD FC DA SA Data FCS ED FS AC: Access Control DA: Destination Address ED: Ending Delimiter FC: Frame Control FCS: Frame Check Sequence FS: Frame Status SA: Source Address SD: Starting Delimiter SFD: Start Frame Delimiter

  50. Name Cable Max. segment Nodes/seg. Advantages 100 30 1024 1024 10BASE5 10BASE2 10BASE-T 10BASE-F Thick coax Thin coax Twisted pair Fiber optics 500 m 200 m 100 m 2000 m Good for backbones Cheapest system Easy maintenance Best between buildings The most common kinds of baseband 802.3 LANS Physical Layer Specificationsfor LAN standards

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