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CPEG 419 Introduction to Networks

CPEG 419 Introduction to Networks. New Homework is on the web. It is due in two weeks. No class on Thursday. Most likely, I will not miss any more classes in October. CRC Example – I was right!. Frame M 1010001101 = x 9 +x 7 +x 3 +x 2 +x 0 . Pattern G 110101.

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CPEG 419 Introduction to Networks

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  1. CPEG 419 Introduction to Networks New Homework is on the web. It is due in two weeks. No class on Thursday. Most likely, I will not miss any more classes in October. EE/CS 450 Spring 00

  2. CRC Example – I was right! • Frame M 1010001101 = x9+x7+x3+x2+x0. • Pattern G 110101. • Dividing (frame*25) by pattern results in 01110. • Thus [M|V] = 1010001101 | 01110. • Receiver can detect errors unless received message Tr is divisible by G. University of Delaware CPEG 419

  3. Fragmentation • A packet is broken up into smaller pieces. Each fragment is sent as a frame. • Link layer has an upper bound on the size of frame. E.g., ethernet frames must be smaller than 1500 bytes. ATM must be 53 bytes, cable modem use 204 bytes. • P1 = (1-Pb)^F. So the smaller the F, the smaller P1. • But the smaller the F, the lower the throughput. The frame has overhead, e.g., the destination address, source address, etc. So a frame of 64 byte Ethernet frame will only contain 38 bytes of data. University of Delaware CPEG 419

  4. Example Data Link Layer Protocol • High-Level Data Link Control (HDLC) • Widely-used (ISO standard). • Single frame format. • Synchronous transmission. University of Delaware CPEG 419

  5. HDLC: Frame Format • Flag: frame delimiters (01111110). • Address field for multipoint links. • 16-bit or 32-bit CRC. • Refer to book (pages 214-221) for more details. flag address flag control data FCS 8 bits 8 ext. 8 or 16 8 variable 16 or 32 University of Delaware CPEG 419

  6. Other DLL Protocols 2 • LLC: Logical Link Control 802.2 • Part of the 802 protocol family for LANs. • Link control functions divided between the MAC layer and the LLC layer. • LLC layer operates on top of MAC layer. Dst. MAC addr LLC ctl. Src. MAC addr Dst. LLC addr Src. LLC addr MAC control Data FCS University of Delaware CPEG 419

  7. Other DLL Protocols 3 • SLIP: Serial Line IP • Dial-up protocol. • No error control. • Not standardized. • PPP: Point-to-Point Protocol • Internet standard for dial-up connections. • Provides framing similar to HDLC. University of Delaware CPEG 419

  8. Multiplexing • Sharing a link/channel among multiple source-destination pairs. • Example: high-capacity long-distance trunks (fiber, microwave links) carry multiple connections at the same time. MUX DEMUX . . . . . . University of Delaware CPEG 419

  9. Multiplexing Techniques • 3 basic types: • Frequency-Division Multiplexing (FDM). • Time-Division Multiplexing (TDM). • Statistical Time-Division Multiplexing (STDM). University of Delaware CPEG 419

  10. FDM 1 • High bandwidth medium when compared to signals to be transmitted. • Widely used (e.g., TV, radio). • Various signals carried simultaneously where each one modulated onto different carrier frequency, or channel. • Channels separated by guard bands (unused) to prevent interference. University of Delaware CPEG 419

  11. FDM 2 University of Delaware CPEG 419

  12. TDM 1 • TDM or synchronous TDM. • High data rate medium when compared to signals to be transmitted. University of Delaware CPEG 419

  13. TDM 2 • Time divided into time slots. • Frame consists of cycle of time slots. • In each frame, 1 or more slots assigned to a data source. U1 U2 ... UN 1 2 ... 1 2 ... N N Time frame University of Delaware CPEG 419

  14. TDM 3 • No control info at this level. • Flow and error control? • To be provided on a per-channel basis. • Use DLL protocol such as HDLC. • Examples: SONET (Synchronous Optical Network) for optical fiber. • +’s: simple, fair. • -’s: inefficient. University of Delaware CPEG 419

  15. Statistical TDM 1 • Or asynchronous TDM. • Dynamically allocates time slots on demand. • N input lines in statistical multiplexer, but only k slots on TDM frame, where k < n. • Multiplexer scans input lines collecting data until frame is filled. • Demultiplexer receives frame and distributes data accordingly. University of Delaware CPEG 419

  16. STDM 2 • Data rate on mux’ed line < sum of data rates from all input lines. • Can support more devices than TDM using same link. • Problem: peak periods. • Solution: multiplexers have some buffering capacity to hold excess data. • Tradeoff data rate and buffer size (response time). • When two sources transmit at the same time, a decision must eb made as to who gets to go first (fairness is an issue). University of Delaware CPEG 419

  17. LAN Systems - Chapter 14 Examples of Link Layer MAC Layer – Medium Access Control LLC – Logical Link Layer EE/CS 450 Spring 00

  18. Local Area Networks 1 • Interconnect devices over short distances. • Within same floor, • Building, • Campus. • Characterized by low delays. University of Delaware CPEG 419

  19. LANs 2 • Typically use common broadcast medium. • Hosts share same communication medium. • Also called multiple-access networks. • LANs are characterized by: • Topology – how nodes are hooked together. • Transmission medium. • Medium access control mechanism. University of Delaware CPEG 419

  20. LAN Protocol Architecture • LAN protocol standards collectively known as IEEE 802 reference model. Application OSI Upper layer protocols Presentation Session Transport IEEE 802 Network LLC Data link MAC Physical Physical University of Delaware CPEG 419

  21. LAN Protocols • MAC sublayer: performs functions that control access to shared medium. • LLC – logical link control (the past two lectures): performs flow and error control and provides services to upper layer. University of Delaware CPEG 419

  22. 802 standards 1 • LLC: IEEE 802.2 • connectionless and connection oriented services. • Reliable and unreliable. • Ethernet (802.3) includes 802.2, but typically does not use the reliable features. So what why is 802.2 included? Just in case you wanted to make the link reliable. University of Delaware CPEG 419

  23. 802 standards 2 • MAC + physical layers • 802.3 802.5 • Bus/tree/star topologies. Ring topology. • CSMA/CD. Token ring. • E.g., Ethernet • 802.4 FDDI • Bus/tree/star topologies. Dual bus (optical). • Token bus. Token ring. • 802.11 • Wireless. • CSMA. University of Delaware CPEG 419

  24. Encapsulation of Frame Application data TCP header IP header LLC header MAC MAC header trailer TCP segment IPdatagram LLC PDU MAC frame University of Delaware CPEG 419

  25. LLC for LANs • Similar functions as general LLCs. • But it has to interface with MAC sublayer. • LLC functions: • Addressing: source and destination. • LLC address versus MAC address. • Control data exchange between 2 users. • User as higher-layer protocol in the station. University of Delaware CPEG 419

  26. LLC Services • 3 different services: • Unacknowledged connectionless (type 1). • No error or flow control. • No delivery guarantees. • Connection-mode (type 2). • Logical connection established. • Flow and congestion control provided. • Acknowledged connectionless (type 3). • No logical connection. • Flow and error control. University of Delaware CPEG 419

  27. LLC (802.2) Protocol • Similar to HDLC (ISO standard). • LLC PDU: 1 byte 1 or 2 bytes 1 byte variable Information DSAP SSAP LLC control University of Delaware CPEG 419

  28. MAC Frame Format Dst. MAC addr Dst. LLC addr Src. MAC addr Src. LLC addr MAC control CRC LLC PDU MAC control: protocol information (protocol type, version #). Destination MAC address: physical address of LAN destination. Source MAC address: physical address of the LAN source. University of Delaware CPEG 419

  29. LAN Topologies Star Ring Tree Central node Bus University of Delaware CPEG 419

  30. Bus Topology • Use of multipoint medium. • Stations attach to bus through tap. • Full-duplex communication allows data to be sent to/received from bus. • Transmission from any station propagates in both directions and is received by all. • Media Access Control is required to gain control of the bus. University of Delaware CPEG 419

  31. Tree Topology • Tree is generalization of bus. • Headend: start of 1 or more cables (branches). • Transmission from one station propagates to all others. University of Delaware CPEG 419

  32. Issues • Inherently, broadcast. • Frames to transmit data. • Need for specifying the destination. • Addresses. • Multi-access. • Need for controlling access to medium. • Avoid collisions. • MAC protocol. University of Delaware CPEG 419

  33. Ring Topology 1 • Stations attach to repeaters. • Repeaters are linked to each other by point-to-point links forming a closed loop. • Links are unidirectional. • Repeaters: receive data from one link and repeat it on the other with no buffering. University of Delaware CPEG 419

  34. Ring 2 • Stations transmit/receive via repeater. • Frames circulate past all stations; destination copies frame as it goes by; source removes frame. • Ring shared by multiple stations. • Need MAC protocol to determine when each station may insert frame. University of Delaware CPEG 419

  35. Star Topology • Each station directly connected to central node via point-to-point link. • Central node’s modes of operation: • Broadcast mode: node broadcasts received frame on all other links; logically works like bus. • Switching mode: node sends frame out only on the link to the destination. • MAC is easy. • Central node as single-point of failure. University of Delaware CPEG 419

  36. Medium Access Control • Control access to shared medium. • Where and how? • Where: centralized versus decentralized. • How: synchronous versus asynchronous. University of Delaware CPEG 419

  37. Centralized versus Distributed MAC • Centralized approaches: • Controller grants access to medium. • Simple, greater control: priorities, QoS. • But, single point of failure and performance bottleneck. • Decentralized schemes: • All stations collectively run MAC to decide when to transmit. University of Delaware CPEG 419

  38. Synchronous versus Asynchronous • Synchronous approaches: • Static channel allocation. • Examples: FDM, TDM. • Simple but inefficient. • Asynchronous or dynamic: • Example: STDM. • 3 categories: round-robin, reservation, and contention. University of Delaware CPEG 419

  39. Round-Robin MAC • Each station is allowed to transmit; station may decline or transmit (bounded by some maximum transmit time). • Centralized (e.g., polling) or distributed control of who is next to transmit. • When done, station relinquishes and right to transmit goes to next station. • Efficient when many stations have data to transmit over extended period (stream). University of Delaware CPEG 419

  40. Reservation • Time divided into slots. • Station reserves slots in the future. • Multiple slots for extended transmissions. • Suited to stream traffic. University of Delaware CPEG 419

  41. Contention • No control. • Stations try to acquire the medium. • Distributed in nature. • Perform well for bursty traffic. • Can get very inefficient under heavy load. • NOTE: round-robin and contention are the most common. University of Delaware CPEG 419

  42. Standardized MACs Topologies Bus Ring Techniques Token bus (802.4) Polling (802.11) Token ring (802.5; FDDI) Round robin Reservation DQDB (802.6) Contention CSMA/CD (802.3) CSMA(802.11) University of Delaware CPEG 419

  43. Wireless LANs • Use wireless transmission media. • Infrared (IR): limited to indoors and single room (IR light doesn’t penetrate walls). • Radio • Narrowband microwave. • Spread Spectrum LANs. University of Delaware CPEG 419

  44. Wireless LAN Applications • Nomadic access (e.g., users roaming around campus). • LAN interconnection (e.g., across buildings). • Ad Hoc Networks (e.g., disaster relief crew). University of Delaware CPEG 419

  45. MAC Protocols • Contention-based • ALOHA and Slotted ALOHA. • CSMA. • CSMA/CD. • Round-robin : token-based protocols. • Token bus. • Token ring. University of Delaware CPEG 419

  46. The ALOHA Protocol • Developed @ U of Hawaii in early 70’s. • Packet radio networks. • “Free for all”: whenever station has a frame to send, it does so. • Station listens for maximum RTT for an ACK. • If no ACK, re-sends frame for a number of times and then gives up. • Receivers check FCS and destination address to ACK. University of Delaware CPEG 419

  47. Collisions • Invalid frames may be caused by channel noise or • Because other station(s) transmitted at the same time: collision. • Collision happens even when the last bit of a frame overlaps with the first bit of the next frame. University of Delaware CPEG 419

  48. ALOHA’s Performance 1 t0+t t0+3t t0 t0+2t Time vulnerable University of Delaware CPEG 419

  49. ALOHA’s Performance 2 • S = G e-2G, where S is the throughput (rate of successful transmissions) and G is the offered load. • S = Smax = 1/2e = 0.184 for G=0.5. University of Delaware CPEG 419

  50. Slotted Aloha • Doubles performance of ALOHA. • Frames can only be transmitted at beginning of slot: “discrete” ALOHA. • Vulnerable period is halved. • S = G e-G. • S = Smax = 1/e = 0.368 for G = 1. University of Delaware CPEG 419

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