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NETE0510 LANs and Hi-speed LANs

NETE0510 LANs and Hi-speed LANs. Dr. Supakorn Kungpisdan supakorn@mut.ac.th. Outline. LAN Overview Ethernet Token Ring FDDI. LAN Topologies. LAN Protocol Architecture. IEEE 802 Layers. Physical encoding/decoding of signals preamble generation/removal bit transmission/reception

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NETE0510 LANs and Hi-speed LANs

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  1. NETE0510LANs and Hi-speed LANs Dr. Supakorn Kungpisdan supakorn@mut.ac.th NETE0510: Communication Media and Data Communications

  2. Outline • LAN Overview • Ethernet • Token Ring • FDDI NETE0510: Communication Media and Data Communications

  3. LAN Topologies NETE0510: Communication Media and Data Communications

  4. LAN Protocol Architecture NETE0510: Communication Media and Data Communications

  5. IEEE 802 Layers • Physical • encoding/decoding of signals • preamble generation/removal • bit transmission/reception • transmission medium and topology • Logical Link Control • interface to higher levels • flow and error control • Media Access Control • on transmit assemble data into frame • on receive disassemble frame • govern access to transmission medium • for same LLC, may have several MAC options NETE0510: Communication Media and Data Communications

  6. LAN Protocols in Context NETE0510: Communication Media and Data Communications

  7. Logical Link Control (LLC) • transmission of link level PDUs between stations • must support multi-access, shared medium • but MAC layer handles link access details • addressing involves specifying source and destination LLC users • referred to as service access points (SAP) • typically higher level protocol NETE0510: Communication Media and Data Communications

  8. LLC Services • based on HDLC • 3 services provided: • Unacknowledged connectionless service • Simple, no flow- and error control, no data delivery guaranteed  rely on higher layer protocols • Connection-mode service • Similar to that offered by HDLC • Need connection setup, provide flow and error control • Acknowledged connectionless service • Hybrid approach • No connection setup required, but require acknowledgement NETE0510: Communication Media and Data Communications

  9. Media Access Control (MAC) • MAC layer receives data from LLC layer • fields • MAC control • destination MAC address • source MAC address • LLC • CRC • MAC layer detects errors and discards frames • LLC optionally retransmits unsuccessful frames (link-to-link retransmission, not end-to-end) NETE0510: Communication Media and Data Communications

  10. Outline • LAN Overview • Ethernet • Token Ring • FDDI NETE0510: Communication Media and Data Communications

  11. Ethernet (CSMA/CD) • most widely used LAN standard • developed by • Xerox - original Ethernet • IEEE 802.3 • Carrier Sense Multiple Access with Collision Detection (CSMA/CD) • random / contention access to media NETE0510: Communication Media and Data Communications

  12. Pure VS Slotted ALOHA Pure ALOHA Slotted ALOHA NETE0510: Communication Media and Data Communications

  13. ALOHA • developed for packet radio nets • when station has frame, it sends • then listens for a bit over max round trip time (RTT) • if receive ACK then fine • if not, retransmit • if no ACK after repeated transmissions, give up • uses a frame check sequence (as in HDLC) to check for errors • 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% NETE0510: Communication Media and Data Communications

  14. Slotted ALOHA • time on channel based on uniform slots equal to frame transmission time • need central clock (or other sync mechanism) • transmission begins only at the beginning of the slot • So, frames either miss or overlap totally • max utilization 37% • both have poor utilization • fail to use fact that propagation time (PT) is much less than frame transmission time (TT) • If PT >> TT, a station may succeed in transmitting a frame • If TT >> PT, none of the stations may not succeed NETE0510: Communication Media and Data Communications

  15. CSMA • stations soon know transmission has started • so first listen for clear medium (carrier sense) • if medium idle, transmit • if two stations start at the same instant, collision • wait reasonable time • if no ACK then retransmit • collisions occur occur at leading edge of frame • max utilization depends on propagation time (medium length) and frame length • shorter PT, longer frame, higher utilization • Also work well for the case that PT << TT • Collision can occur only more than one user begins transmitting within PT NETE0510: Communication Media and Data Communications

  16. CSMA Persistence and Backoff NETE0510: Communication Media and Data Communications

  17. Non-persistent CSMA • Non-persistent CSMA rules: • if medium is idle, transmit • if medium is busy, wait for amount of time drawn from probability distribution (retransmission delay) & retry • random delays reduces probability of collisions • capacity is wasted because medium will remain idle following end of transmission even stations are waiting to transmit frames NETE0510: Communication Media and Data Communications

  18. 1-persistent CSMA • 1-persistent CSMA avoids idle channel time • 1-persistent CSMA rules: • if medium idle, transmit; • if medium busy, listen until idle; then transmit immediately • 1-persistent stations are selfish • if two or more stations waiting, a collision is guaranteed NETE0510: Communication Media and Data Communications

  19. P-persistent CSMA • a compromise to try and reduce collisions and idle time • p-persistent CSMA rules: • if medium idle, transmit with probability p, and delay one time unit (equal to max propagation delay) with probability (1–p) • if medium busy, listen until idle and repeat step 1 • if transmission is delayed one time unit, repeat step 1 • issue of choosing effective value of p to avoid instability under heavy load NETE0510: Communication Media and Data Communications

  20. CSMA/CD • with CSMA, collision occupies medium for duration of transmission • better if stations listen whilst transmitting • CSMA/CD rules: • if medium idle, transmit • if busy, listen for idle, then transmit • if collision detected, jam and then cease transmission • after jam, wait random time (backoff period) then retry NETE0510: Communication Media and Data Communications

  21. CSMA/CD (cont’d) NETE0510: Communication Media and Data Communications

  22. 1. Adaptor receives datagram from net layer & creates frame 2. If adapter senses channel idle (no signal energy entering adapter for 96 bit times), it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits 3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame ! 4. If adapter detects another transmission while transmitting, aborts and sends 48-bit jam signal 5. After aborting, adapter enters exponential backoff: after the mth collision, adapter chooses a K at random from {0,1,2,…,2m-1}. Adapter waits K·512 bit times and returns to Step 2 CSMA/CD Algorithm NETE0510: Communication Media and Data Communications

  23. Jam Signal: make sure all other transmitters are aware of collision; 48 bits Bit time: time to send 1 bit of data = 0.1 microsec for 10 Mbps Ethernet ;for K=1023, wait time is about 50 msec (1023 x 512 x 0.1 = 52378 microsec = 52.38 msec) Exponential Backoff: Goal: adapt retransmission attempts to estimated current load heavy load: random wait will be longer first collision: choose K from {0,1}; delay is K· 512 bit transmission times after second collision: choose K from {0,1,2,3}… after ten collisions, choose K from {0,1,2,3,4,…,1023} CSMA/CD (cont’d) NETE0510: Communication Media and Data Communications

  24. IEEE 802.3 Frame Format NETE0510: Communication Media and Data Communications

  25. 10Mbps Specification (Ethernet) NETE0510: Communication Media and Data Communications

  26. 100Mbps Fast Ethernet NETE0510: Communication Media and Data Communications

  27. 100BASE-X • uses a unidirectional data rate 100 Mbps over single twisted pair or optical fiber link • encoding scheme same as FDDI • 4B/5B-NRZI • two physical medium specifications • 100BASE-TX • uses two pairs of twisted-pair cable for tx & rx • STP and Category 5 UTP allowed • MTL-3 signaling scheme is used • 100BASE-FX • uses two optical fiber cables for tx & rx • convert 4B/5B-NRZI code group into optical signals NETE0510: Communication Media and Data Communications

  28. MTL-3 Encoding • An MLT-3 interface emits less electromagnetic interference and requires less bandwidth than most other binary or ternary interfaces that operate at the same data rate NETE0510: Communication Media and Data Communications

  29. 100BASE-T Options NETE0510: Communication Media and Data Communications

  30. Gigabit Ethernet Configuration NETE0510: Communication Media and Data Communications

  31. Gigabit Ethernet – Physical NETE0510: Communication Media and Data Communications

  32. 10Gbps Ethernet Options NETE0510: Communication Media and Data Communications

  33. Outline • LAN Overview • Ethernet • Token Ring • FDDI NETE0510: Communication Media and Data Communications

  34. Many types of token ring technology: IBM’s Token Ring IEEE802.5 Token Ring FDDI (Fiber Distribution Data Interface) IEEE802.17 Resilient Packet Ring A token ring network consists of nodes connected in a ring. Data always flows in a particular direction around the ring, with each node receiving frames from its upstream neighbor and then forwarding them to its downstream neighbor. Token Ring NETE0510: Communication Media and Data Communications

  35. Token Ring (cont’d) • Different from Ethernet: ring-based VS bus topology • Same  single shared-medium network • Two common features of Token Ring and Ethernet • Involve a distributed algorithm that controls when each node is allowed to transmit • All nodes see all frames; only the node identified in a frame as the destination will save a copy of the frame as it flows past. NETE0510: Communication Media and Data Communications

  36. Token Ring specifications • Data transfer rate is 4 or 16 Mbps • Uses Twisted Pair cabling (Cat 3 for 4 MB/s, Cat 5 for 16 Mb/s) for IBM’s Token Ring, but not specified in IEEE802.5 • Use Manchester encoding • Access method is token passing • Logical topology ring, physical topology is star • Connector type is RJ-45 • Maximum attachments per segment is 250 (IEEE 802.5) and 260 (IBM) per ring NETE0510: Communication Media and Data Communications

  37. Token • Token is a special sequence of bits circulating around the ring • Token Ring operation: • Each node receives and forwards the token • When a node that has a frame to transmit sees the token, it takes the token off the ring, and insert its frame into the ring • Each node along the way simply forwards the frame, with the destination node saving a copy and forwarding the message onto the next node on the ring. • When the frame makes its way back around to the sender, this node strips its frame off the ring and reinserts the token. • The media access algorithm is fair  the token circulates around the ring, each node gets a chance to transmit. NETE0510: Communication Media and Data Communications

  38. Physical Properties • Link or node failure would render the whole network useless • Solved by connecting each station into the ring using an electromechanical relay. NETE0510: Communication Media and Data Communications

  39. Physical Properties (cont’d) • Several of these relays are usually packed into a single box, known as a multi-station access unit (MSAU or MAU) NETE0510: Communication Media and Data Communications

  40. Token Ring Frame Format NETE0510: Communication Media and Data Communications

  41. Token Ring Frame Format (Cont’d) Access Control • T is token bit, set to specify the token frame • M is monitor bit, set by Active Monitor Frame Status • A=1, Address recognized • C = 1, Frame copied NETE0510: Communication Media and Data Communications

  42. Token Ring Media Access Control • As the token circulates around the ring, any station that has data to send may seize the token by simply modifying 1 bit (T bit) in the second byte token • The first 2 bytes of the modified token now become the preamble for the subsequent data packet. NETE0510: Communication Media and Data Communications

  43. Token Holding Time (THT) • Specify how long a given node is allowed to hold the token • How much data a given node is allowed to transmit each time it possesses the token • Time limit, data limit, or no limit? • Default THT for IEEE802.5 is 10 ms NETE0510: Communication Media and Data Communications

  44. Token Rotation Time (TRT) • The amount of time it takes a token to traverse the tine as viewed by a given node TRT ≤ ActiveNodes x THT + RingLatency • Where, • RingLatency denotes how long it takes to circulate around the ring where no one has data to send, • ActiveNodes denotes the number of nodes that have data to send NETE0510: Communication Media and Data Communications

  45. Reliable Delivery • 802.5 provides a form of reliable delivery using 2 bits in the frame status field, A and C bits • Initially A and C are 0s • When a destination station sees a frame, it sets A bit • When it copies the frame into its adaptor, it sets C bit • If the sending station receives the frame with A bit still 0, the recipient is not functioning or absent • If A bit is set, but C bit is 0, the destination could not accept the frame (may be the buffer is full). The sender may retransmit the frame NETE0510: Communication Media and Data Communications

  46. Token Ring Priority • The token contains a 3-bit priority field. It has certain priority n at any time • Each station that has data to send assigns priority to that frame, and the station can only seize the token to transmit a packet if the packet’s priority is at least as great as the token’s • The token’s priority changes over time due to 3 reservation bitsin Access Control field NETE0510: Communication Media and Data Communications

  47. Token Ring Priority (cont’d) • For example, Station X waiting to send a priority n packet may set the reservation bit to n if it sees the a data frame going past an the bits have not been set these bits to a higher value • So, the station that currently holds the token must reduce the priority of the token to n when it releases the token. NETE0510: Communication Media and Data Communications

  48. Token Release • Early release or delayed release • Early release allows better bandwidth utilization NETE0510: Communication Media and Data Communications

  49. Active and Standby Monitors • Every station in a token ring network is either an active monitor(AM) or standby monitor(SM) station. • However, there can be only one active monitor on a ring at a time. • Becoming an AM is chosen by election. Once an AM is chosen, every other station becomes a standby monitor. All stations must be capable of becoming an active monitor station if necessary. NETE0510: Communication Media and Data Communications

  50. Active Monitor Election • Electing AM is done when the ring is first connected or on the failure of the current AM. • The active monitor is chosen through an election or monitor contention process. • a loss of signal on the ring is detected, • an active monitor station is not detected by other stations on the ring, or • when a particular timer on an end station expires such as the case when a station hasn't seen a token frame in the past 7 seconds. • The station that detects the above situation will try to become a new AM by performs the following: NETE0510: Communication Media and Data Communications

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