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Data/Link Layer Issues

Data/Link Layer Issues

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Data/Link Layer Issues

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  1. Data/Link Layer Issues • Protocol & Services • Topology • Error Detection & Recovery

  2. Topology vs Geography Physical Layout How the signal actually travels Logical Layout "How devices talk to each other" -or- "How devices hear each other"

  3. Topologies

  4. BUS • Every node hears every other node's transmission directly.

  5. Ring • Series of unidirectional point-to-point links without "store & forward", usually with a bypass ability.

  6. Star • Switching functions all in central node

  7. Mesh • Each node independently routes over (bi-directional) point-to-point links.

  8. IEEE & OSI LLC 2 MAC 1 PHY LLC = Logical Link Control MAC = Media Access Control PHY = Physical

  9. Link/Physical Layer Standards • Ethernet • 10BASET, Fast Ethernet, Gigabit Ethernet • Token Ring • 4/16MB • FDDI • ATM

  10. Ethernet & IEEE 802.3 What the IEEE standard covers- Physical layer and interface to the link layer. IEEE 802.2 is the Link layer standard. History- DEC/Intel/Xerox came up with it, then submitted to IEEE for standardization. Some changes were made so Ethernet is not identical to IEEE 802.3 Differences between Ethernet and 802.3 There are some electrical and connector differences; most equipment uses IEEE 802.3. There is difference in the header. DIX uses TYPE, 802.3 uses LENGTH. SInce the frame is limited in size, the two coexist. Most people use the DIX format.

  11. Ethernet • Work started back in 1973 by Bob Metcalfe and David Boggs from Xerox Palo Alto Research Center (PARC). • He studied the Aloha network and "fixed" the mathematics. • Experimental Ethernet implemented in 1975. • Cooperative effort between Digital, Intel, and Xerox produced Ethernet Version 1.0 in 1980. • This also became known as the Blue Book specification or DIX standard. Ethernet V2.0 adopted in 1982. • Ethernet was adopted with modifications by the standards committees IEEE 802.3 and ANSI 8802/3. • Ethernet allows for only connectionless communication.

  12. CSMA/CD "Carrier Sense/Multiple Access with Collision Detection" "Driving in Boston" BUS! 51.2 microseconds "Many stations; Listen before talking; listen while talking; if a collision, backoff and try again"

  13. B C Address mismatch packet discarded Address mismatch packet discarded Send data to node D Address match packet processed Transmitted packet seen by all stations on the LAN (broadcast medium) A D Data Normal Ethernet Operation

  14. Ethernet Collisions B C Collision Data transmission for C Data transmission for A A D

  15. CSMA/CD - A Simple Definition • A network station wishing to transmit will first check the cable plant to ensure that no other station is currently transmitting (CARRIER SENSE). • The communications medium is one cable, therefore, it does allow multiple stations access to it with all being able to transmit and receive on the same cable (MULTIPLE ACCESS). • Error detection is implemented throughout the use of a station "listening" while it is transmitting its data. • Two or more stations transmitting causes a collision (COLLISION DETECTION) • A jam signal is transmitted to network by the transmitting stations that detected the collision, to ensure that all stations know of the collision. All stations will "backoff" for a random time. • Detection and retransmission is accomplished in microseconds.

  16. Frame/Packet Format Preamble SFD Dst Src Type Data/Pad FCS Size 7 1 6 6 2 46-1500 4 (octets) In IEEE 802.3, the Type field is used as a Length field. Addresses are generally (3) octets vendor code, (3) octets device number.

  17. Ethernet Addressing Each station recognizes three classes of addresses. • Own address • Broadcast address (all 1's) • Optionally, one or more multicast addresses Major reason for broadcast is address discovery. Multicast addresses are used for specialized link layer functions.

  18. Ethernet Cable Names Name Thick coaxial Thin coaxial Unshielded Twisted Pair Fiber RG-8 Wire Type 22 - 26 AWG 62.5/125 micron RG-58 10BASE5 10BASE2 10BASEF 10BASET IEEE Name N/A Standard Number IEEE 802.3 IEEE 802.3a IEEE 802.3i Other names Thick net Thin net UTP

  19. Thick braid for EMI Foil Thin braid for EMI Thin foil bonded to insulation Jacket of PVC or Teflon Thick Coax Makeup Center conductor of tin plated solid copper conductor Teflon is used for fire code regulations

  20. 500 meter maximum cable run Black marks every 2.5 meters to show transceiver placement Transceiver cable Transceiver Thick Coaxial Connection Pierce clamp

  21. Transceivers • Transmitter/Receiver: AUI on one side, media on the other • Used on all Ethernet networks and is the device that allows data to flow between the controller card and the network. • Detects errors on the bus cable plant and reports them to the station's controller card. • For thick coaxial cable, the transceiver is external to the controller card and attaches directly to the thick coaxial cable via a special cable known as the transceiver cable. • External transceivers have a SQE function that enables the controller to determine the status of the transceiver. • Usually has status indicators (LEDs) physically located on it to indicate the state of the transceiver (transmitting, receiving, collision, and power.)

  22. Thin Coaxial Cable Makeup Polyethylene foam Tinned copper wire Jacket made of PVC or Teflon EMI braided shielding

  23. Thin Coaxial Connection Concatenation of network attachments Direct connection to card T connector BNC connector at each cable end

  24. On-board transceiver logic ASIC 02608C Thin Coaxial Connection (cont.) AUI connector T connector for connection to cable plant BNC connector Interface to computer bus

  25. UTP Makeup • UTP was standardized by the IEEE 802.3 committee in October of 1990. • Standardized by the EIA under TIA 568A. • UTP for LANs is now classified as: • Category 3 - used for LANs up to 10 Mbps. • Category 4 - used for LANs up to 16 Mbps. • Category 5 - used for LANs up to 100 Mbps. • Cable is made up of 8 strands of 24 AWG wire. • Only 2 pair are used for single 10BASET connection.

  26. Unshielded Twisted Pair Unshielded twisted pair cable Repeater unit required 100m max cable run Straight through pins 1, 2, 3 and 6 Unshielded twisted pair atleast two (2) twists per foot RJ-45 connector RJ-45 Connector 8 pin 8 pin

  27. Concentrator (Hub) Management • With the concentration of the wiring into a common point, network managers can manage the hub with specialized software. • Network management software resides not only in the concentrator but on an external workstation’s device (a PC, for example). • The workstation can query the concentrator for information. • Concentrators also allow the control of individual ports. • This software allows managers to extract information from each card that is inserted in the repeater. You could query the hub for statistics such as: • number of packets (bytes), • number of collisions (single and multiple), • number of framing errors, • number of time the particular card de-inserted itself from the network, • ability to turn on/off any repeater card in the hub, and • all information is time and date stamped. • With 10BASET, all information is provided on an individual-connection basis, giving a manager information right from the desktop.

  28. Ethernet Repeaters • Extend the network by interconnecting multiple segments • Extend the physical domain of the network • Governed by the IEEE 802.3c working group standard. • This governs the electrical specifications of a repeater. • The physical configurations of a repeater varied from vendor to vendor. • Some repeaters contain the intelligence to: • detect collisions per cable plant (will not repeat collision fragments to other cable plants). • de-insert themselves from a wiring concentrator (when there are excessive errors on the cable plant). • submit network management information to a central controller. • Repeaters have been transformed into wiring concentrators or hubs • Repeaters can be used to interconnect different wiring types but not different access methods (i.e., not Token Ring to Ethernet).

  29. IEEE802.3 Efficiency "WARNING: Opinion" % Utilization Status 0 - 10 Great! 10 - 40 OK 40 - 60 Performance Problems -- look at it 60+ "Utilization" Signal On Time

  30. Token Ring - IEEE 802.5 What the IEEE standard covers History Differences between 802.5 and 802.3 "Physical layer standard (gives link layer format)" Essentially an IBM standard 'given' to the industry" "Guaranteed response Priorities Controlled delays"

  31. Token Ring History • Presented by IBM in 1982 to IEEE 802 committee. • First prototype developed in 1983 in Geneva, Switzerland. • Cabling System was announced in 1984. • Officially announced in 1985. • Standardized by IEEE in 1985. • Only one adopted by the IEEE 802.5 committee.

  32. Token Ring Technology Summary • Access method by which network attachments gain access to the cable plant by acquiring a special frame called the token. {Token is a special 24-bit pattern that continuously circulates the ring.} • Token Ring is a broadcast medium. {To receive data, a destination station performs an address match.} • The destination station merely copies the frame as it repeats it back to the ring. • When the frame arrives back to the source station, it strips the frame from the ring and then releases the token (4 megabit operation only). • The token is allowed to be released prior to frame reception on 16-megabit rings. • Token Ring originally ran at 4 Mbps. Upgraded in 1989 to 16 Mbps • Maximum frame size for 4 Mbps is 4472. • This is based only on the fact a station cannot hold the token longer than 10 milliseconds. • Maximum frame size for 16 Mbps is 17,800.

  33. TRN Features "data rate of 4 or 16Mbps" Traffic usually (always in 802.5) unidirectional RAR (802.5) vs RAT (FDDI) for Token Passing Recovery from lost token Priorities Frame Structure "one frame on the net at a time..."

  34. Controller Attachment to a MAU The IBM 8228 MAU Shielded or UTP cable Lobe cables

  35. Token Ring controller Cable Connectors Hermaphroditic or RJ-45 connectors on MAU DB-9 connector MAU Media filter for UTP only RJ-11 or RJ-45 connector Media filter can be on-board

  36. Ring out Ring in MAU Ring out Ring in MAU Type 6 patch cables Ring out MAU Ring in Multiple MAU Connection

  37. Closed Closed Closed MAU Operation Lobe cables Relays MAU top view Ring out Ring in MAU bus All stations are active

  38. MAU Operation (Inactive Station) Lobe cables Relays Closed Closed Closed MAU top view Ring out Ring in MAU bus Inactive station

  39. Token Ring Cable Types • Type 1 • A shielded data grade cable with two solid wire twisted pairs. • Available in indoor and outdoor versions. • Type 2 • A Type 1 indoor cable with four solid twisted pairs of 24 AWG wire. • Contains four voice grade wires along with four data grade wires. • Type 3 • Unused existing telephone wire or EIA category 3 wire (4 Mbps operation). • Category 4 is needed for 16 Mbps (speed of the Token Ring) operation. • Must use a special media filter. • Type 5 • 100/140 micron fiber cable used for fiber optic repeater links. • Type 6 • Often used for patch cables. • Patch cables can be used for MAU-to-MAU connection or from a wall outlet to a network attachment.

  40. Type 3 Media Filter • Type 3 cable requires a device known as a media filter. • Its purpose is to filter out any unwanted signals. • It is a small rectangular device that is usually part of the UTP cable itself. • It can be a separate device that attaches to the UTP cable at the end of the cable that attaches to the controller card. • It can be used on 16- or 4-mb Token Rings. • It is only used with Type 3 (UTP) cable.

  41. 802.5 Framing • IEEE 802.5 uses special characters, but does not use bit stuffing! Manchester “1” bit “0” bit Violations!

  42. Physical header no preset size Physical trailer Routing Information Fields IEEE 802.2 FC DA SA Data FCS ED FS SD AC MAC or LLC Frame Token frame SD AC ED 1 byte 1 byte 1 byte Abort frame SD ED Token Ring Frames 1 byte 1 byte

  43. Token Ring Frame Field Definitions SD - Starting Delimiter AC - Access Control FC - Frame Control DA - Destination Address SA - Source Address FCS - Frame Control Sequence ED - Ending Delimiter FS - Frame Status no preset size Routing Information Fields IEEE 802.2 SD AC DA Data FCS ED FS FC SA 4 bytes 1 byte 1 byte 1 byte 1 byte 1 byte 6 bytes 6 bytes <= 18 bytes DSAP SSAP Control Legend 1 or 2 bytes 1 byte 1 byte

  44. Field Bit 0 Bit 7 SD J K 0 J K 0 0 0 PPP - priority bits P P P T M R R R T - Token bit AC M - Monitor bit RRR - Reservation bits The SD and the AC Fields

  45. The FC, ED, and FS Fields Field Bit 0 Bit 7 FF - indicates a MAC or LLC frame. ZZZZ - indicates the type of MAC frame. FC F F r r Z Z Z Z I - Intermediate bit ED J K 1 J K 1 I E E - Error bit A - Address recognized bits FS A C r r A C r r C - Frame copied bits

  46. Bit Order Transmissionfor Token Ring • Bit 0 is the first bit transmitted. • Bit 0 is the left most bit of the byte. • Unlike Ethernet, the bits in the bytes are not reversed as they are transmitted. • Example: • 40-00-12 are the first three bytes of a MAC address. • Translated to binary: 01000000-00000000-00010010 • As transmitted on a Token Ring: 01000000-00000000-00010010 • Compared to Ethernet transmission: 00000010-00000000-01001000

  47. Token Passing Policies (Defn) • Multiple Token • RAT (FDDI): free token is appended to tail of last packet • Single Token • ?: Token is released upon receipt of leading edge of own packet • Single Packet • RAR (802.5):Token is released upon receipt of trailing edge of own packet

  48. Token Passing Policies (Usage) • Multiple Token • Allows multiple packets on the segment at one time. Good when packet length is less than ring latency • Single Token • More efficient than RAR; when packet length is about the same as ring latency • Single Packet • Least efficient, but allows controlling station knowledge of (un)successful transfer before the token is released (see pg. 224, 1st paragraph)

  49. Token Passing Policies (Perf.) • Multiple Token • Always the best performer, but more complex • Single Token • Closer to RAR than RAT • Single Packet • ‘Worst’ performance KEY POINT: Ratio of ring latency to packet length, a, is real determiner of performance. For a << 1, RAR is OK.

  50. Controller Operation - Phases 0 and 1 • Five-phase initialization • Phase 0 - Lobe test • The controller transmits frames between the controller card and the cable attached between the controller card and the MAU. • The controller tests to ensure that the lobe cable can successfully transmit and receive frames. • Phase 1 - Monitor Check • Station inserts into the ring (flips the relay in the MAU) and looks for special frames that are transmitted by the monitors. • Sets a timer to wait for these frames. • If the station does not receive any of the frames, the controller assumes: • it is the first ring station on the network, • there is not an Active Monitor present, or • inserting into the ring disrupted the ring. • The controller may initiate the token claim process.