Local Area Networks. Ethernet LAN

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1 Local Area Networks Ethernet LAN - 7-1

2 Local Area Networks (Lokale Netze) Wide Area Network LAN - 7-2

3 Local Area Networks What is a LAN? Multiple systems attached to an often shared medium high total bandwidth, shared by the stations low delay low error rate broadcast/multicast capability single message (frame) transmitted once and received by multiple recipients limited geography (max. some km) limited number of stations (max. few hundred) all stations are equivalent (no master/slave) privately operated, not governed by telecommunications regulations LAN - 7-3

4 LAN IEEE 802 Standards (Institute of Electrical and Electronics Engineers, USA) IEEE 802 committee standardizes LANs: General Issues, Addressing, Management LLC (Logical Link Control) Type 1: datagram (no functionality) Type 2: reliable, connection oriented HDLC (high-level data link control) on top of LAN frames MAC (Medium Access Control) Layer CSMA/CD (similar to Ethernet) Token Bus Token Ring DQDB WLAN CSMA/CD DQDB Carrier Sense Multiple Access / Collision Detection Distributed Queue Dual Bus LAN - 7-4

5 IEEE 802 Standards IEEE Higher layer LAN protocols IEEE Logical Link Control IEEE Ethernet IEEE Token bus (disbanded) IEEE Token Ring IEEE Metropolitan Area Networks (disbanded) IEEE Broadband LAN using Coaxial Cable (disbanded) IEEE Fiber Optic TAG (disbanded) IEEE Integrated Services LAN (disbanded) IEEE Interoperable LAN Security (disbanded) IEEE Wireless LAN IEEE Demand priority IEEE (not used) IEEE Cable modems (disbanded) IEEE Wireless PAN IEEE Broadband Wireless Access IEEE Resilient packet ring IEEE Radio Regulatory TAG IEEE Coexistence TAG IEEE Mobile Broadband Wireless Access (disbanded) IEEE Media Independent Handoff IEEE Wireless Regional Area Network LAN - 7-5

6 LOGICAL LINK CONTROL MEDIUM ACCESS CONTROL PHYSICAL SIGNALING DATA LINK LAYER PHYSICAL LAYER ACCESS UNIT INTERFACE PHYSICAL MEDIA ATTACHMENT MEDIUM OSI-model for local networks with partition of the link layer in media access control and logical link control, as defined by IEEE 802. LAN - 7-6

7 IEEE Logical Link Control (LLC) Independent of network topologies General interface between different protocols (IPX, TCP/IP, etc.) and different network types (Ethernet, Token Ring, etc.) Responsible for logical link functions of one or more logical links, hence the name LLC provides the following services to the Network Layer: Unacknowledged connectionless-mode (Type 1 Operation) Connection-mode (Type 2 Operation) Complete spec., Ver. 1998: pdf LAN - 7-7

8 IEEE LLC Logical Link Control UNITDATArequest Parameter Name Source Address Destination Address Data Priority UNITDATAindication Parameter Function: local SAP + MAC Address remote SAP + MAC Address link SDU requested priority Destination service access point (8 bit) LLC PDU Source service access point (8 bit) Control (8 / 16 bit) Data (variable) LAN - 7-8

9 IEEE MAC sub-layer IEEE 802.3: collection of standards defining PHY and MAC layer, different standards Connectionless service, service primitives: DATArequest, DATAindication DATArequest Parameter Destination Address MAC Address LAN Data MAC SDU Service Class QoS in LANs only one class for CSMA/CD LAN - 7-9

10 Network Topologies (Netztopologien) Star (Stern) Ring Tree (Baum) Complete mesh, (vollständig vernetzt) Bus Satellite, Radio Cell (Satellit, Funkzelle) LAN

11 Addresses In communication from a source to a destination: Name identifies a resource ( identifier ) independent of location of both source and destination Address tells where something is may depend on the location of the destination Route tells how to get from a source to a destination depends on locations of source and destination ( go left, then take the third turn right ) [Perlman 1999] LAN

12 extended frame standard frame MAC Frame Preamble (7 octets) Start frame delimiter (1 octet) Destination address (6 octets) Source address (6 octets) Length / Type (2 octets) Data Padding Frame Check Sequence (4 octets) Extension LAN

13 IEEE (cont.) Preamble: alternating 0/1 Start frame delimiter: ends with 11 Synchronization of frames Destination address / Source address (6 octets each) Hardware Address NIC (Network Interface Card) 3 octets: specified by IEEE, unique identification of manufacturer 3 octets: specified by manufacturer unique address for every card Length / Type: Number of octets in frame / Type specification Minimum length of frame? 64 octets = 512 bit! Why? Padding if less than 64 octets Maximum length of frame? 1518 octets Extension field (Gigabit Ethernet) 48-bit MAC address space is expected to exhaust by the year 2100 LAN

14 IEEE LAN Addresses 16 bit addresses (option in the IEEE standards) enough for any LAN, if configured during network start-up not used 48 bit addresses for Ethernet / LAN interfaces initialized by hardware manufacturers in a globally unique way first 3 octets: vendor code (organizationally unique identifier, OUI) last 3 octets: unique hardware ID e.g c-f9-cf-30 Vendor: 3com Interface ID see Bit and byte orders depend on LAN standard LAN

15 LAN Addresses LAN Addresses must be unique on a LAN ensured e.g. by globally unique 48 bit IEEE addresses host interface listens to all frames interface card generates interrupt only for frames with destination address = local address or destination address = broadcast address or destination address = supported multicast address Sending point-to-point higher layer information in broadcast packets generates excessive interrupt load on all machines! LAN

16 Length DSAP SSAP CTL Frame Formats Ethernet Frame (not to scale) Preamble Octets: Destination Address Source Address Protocol Frame Data IEEE Frame (not to scale) Preamble Octets: Destination Address Source Address Frame Data DSAP=SSAP=170 (dec) -> SNAP FCS FCS CTL DSAP FCS SNAP SSAP Control Field Destination Service Access Point Frame Check Sequence Subnetwork Access Protocol Source Service Access Point LAN

17 CSMA/CD Carrier Sense listen before transmission Multiple Access medium is shared between multiple stations Collision Detect monitor while transmitting detect multiple simultaneous transmissions back off (random time, increased after collisions) Minimum Frame length determined by collision detection! shorter data must be padded CSMA Carrier Sense Multiple Access CD Collision Detection LAN

18 CSMA/CD if frames are too short S1 S1 S2 S2 S1 starts sending a very short frame S1 S2 S2 starts sending S1 S2 S2 detects collision S1 S2 LAN

19 CSMA/CD Operational Parameters Maximum Wire Length 2500m Transmission Speed 10 Mbit/s Speed of Electricity on Wire at least km/s (2.3 x km/s) Attempt Limit 16 Backoff Limit 10 Jam Size 32 bits Maximum Frame Size 1518 octets = bits 1 octet =? Minimum Frame Size? Slot Time? LAN

20 LAN

21 LAN

22 10 Mbit/s Ethernet CSMA/CD Interframe Gap: 9.6 s distance time for stations to change from sending to receiving mode (today cards sometimes have a smaller Interframe Gap, which can lead to incompatibilities) LAN

23 CSMA/CD Protocol LAN

24 100 Mbit/s Fast Ethernet IEEE Cost-efficient high speed network Easy migration from Ethernet to Fast Ethernet 100baseT (100 Mbit/s, base band, Twisted Pair or fiber) 100baseX (100baseTX: 2xTwisted Pair,100baseFX: 2xfiber) 100baseT4: 4x UTP (Unshielded Twisted Pair) Unshielded Twisted Pair: uses existing telephone cabling Is based directly on the Ethernet Standard: Access Method, Addressing, Frame Format and Installation almost identical Specific Extensions: Cable, Cable Length, Repeater Maximum UTP cable length for 100Mbit/s Ethernet with 512 bit min. frame length and 1 repeater in the path (1.3 s delay per repeater)? LAN

25 LAN

26 Gigabit Ethernet Start Nov. 1995: Gigabit Ethernet Task Force Requirements: Data rate 1000 Mbit/s frame format CSMA/CD access method Backwards compatible with 10baseT and 100baseT technology 1000baseX Maximum cable length for 1000 Mbit/s Ethernet with 512 bit min. frame length without repeater? Extension octets, new min. frame length? Maximum cable length? Min. frame length for 1000 Mbit/s, star topology, segment length 100m and 1 repeater (0.5 s delay)? LAN

27 LAN

28 Twisted Pair Ethernet (a) hub Single collision domain (b) switch High-Speed Backplane or Interconnection Fabric Copyright 2000 The McGraw Hill Companies Figure 6.56 LAN

29 Switched Ethernet * Basic idea: improve on the Hub concept The switch learns destination locations by remembering the ports of the associated source address in a table. The switch may not have to broadcast to all output ports. It may be able to send the frame only to the destination port. a big performance advantage over a hub, if more than one frame transfer can go through the switch concurrently (http://www.cs.wpi.edu/~rek/undergrad_nets/c04/ethernet.ppt) LAN

30 Switched Ethernet Advantages when the switched Ethernet backplane is able to repeat more than one frame in parallel Under this scheme collisions are still possible when two concurrently arriving frames are destined for the same station. Each parallel transmission can take place at 10 Mbit/s LAN

31 Ethernet Switch Architecture Ethernet Plug-In Cards BACK PLANE High-Speed Back Plane interconnects the Cards LAN

32 A More Detailed Architecture Back Plane A switch is just a special-purpose computer LAN

33 Back Plane Issues If backplane acts as a bus, performance degrades Need a way to move frames in parallel LAN

34 Example: Crossbar Switch Input Ports Output Ports LAN

35 Switched Ethernet Since servers are often shared by multiple nodes, one can employ a switching hub with a port which operates at a higher rate than the other ports. Extra buffering inside hub to handle speed mismatches Can be further enhanced by higher rated port full-duplex LAN

36 Fast Ethernet Switch Server Ethernet Switch 100 Mbit/s links Ethernet Switch 10 Mbit/s links Copyright 2000 The McGraw Hill Companies Leon-Garcia & Widjaja: Communication Networks Figure 6.57 LAN

37 Switched Ethernet Reduction of Collision Domains Each Port of a Switch corresponds to a Collision Domain Full-Duplex and Half-Duplex operation possible Full-Duplex especially for servers Longer distances possible by elimination of collisions Additional Flow Control in MAC sub-layer: Sender is slowed down with pause frame Length indication in a 2 octet field, Quanta units of 512 bit times, e.g., 10 Mbit/s: 51.2 s 100 Mbit/s: 5.12 s 1000 Mbit/s: 512 ns LAN

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