Computer Networks. Unit 5: Local Area Networks. LANs basics The IEEE 802 standard Ethernet MAC protocols Internetworking: repeaters and bridges

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1 Computer Networks Unit 5: Local Area Networks LANs basics The IEEE 802 standard Ethernet MAC protocols Internetworking: repeaters and bridges unit 5 1

2 Introduction A local area network is a communication network that interconnects a variety of data devices within a small geographic area and broadcasts data at high data transfer rates with very low error rates. Since the local area network first appeared in the 1970s, its use has become widespread in commercial and academic environments. Functions of a LAN: a few examples File server - A large storage disk drive that acts as a central storage repository. Print server - Provides the authorization to access a particular printer, accept and queue print jobs, and provides a user access to the print queue to perform administrative duties. Interconnection - A LAN can provide an interconnection to other LANs and to wide area networks Manufacturing support - LANs can support manufacturing and industrial environments. Distributed processing - LANs can support network operating systems which perform the operations of distributed processing. unit 5 2

3 Types and Topologies Types point-to-point broadcast channel sharing problem Topologies Tree Bus Ring Star unit 5 3

4 The IEEE Standards for Local and Metropolitan Area Networks 802 Architecture and Overview Specification (originally known as IEEE Std 802.1A) series Glossary, Network Management and Internetworking Logical Link Control (LLC) CSMA/CD Access Method and Physical Layer Specifications Token-Passing Bus Access Method and Physical Layer Specifications Token-Passing Ring Access Method and Physical Layer Specifications Wireless Access Method and Physical Layer Specification Network Data Link L L C M A C Physical Ethernet v2.0 CSMA/CD IEEE ISO Network IEEE LLC ISO Data Link IEEE ISO ANSI X3T9.5 ISO CSMA/CD Token Ring FDDI unit 5 4

5 The IEEE 802 standard vs OSI Interface to higher levels Flow and error control Encoding/decoding Preamble generation/removal Bit transmission/reception Transmission medium and topology unit 5 5

6 The MAC (Media Access Control) Assembly of data into frame with address and error detection fields Disassembly of frame Address recognition Error detection Governs access to transmission medium Not found in traditional layer 2 data link control For the same LLC, several MAC options may be available N layer (e.g., IP) LLC MAC PH layer unit 5 6

7 48-Bit Universal LAN MAC Addresses Described in IEEE Std , pg bits burned into ROM on adaptor Octet: First bit transmitted on the LAN medium. (Also the I/G Address Bit: 0=Individual; 1=Group) Second bit, the Universally or Locally Administered, U/L Address Bit, indicates whether the address has been assigned by a local (=1) or universal administrator (=0). The hexadecimal representation is: AC:DE:48:00:00:80 Organizationally Unique Identifier as assigned by the IEEE Unicast address: destined for individual receiver Broadcast address: all 1 s in address- for every node Multicast address: leading 1 in address subset of nodes The adaptor can also be put in promiscuous mode receives all frames unit 5 7

8 Ethernet: some historical notes : Robert Metcalfe designs the first prototype at the Xerox PARC 1980: the first standard appears, by DEC-Intel-Xerox 1985: IEEE802.3 standard is released. The complete name is: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications IEEE supplements 802.3d-1987: FOIRL fiber link 802.3i-1990: 10BASE-T twisted pair 802.3z-1998: Gigabit Ethernet 802.3ab-1999: 1000BASE-T Gigabit Ethernet over twisted pair 802.3ae-2002: 10Gb/s Ethernet unit 5 8

9 The Ethernet family tree 10Mbps 10Base2 10Base5 10Base-T FOIRL 10Base-F 10Base-FB 10Base-FP 10Base-FL 100 Mbps (100Base-T) 100Base-X 100Base-TX 100Base-FX 100Base-T Mbps 1000Base-X 1000Base-SX 1000Base-LX 1000Base-CX 1000Base-T 22 = thin thin coaxial; coaxial; 55 = thick thick coaxial; coaxial; T = twisted twisted pair; pair; F F = fiber fiber optics; optics; S = short short wavelength; wavelength; L L = long long wavelength; wavelength; C = short short copper copper cable; cable; unit 5 9

10 Baseband Coaxial Cable Few kilometers range Entire frequency spectrum of cable used: single, half-duplex channel on cable Manchester or Differential Manchester encoding we saw the details about this point in Unit 3!! 10Base5 Ethernet and originally used 0.4 inch diameter cable at 10Mbps Max cable length 500m Max 100 taps 10Base inch cable Shorter distance (185m) More flexible Easier to bring to workstation Cheaper electronics Fewer taps (30) unit 5 10

11 MANCHESTER encoding Computer Networks (2005/2006) unit 5 11

12 Ethernet 10Base2 Computer Networks (2005/2006) unit 5 12

13 Ethernet 10Base5 Computer Networks (2005/2006) 10Base5 transceiver unit 5 13

14 10BaseT: twisted pair R R R R R R R R Computer Networks (2005/2006) unit 5 14

15 10BaseT: the RJ45 jack cross-over connection Computer Networks (2005/2006) unit 5 15

16 Twisted pair configuration Computer Networks (2005/2006) unit 5 16

17 10BaseT and 100BaseT 10/100 Mbps rate; latter called fast ethernet (IEEE 802.3u) Fast Ethernet requires Higher quality cabling STP, UTP Cat. 5 and fiber optics (100Base-FX). Two pair of cables (or two fibers). Based on star topology CSMA/CD implemented at hub Max distance from node to Hub is 100 meters Hub can disconnect jabbering adapters Hub can gather monitoring information, statistics for display to LAN administrators unit 5 17

18 Gigabit Ethernet use standard Ethernet frame format allows for point-to-point links and shared broadcast channels in shared mode, CSMA/CD is used; short distances between nodes to be efficient uses hubs, called here Buffered Distributors Full-Duplex at 1 Gbps for point-to-point links unit 5 18

19 MAC protocols We consider broadcast channels Problem: how to determine who gets to use the channel when there is competition for it There are two basic schemes: Static channel allocation and Dynamic channel allocation Frequency Division Multiplexing (FDM) Time Division Multiplexing (TDM) Aloha Carrier Sense Multiple Access (CSMA) Collision-free protocols: Bit-map protocol Binary countdown Limited-contention protocols: Adaptive tree walk... unit 5 19

20 The CSMA protocol Three types: 1 persistent while there are are frames to to be be sent while (channel is is busy) loop; /* /* the the channel is is available (idle) */ */ << << sending >> >> if if a collision is is detected then wait a random amount of of time whend non persistent p persistent unit 5 20

21 The CSMA with Collision Detection protocol while there are are frames to to be be sent while (channel is is busy) loop; /* /* the the channel is is available (idle) */ */ << << sending >> >> if if a collision is is detected then abort the the transmission wait a random amount of of time whend unit 5 21

22 Binary Exponential Backoff -> -> i th th collision <- <- if if i then choose a number n between 0 and and (2 (2 i i -1) -1) wait n time-slots try try sending again else if if i then choose a number n between 0 and and (2 ( ) -1) wait n time-slots try try sending again else if if i > then fi fi error unit 5 22

23 CSMA/CD: a few details A B C data interframe gap (IFG): 96 bits collision enforcement jam signal: 32 bits collision time backoff time: multiples of the slot time: 512 bits unit 5 23

24 Why collisions occur? A B C D A B C D A B C D t0 t1 t2 Computer Networks (2005/2006) A B C D t3 unit 5 24

25 Collisions and maximum frame size A B A B t=0 t = τ ε A B A B Computer Networks (2005/2006) t = τ t = 2τ unit 5 25

26 The frame format IEEE 802.3: type [ ] 0x600..0xFFFF Ethernet DIX: length [ ] 8 preamble preamble 6 6 Dest. Dest. address address Source Source address address If payload is <46 then this field is padded up to 46 bytes data data CRC CRC Start Startof offrame Delimiter (SFD) 2 bits bits unit 5 26

27 802.3 Efficiency Computer Networks (2005/2006) unit 5 27

28 Advantages and disadvantages Advantages Widely used Simple and cheap Hosts can be added and extracted dynamically With a low network load the access is almost immediate Disadvantages Minimum packet size is 64 bytes No deterministic access No priorities unit 5 28

29 Logical Link Control (LLC) Transmission of link level PDUs between two stations Must support multiaccess, shared medium Relieved of some link access details by MAC layer Addressing involves specifying source and destination LLC users Referred to as service access points (SAP) Typically higher level protocol unit 5 29

30 Internetworking The objective is to extend/connect networks using the same or a different technology. Why LAN LAN Why not not just just one one big big LAN? LAN? LAN WAN share share bandwidth bandwidth WAN WAN limited limited length: length: specifies specifies maximum maximum cable cable length length LAN WAN LAN large collision domain (can collide with many stations) Limited Limited amount amount of of supportable supportable traffic: traffic: on on single single LAN, LAN, all all stations stations must must large collision domain (can collide with many stations) limited limited number number of of stations: stations: We will use 3 types of black boxes that take charge of moving information among the different networks REPEATERS; BRIDGES; ROUTERS or GATEWAYS unit 5 30

31 Repeaters Work at layer 1 Amplify the signal Transmits in both directions Joins two segments of cable No buffering No logical isolation of segments If two stations on different segments send at the same time, packets will collide Only one path of segments and repeaters between any two stations: NO LOOPS Re Re Re Re unit 5 31

32 Bridges Work at layer 2 Store and forward strategy Checks whether the frame has errors (CRC) There must be a compatible addressing scheme They can join different technologies LANs Used to build extended LANs P unit 5 32

33 Ethernet repeater HUBs Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top. Each connected LAN referred to as LAN segment Hub Advantages: simple, inexpensive device Multi-tier provides graceful degradation: portions of the LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m per Hub) unit 5 33

34 Ethernet switching HUBs Repeater HUBs limitations single collision domain results in no increase in max throughput individual LAN restrictions pose limits on number of nodes in same collision domain and on total allowed geographical coverage cannot connect different Ethernet types (e.g., 10BaseT and 100baseT) We have to move to switching HUBs (i.e., bridges) They are link-layer devices: operate on Ethernet frames, examining frame header and selectively forwarding frame based on its destination Switches isolates collision domains since it buffers frames When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit Advantages: Isolates collision domains resulting in higher total max throughput, and does not limit the number of nodes nor geographical coverage Can connect different type Ethernet since it is a store and forward device Transparent: no need for any change to hosts LAN adapters unit 5 34

35 Backbone Bridge Interconnection without a backbone is not recommended for two reasons: - single point of failure at Computer Science hub - all traffic between EE and SE must path over CS segment unit 5 35

36 Example 100Mbps Ethernet Backbone strategy Computer Networks (2005/2006) unit 5 36

37 Bridges: frame filtering, forwarding bridges filter packets same-lan -segment frames not forwarded onto other LAN segments forwarding: how to know which LAN segment on which to forward frame? looks like a routing problem (more shortly!) bridges learn which hosts can be reached through which interfaces: maintain filtering tables when frame received, bridge learns location of sender: incoming LAN segment records sender location in filtering table filtering table entry: (Node LAN Address, Bridge Interface, Time Stamp) stale entries in Filtering Table dropped (TTL can be 60 minutes) unit 5 37

38 Learning bridge B A A B A B A B C D E F B A C Listen in promiscuous mode. Use the source address to update the cache memory associated to the relative input port. If the destination address is present in any of the caches associated with all the ports then if the output port would be same as the input port then do not propagate the frame else send the frame to the corresponding output port else send the frame to all the ports, except the incoming one. Clear the caches entries older than the aging time. D A B C D E F E B A F unit 5 38

39 About loops A solution? Avoid loops!!! A B Forbid the creation of loops. If they do exist the network administrator is to blame! Extracting a spanning tree from the original topology unit 5 39

40 Bridges Spanning Tree for increased reliability, desirable to have redundant, alternate paths from source to destination with multiple simultaneous paths, cycles result - bridges may multiply and forward frame forever solution: organize bridges in a spanning tree by disabling subset of interfaces Disabled unit 5 40

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