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Computer Networking LAN LANs Our goals: Overview: Computer Networking ß understand principles ß multiple access protocols behind LANs: ß example LANs: ß sharing a broadcast ß Ethernet channel: multiple ß 802.11 Local Area Networks access ß token ring ß link layer addressing ß token bus ß LAN interconnection ß link layer addressing Prof. Andrzej Duda ß instantiation and ß LAN interconnection [email protected] implementation of ß hubs, bridges, switches various LAN technologies http://duda.imag.fr 1 2 Characteristics Data link layer in LANs ß Shared channel ß multiplexing (TDM, FDM, or CDM) ß fixed allocation: wasted badwidth if no active sources ß statistical multiplexing (multiple access) ß suitable for bursty traffic - channel used at the full capacity ß Most of LANs ß no retransmission (up to upper layers) Metcalfe’s Etheret ß WLANs ß Short distances (100 m - 1 km) sketch ß ACK of delivery ß High bit rate (10 Mb/s, 100 Mb/s, 1 Gb/s) ß Shared communication channel ß Used in a distributed environment ß shared equipment, shared data 3 4 Multiple Access protocols Multiple Access Protocols ß single shared communication channel ß two or more simultaneous transmissions Three broad classes: by nodes: interference ß Random Access (Ethernet, 802.11) ß only one node can send successfully at a time ß allow collisions ß multiple access protocol: ß “recover” from collisions ß distributed algorithm that determines how stations share channel, i.e., determine when ß Tokens - “Taking turns” (Token Ring, FDDI) station can transmit ß tightly coordinate shared access to avoid collisions ß communication about channel sharing must ß Distributed Queue (DQDB) use channel itself! ß use the channel in the arrival order ß what to look for in multiple access protocols: ß synchronous or asynchronous ß Goal: efficient, fair, simple, decentralized ß information needed about other stations ß robustness (e.g., to channel errors) ß performance 5 6 1 LAN LAN technologies LAN Reference model ß Data link layer: LLC 802.2 ß services, multiple access Data link ß LAN technologies MAC MAC MAC ß addressing Physical 802.3 802.4 802.5 ß Ethernet, 802.11 ß repeaters, hubs, bridges, switches ß virtual LANs ß LLC - Logical Link Control: IEEE 802.2 (ISO 8802.2) ß MAC - Medium Access Control ß IEEE 802.3 (ISO 8802.3): CSMA/CD ß IEEE 802.4 (ISO 8802.4): token bus ß IEEE 802.5 (ISO 8802.5): token ring ß IEEE 802.11: CSMA/CA 7 8 IEEE 802.3 - Ethernet Coding 100 ns time host transceiver ß Synchronous transmission ß receiving station locks on 10 MHz - preamble ß Manchester coding repeater terminator 9 10 Random Access protocols CSMA/CD (Collision Detection) ß When node has packet to send ß CSMA/CD (Carrier Sense Multiple Access/ Collision Detection) ß transmit at full channel data rate R. ß carrier sensing, deferral if ongoing transmission ß no a priori coordination among nodes ß collisions detected within short time ß two or more transmitting nodes -> “collision”, ß colliding transmissions aborted, reducing channel wastage ß random access protocol specifies: ß persistent transmission ß how to detect collisions ß collision detection: ß how to recover from collisions (e.g., via delayed ß easy in wired LANs: measure signal strengths, compare retransmissions) transmitted, received signals ß Examples of random access protocols: ß difficult in wireless LANs: receiver shut off while transmitting ß ALOHA, slotted ALOHA ß CSMA, CSMA/CD (Ethernet), CSMA/CA (802.11) 11 12 2 LAN CSMA/CD algorithm CSMA / CD Collision i = 1 while (i <= maxAttempts) do ß A senses idle listen until channel is idle channel, starts A B transmitting transmit and listen 0 wait until (end of transmission) or ß shortly before T, T B senses idle (collision detected) channel, starts if collision detected then transmitting stop transmitting, send jam bits (32 bits) else wait for interframe delay (9.6 ms) leave wait random time increment i end do 13 14 CSMA / CD Jam Signal Random retransmission interval ß B senses r = random (0, 2k -1) collision, continues to k = min (10, AttemptNb) A B transmit the jam k signal (32-bit) 0 tr = r ¥ 51.2ms, r Œ[0, 2 -1] ß A senses T collision, continues to ß slot time = 51.2 ms transmit the jam ß 1st collision, r = 0, 1 signal t2 ß 2nd collision, r = 0, 1, 2, 3 ß 10th, r = 0, 1, …, 1023 ß 15th, stop 15 16 CSMA / CD Retransmission CSMA/CD performance A B 0 ß Maximum utilization of Ethernet (approximation) ß A waits random T time t1 ß B waits random q ª 1 / ( 1 + C a ) time t2=slottime t2 < t1 =2*slottime where a = 2Db / L, ß B senses channel D = propagation delay, b = bit rate, idle and transmits L = frame size A senses channel ß C is a constant: busy and defers to B ß C = 3.1 is a pessimistic value; ß A now waits until ß C = 2.5 is an approximate value based on simulations channel is idle t1 17 18 3 LAN Frame format (Ethernet v.2) Frame format (802.3) preamble dest source length data pad CRC preamble dest source type data CRC 8 bytes 6 bytes 6 bytes 2 bytes 46 - 1500 bytes 4 bytes 8 bytes 6 bytes 6 bytes 2 bytes 46 - 1500 bytes 4 bytes ß Preamble LLC frame DSAP SSAP control data • synchronization : 10101010….0101011 1 byte 1 byte 1 byte • Addresses (xAA) (xAA) (x03) • unique, unicast and multicast (starts with the first bit 1) SNAP frame • broadcast: 11111…11111 prot. id type data 3 bytes • Type 2 bytes • upper layer protocol (IP, IPX, ARP, etc.) (x00) ß SNAP (Subnet Access Protocol) used in bridge management (any length of data: 0 - 1492) 19 20 Addressing Addressing ß MAC address: 48 bits = adapter identifier ß Data on Ethernet is transmitted least significant bit of ß sender puts destination MAC address in the frame first byte first (a bug dictated by Intel processors) ß all stations read all frames; keep only if destination ß Canonical representation thus inverts the order of bits address matches inside a byte (the first bit of the address is the least ß all 1 address (FF:FF:FF:FF:FF:FF) = broadcast significant bit of the first byte) ß examples of addresses: ß 01:00:5e:02:a6:cf (a group address) ß 08:00:20:71:0d:d4 (a SUN machine) ß 00:00:c0:3f:6c:a4 (a PC ) B C ß 00:00:0c:02:78:36 (a CISCO router) ß FF:FF:FF:FF:FF:FF the broadcast address MAC address A D 08:00:20:71:0d:d4 00:00:c0:3f:6c:a4 01:00:5e:02:a6:cf (group address) 21 22 Interconnecting LANs Repeaters ß Function of a simple, 2 port Why not just one big LAN? repeater: ß Limited amount of supportable traffic: on single LAN, all stations ß repeat bits received on one port must share bandwidth to other port ß limited distance ß if collision sensed on one port, ß large “collision domain” (can collide with many stations) repeat random bits on other port Repeater ß processing broadcast frames ß One network with repeaters = LAN evolution one collision domain ß increase the bit rate: 10Mb/s, 100Mb/s, 1 Gb/s ß Repeaters perform only ß from hubs to switches physical layer functions (bit repeaters) 23 24 4 LAN From Repeaters to Hubs 10 BASE T Hubs Multiport ß Multiport repeater (n ports), Repeater logically equivalent to: hub ß n simple repeater ß connected to one internal Ethernet segment hub hub ß Multi-port repeaters make it possible to use point-to-point segments (Ethernet in the Ethernet Hub box) S1 ß ease of management Multi- port ß Tree topology (star) ß fault isolation S2 Re- ß hub (répéteur multiport) UTP segment peater S3 ß max. 4 hubs to other hub 25 26 10 BASE T 10BaseT and 100BaseT ß 10/100 Mbps rate; latter called “fast ethernet” hub ß T stands for Twisted Pair ß Hub to which nodes are connected by twisted pair, thus “star topology” ß CSMA/CD supported by hubs host ß Two pairs ß Hub - host ß emission ß straight cable ß reception ß Hub - hub ß RJ-45 jack ß inversed cable 27 28 Gigabit Ethernet Gigabit Ethernet ß use standard Ethernet frame format ß 1000 BASE T ß allows for point-to-point links and shared broadcast ß over twisted pair (25 m) channels ß 1000 BASE SX ß in shared mode, CSMA/CD is used; short distances ß short wavelength (850 nm) over multimode (500 m) between nodes to be efficient ß 1000 BASE LX ß Full-Duplex at 1 Gbps for point-to-point links ß long wavelength (1300 nm) over multimode (550 m) and single- mode fiber (10 km) ß 1000 BASE LH (Long Haul) ß greater distance over 10 µm single-mode (500 m) ß 1000 BASE ZX ß extended wavelength (1550 nm) over 10 µm single-mode (70 km) 29 30 5 LAN Bridges Bridges – interconnection at layer 2 ß Link Layer devices: operate on Ethernet frames, Forwarding Table port 1 port 3 examining frame header and selectively forwarding Bridge Dest Port frame based on its destination A C MAC Nb ß Bridge isolates collision domains since it buffers port 2 addr Repeater frames A 1 B 2 ß When needs to forward a frame on a segment, B C 3 bridge uses CSMA/CD to access the segment and D D 2 transmit ß Can connect different type Ethernets, since it is a buffering device ß Bridges are intermediate systems, or switches, that ß Two main types of bridges: transparent bridges and forward MAC frames to destinations based on MAC spanning tree bridges (guarantee no loops) addresses ß Transparent bridges: learn the Forwarding Table 31 32 Bridges vs.
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