The Data Link Layer Last time Our goals: � link layer services � understand principles � error detection, correction behind data link layer services: � multiple access protocols and LANs � error detection, correction � link layer addressing, ARP, DHCP � sharing a broadcast channel: multiple access Today � link layer addressing � Ethernet � reliable data transfer, � (Token Ring and FDDI) flow control: done! � hubs and switches � instantiation and � PPP implementation of various link layer technologies 8/5-06 Datakommunikation - Jonny Pettersson, UmU 1 Addressing � different address scheme in different layers � application layer: host names � (transport layer: port number) � network layer: IP-addresses � link layer: LAN addresses � translation mechanisms � DNS � ARP 8/5-06 Datakommunikation - Jonny Pettersson, UmU 2 Ethernet : IEEE802.3 standard “dominant” wired LAN technology: � cheap, less than $20 for 100Mbs! � first widely used LAN technology � Simpler, cheaper than token LANs and ATM � Kept up with speed race: 10 Mbps – 10 Gbps Metcalfe’s Ethernet sketch 8/5-06 Datakommunikation - Jonny Pettersson, UmU 3 1
Star topology � Bus topology popular through mid 90s � Now star topology prevails � Connection choices: hub or switch (more later) hub or switch 8/5-06 Datakommunikation - Jonny Pettersson, UmU 4 Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: � 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 � used to synchronize receiver, sender clock rates 8/5-06 Datakommunikation - Jonny Pettersson, UmU 5 Ethernet Frame Structure (more) � Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not match � Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk) � Data: 46 – 1500 (MTU) byte � CRC: checked at receiver, if error is detected, the frame is simply dropped 8/5-06 Datakommunikation - Jonny Pettersson, UmU 6 2
Unreliable, connectionless service � Connectionless: No handshaking between sending and receiving adapter. � Unreliable: receiving adapter doesn’t send ACKs or NACKs to sending adapter � stream of datagrams passed to network layer can have gaps � gaps will be filled if app is using TCP � otherwise, app will see the gaps 8/5-06 Datakommunikation - Jonny Pettersson, UmU 7 Ethernet uses CSMA/CD � No slots � Before attempting a retransmission, � adapter doesn’t transmit adapter waits a if it senses that some random time, that is, other adapter is random access transmitting, that is, carrier sense � transmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detection 8/5-06 Datakommunikation - Jonny Pettersson, UmU 8 Ethernet CSMA/CD algorithm 1. Adaptor receives 4. If adapter detects datagram from net layer & another transmission while creates frame transmitting, aborts and sends jam signal 2. If adapter senses channel idle, it starts to transmit 5. After aborting, adapter frame. If it senses enters exponential channel busy, waits until backoff : after the m’th channel idle and then collision, adapter chooses transmits a K at random from {0,1,2,…,2 m -1}. Adapter 3. If adapter transmits waits K · 512 bit times and entire frame without returns to Step 2 detecting another transmission, the adapter is done with frame ! 8/5-06 Datakommunikation - Jonny Pettersson, UmU 9 3
Ethernet’s CSMA/CD (more) Jam Signal: make sure all Exponential Backoff: other transmitters are � Goal : adapt retransmission aware of collision; 48 bits attempts to estimated Bit time: .1 microsec for 10 current load Mbps Ethernet ; � heavy load: random wait for K=1023, wait time is will be longer about 50 msec � first collision: choose K from {0,1}; delay is K · 512 bit transmission times � after second collision: choose K from {0,1,2,3}… See/interact with Java � after ten collisions, choose applet on AWL Web site: K from {0,1,2,3,4,…,1023} highly recommended ! � m = min(n,10) 8/5-06 Datakommunikation - Jonny Pettersson, UmU 10 CSMA/CD efficiency � T prop = max prop between 2 nodes in LAN � t trans = time to transmit max-size frame 1 = efficiency + 1 5 / t prop t trans � Efficiency goes to 1 as t prop goes to 0 � Goes to 1 as t trans goes to infinity � Much better than ALOHA, but still decentralized, simple, and cheap 8/5-06 Datakommunikation - Jonny Pettersson, UmU 11 10BaseT and 100BaseT � 10/100 Mbps rate; latter called “fast ethernet” � T stands for Twisted Pair � Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub twisted pair hub 8/5-06 Datakommunikation - Jonny Pettersson, UmU 12 4
10BaseT and 100BaseT (more) � max distance from node to Hub is 100 meters � Hub acts similar to repeater � Hub can disconnect “jabbering” adapter � Hub can gather monitoring information, statistics for display to LAN administrators � 10Base2 and 10BaseT uses Manchester encoding � 100BaseT uses 4B5B (five clock periods for four bits) encoding � many Ethernet adapters today are 10/100 Mbps adapters 8/5-06 Datakommunikation - Jonny Pettersson, UmU 13 Gbit Ethernet � uses 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 required for efficiency � uses hubs, called here “Buffered Distributors” � Full-Duplex at 1 Gbps for point-to-point links � 10 Gbps now ! 8/5-06 Datakommunikation - Jonny Pettersson, UmU 14 Token Passing: IEEE802.5 standard � IBM Token Ring ≈ IEEE802.5 � more different types of Token Rings than there are types of Ethernet � ring of nodes, unidirectional � key features � distributed algorithms for sending � all nodes sees all frames � token, sequence of bites is sent around � max token holding time: 10 ms, limiting frame length � Manchester encoding � up to 260 nodes in a ring � - 100 Mbps 8/5-06 Datakommunikation - Jonny Pettersson, UmU 15 5
FDDI � Fiber Distributed Data Interface � dual rings � - 200 Mbps � backbones � max 100 km (a) (b) 8/5-06 Datakommunikation - Jonny Pettersson, UmU 16 Performance � CSMA/CD: okay up to approximately 30% load � Token Ring: okay up to approximately 60% load � Token Ring better then CSMA/CD, but more expensive and more complex 8/5-06 Datakommunikation - Jonny Pettersson, UmU 17 Interconnecting LANs Q: Why not just one big LAN? � limited amount of supportable traffic: on single LAN, all stations must share bandwidth � limited length: 802.3 specifies maximum cable length � large “collision domain” (can collide with many stations) � limited number of stations: 802.5 have token passing delays at each station 8/5-06 Datakommunikation - Jonny Pettersson, UmU 18 6
Interconnecting with hubs � backbone hub interconnects LAN segments � extends max distance between nodes � but individual segment collision domains become one large collision domain � can’t interconnect 10BaseT & 100BaseT hub hub hub hub 8/5-06 Datakommunikation - Jonny Pettersson, UmU 19 Hubs Hubs are essentially physical-layer repeaters: � bits coming from one link go out all other links � at the same rate � no frame buffering � no CSMA/CD at hub: adapters detect collisions � provides net management functionality twisted pair hub 8/5-06 Datakommunikation - Jonny Pettersson, UmU 20 Switch � Link layer device � stores and forwards Ethernet frames � examines frame header and selectively forwards frame based on MAC dest address � when frame is to be forwarded on segment, uses CSMA/CD to access segment � transparent � hosts are unaware of presence of switches � plug-and-play, self-learning � switches do not need to be configured 8/5-06 Datakommunikation - Jonny Pettersson, UmU 21 7
Forwarding switch 1 3 2 hub hub hub • How do determine onto which LAN segment to forward frame? • Looks like a routing problem... 8/5-06 Datakommunikation - Jonny Pettersson, UmU 22 Self learning � a switch has a switch table � entry in switch table: � (MAC Address, Interface, Time Stamp) � stale entries in table dropped (TTL can be 60 min) � switch learns which hosts can be reached through which interfaces � when frame received, switch “learns” location of sender: incoming LAN segment � records sender/location pair in switch table 8/5-06 Datakommunikation - Jonny Pettersson, UmU 23 Filtering/Forwarding When switch receives a frame: index switch table using MAC dest address if entry found for destination then{ if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived 8/5-06 Datakommunikation - Jonny Pettersson, UmU 24 8
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