The Data Link Layer Addressing Last time Our goals: � different address scheme in different � link layer services � understand principles layers � error detection, correction behind data link layer services: � multiple access protocols and � application layer: host names LANs � error detection, � (transport layer: port number) correction � link layer addressing, ARP, DHCP � sharing a broadcast � network layer: IP-addresses channel: multiple access Today � link layer: LAN addresses � link layer addressing � Ethernet � translation mechanisms � reliable data transfer, � (Token Ring and FDDI) flow control: done! � hubs and switches � DNS � instantiation and � PPP implementation of various � ARP link layer technologies 3/5-07 Datakommunikation - Jonny Pettersson, UmU 3/5-07 Datakommunikation - Jonny Pettersson, UmU Star topology Ethernet : IEEE802.3 standard “Dominant” wired LAN technology: � Bus topology popular through mid 90s � Cheap, less than $20 for 100Mbs! � Now star topology prevails � First widely used LAN technology � Connection choices: hub or switch (more later) � Simpler, cheaper than token LANs and ATM � Kept up with speed race: 10 Mbps – 10 Gbps Metcalfe’s Ethernet sketch hub or switch 3/5-07 Datakommunikation - Jonny Pettersson, UmU 3/5-07 Datakommunikation - Jonny Pettersson, UmU Ethernet Frame Structure Ethernet Frame Structure (more) Sending adapter encapsulates IP datagram (or other � Addresses: 6 bytes, frame is received by all network layer protocol packet) in Ethernet frame 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) Preamble: � Data: 46 – 1500 (MTU) byte � 7 bytes with pattern 10101010 followed by one � CRC: checked at receiver, if error is detected, the byte with pattern 10101011 frame is simply dropped � used to synchronize receiver, sender clock rates 3/5-07 3/5-07 Datakommunikation - Jonny Pettersson, UmU Datakommunikation - Jonny Pettersson, UmU 1
Unreliable, connectionless service Ethernet uses CSMA/CD � Connectionless: No handshaking between sending � No slots � Before attempting a and receiving adapter. retransmission, � adapter doesn’t transmit adapter waits a � Unreliable: receiving adapter doesn’t send ACKs or if it senses that some random time, that is, NACKs to sending adapter other adapter is random access transmitting, that is, � stream of datagrams passed to network layer can have carrier sense gaps � gaps will be filled if app is using TCP � transmitting adapter � otherwise, app will see the gaps aborts when it senses that another adapter is transmitting, that is, collision detection 3/5-07 Datakommunikation - Jonny Pettersson, UmU 3/5-07 Datakommunikation - Jonny Pettersson, UmU Ethernet CSMA/CD algorithm Ethernet’s CSMA/CD (more) 1. Adaptor receives 4. If adapter detects another transmission while Jam Signal: make sure all Exponential Backoff: datagram from net layer & transmitting, aborts and other transmitters are � Goal : adapt retransmission creates frame aware of collision; 48 bits attempts to estimated sends jam signal current load 2. If adapter senses channel Bit time: .1 microsec for 10 5. After aborting, adapter � heavy load: random wait Mbps Ethernet ; idle, it starts to transmit will be longer enters exponential for K=1023, wait time is frame. If it senses � first collision: choose K backoff : after the n’th about 50 msec from {0,1} channel busy, waits until collision, adapter chooses � after second collision: channel idle and then a K at random from choose K from {0,1,2,3}… transmits {0,1,2,…,2 m -1}, m = � after ten collisions, choose K from {0,1,2,3,4,…,1023} min(n,10). Adapter waits 3. If adapter transmits See/interact with Java � after the n’th collision, K · 512 bit times and entire frame without applet on AWL Web site: adapter chooses a K at returns to Step 2 random from {0,1,2,…,2 m -1}, highly recommended ! detecting another m = min(n,10) transmission, the adapter � delay is K · 512 bit transmission times is done with frame! 3/5-07 Datakommunikation - Jonny Pettersson, UmU 3/5-07 Datakommunikation - Jonny Pettersson, UmU 10BaseT and 100BaseT CSMA/CD efficiency � 10/100 Mbps rate; latter called “fast ethernet” � T stands for Twisted Pair � T prop = max prop between 2 nodes in LAN � Nodes connect to a hub: “star topology”; 100 m � t trans = time to transmit max-size frame max distance between nodes and hub 1 = efficiency + 1 5 / t prop t trans twisted pair � 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, hub simple, and cheap 3/5-07 3/5-07 Datakommunikation - Jonny Pettersson, UmU Datakommunikation - Jonny Pettersson, UmU 2
10BaseT and 100BaseT (more) Gbit Ethernet � Max distance from node to Hub is 100 meters � uses standard Ethernet frame format � Hub acts similar to repeater � allows for point-to-point links and shared broadcast channels � Hub can disconnect “jabbering” adapter � in shared mode, CSMA/CD is used; short distances � Hub can gather monitoring information, statistics between nodes required for efficiency for display to LAN administrators � uses hubs, called here “Buffered Distributors” � 10Base2 and 10BaseT uses Manchester encoding � Full-Duplex at 1 Gbps for point-to-point links � 100BaseT uses 4B5B (five clock periods for four bits) encoding � 10 Gbps now ! � Many Ethernet adapters today are 10/100 Mbps adapters 3/5-07 Datakommunikation - Jonny Pettersson, UmU 3/5-07 Datakommunikation - Jonny Pettersson, UmU FDDI Token Passing: IEEE802.5 standard � IBM Token Ring ≈ IEEE802.5 � more different types of Token Rings � Fiber Distributed Data than there are types of Ethernet Interface � ring of nodes, unidirectional � dual rings � key features � backbones � 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) (a) (b) 3/5-07 Datakommunikation - Jonny Pettersson, UmU 3/5-07 Datakommunikation - Jonny Pettersson, UmU Performance Interconnecting LANs � CSMA/CD: okay up to approximately 30% Q: Why not just one big LAN? load � limited amount of supportable traffic: on single LAN, all stations must share bandwidth � Token Ring: okay up to approximately 60% � limited length: 802.3 specifies maximum cable load length � large “collision domain” (can collide with many � Token Ring better then CSMA/CD, but stations) more expensive and more complex � limited number of stations: 802.5 have token passing delays at each station 3/5-07 3/5-07 Datakommunikation - Jonny Pettersson, UmU Datakommunikation - Jonny Pettersson, UmU 3
Interconnecting with hubs Hubs Hubs are essentially physical-layer repeaters: � backbone hub interconnects LAN segments � bits coming from one link go out all other links � extends max distance between nodes � at the same rate � but individual segment collision domains become one � no frame buffering large collision domain � no CSMA/CD at hub: adapters detect collisions � can’t interconnect 10BaseT & 100BaseT � provides net management functionality hub twisted pair hub hub hub hub 3/5-07 Datakommunikation - Jonny Pettersson, UmU 3/5-07 Datakommunikation - Jonny Pettersson, UmU Switch Forwarding switch � Link layer device 1 3 � stores and forwards Ethernet frames 2 � examines frame header and selectively forwards frame based on MAC dest address hub � when frame is to be forwarded on segment, hub hub uses CSMA/CD to access segment � transparent � hosts are unaware of presence of switches � plug-and-play, self-learning • How do determine onto which LAN segment to � switches do not need to be configured forward frame? • Looks like a routing problem... 3/5-07 Datakommunikation - Jonny Pettersson, UmU 3/5-07 Datakommunikation - Jonny Pettersson, UmU Filtering/Forwarding Self learning When switch receives a frame: � a switch has a switch table index switch table using MAC dest address � entry in switch table: if entry found for destination � (MAC Address, Interface, Time Stamp) then{ � stale entries in table dropped (TTL can be 60 min) if dest on segment from which frame arrived � switch learns which hosts can be reached through then drop the frame which interfaces else forward the frame on interface indicated � when frame received, switch “learns” location of } sender: incoming LAN segment else flood � records sender/location pair in switch table forward on all but the interface on which the frame arrived 3/5-07 3/5-07 Datakommunikation - Jonny Pettersson, UmU Datakommunikation - Jonny Pettersson, UmU 4
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