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10BaseT, 100BaseT, hub T= Twisted pair (copper wire) Nodes connected to a hub, 100m max distance Hub: physical layer repeaters repeat received bits on one interface to all other interfaces; no buffering Transmission by one node may collide with any node residing at any segment connected to the same hub twisted pair hub 5/31/05 1 CS118/Spring05 Interconnecting using hubs Can use a backbone hub to interconnect LAN segments Extends max distance between nodes Create a single large collision domain Can’t interconnect 10BaseT & 100BaseT hub hub 5/31/05 hub 2 hub CS118/Spring05 Ethernet Switch Link layer device: stores and forwards Ethernet frames forwards frame based on MAC dest address uses CSMA/CD to access segment Transparent: hosts are unaware of presence of switches plug-and-play: switches do not need to be configured switch 1 3 2 hub 5/31/05 hub 3 hub CS118/Spring05 Building a forwarding table by self learning When receive data frame: associate sender address with incoming interface record sender/interface pair in a forwarding table Each entry: MAC Address, Interface, Time Stamp stale entries in table dropped (TTL can be 60 min) Data forwarding algorithm: when receive a frame if entry found for destination then{ if destination on interface from which frame arrived then drop the frame else forward the frame on interface indicated } else flood (forward to all but the interface the frame came from) 5/31/05 4 CS118/Spring05 Switch example Suppose C sends a data frame to D address A B E G C D switch 1 3 2 hub hub hub A interface 1 1 2 3 1 2 I B C F D E G H Switch receives from C Add to forwarding table: C is on interface 1 D is not in table: forwards to interfaces 2 and 3 frame received by D When D replies back with a frame to C Add to forwarding table: D is on interface 2 Forward to C 5/31/05 5 CS118/Spring05 More on Switch Traffic isolation: same-LAN-segment frames (usually) not forwarded onto other LAN segments segments become separate collision domains switch collision domain hub Collision domain hub collision domain hub cut-through switching: frame forwarded from input to output port without first collecting entire frame can combine 10/100/1000 Mbps interfaces 5/31/05 6 CS118/Spring05 Switches vs. Routers both are store-and-forward devices routers: network layer devices (examine network layer headers) Switches: link layer devices routers maintain routing tables, implement routing algorithms switches maintain switch tables, implement selflearning algorithms Switch 5/31/05 7 CS118/Spring05 Switches: advantages and limitations Transparent: no need for any change to hosts Isolates collision domains resulting in higher total max throughput Can connect different types of Ethernet because it is a store and forward device Constrained topology: tree only all inter-segment traffic concentrated on a single tree (all multicast traffic forwarded to all LAN’s) 5/31/05 8 CS118/Spring05 Routers: advantages and limitations Support arbitrary topologies Efficient support for multicast routing And can prevent broadcast storms Require IP address configuration (not plug and play) More complex data processing than switches bridges do well in small setting (few hundred hosts), routers are used in large networks 5/31/05 9 CS118/Spring05 Point to Point Data Link Control One sender, one receiver, one link e.g., dialup link, ISDN line easier than broadcast link: no Media Access Control no need for explicit MAC addressing popular point-to-point DLC protocols: PPP (point-to-point protocol) HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack! 5/31/05 10 CS118/Spring05 PPP Design Requirements [RFC 1661, 1662] packet framing: encapsulation of network-layer datagram in data link frame carry data of any network layer protocol (not just IP) ability to de-multiplex upwards bit transparency: must carry any bit pattern in data field error detection connection liveness: detect, signal link failure to network layer network layer address negotiation: endpoint can learn/configure each other’s network address Non-requirements no error correction/recovery no flow control out of order delivery OK 11 5/31/05 CS118/Spring05 PPP Data Frame Flag: delimiter (framing) Address: does nothing (only one option) Control: does nothing; in the future possible multiple control fields Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, IP, IPCP, etc) info: upper layer data being carried check: cyclic redundancy check for error detection 5/31/05 12 CS118/Spring05 Byte Stuffing “data transparency” requirement: data field must be allowed to include flag pattern <01111110> Q: is received <01111110> data or flag? Define the Control Escape octet as 01111101 Sender: adds (“stuffs”) < 01111101> byte after each < 01111110> data byte Receiver: < 01111101> followed by <01111110>: discard first byte, continue data reception single 01111110: flag byte 5/31/05 13 CS118/Spring05 Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data 5/31/05 14 CS118/Spring05 PPP Data Control Protocol Before exchanging networklayer data, data link peers must configure PPP link (max. frame length, authentication) learn/configure network layer information for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address 5/31/05 15 CS118/Spring05