10 Base T 100 Base T hub r
10 Base. T, 100 Base. T, hub r T= Twisted pair (copper wire) r Nodes connected to a hub, 100 m max distance r Hub: physical layer repeaters v repeat received bits on one interface to all other interfaces; no buffering v 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 CS 118/Spring 05
Interconnecting using hubs r Can use a backbone hub to interconnect LAN segments v Extends max distance between nodes r Create a single large collision domain r Can’t interconnect 10 Base. T & 100 Base. T hub 5/31/05 hub 2 hub CS 118/Spring 05
Ethernet Switch r Link layer device: stores and forwards Ethernet frames v forwards frame based on MAC dest address v uses CSMA/CD to access segment r Transparent: hosts are unaware of presence of switches r plug-and-play: switches do not need to be configured 1 switch 3 2 hub 5/31/05 hub 3 hub CS 118/Spring 05
Building a forwarding table by self learning r When receive data frame: associate sender address with incoming interface r record sender/interface pair in a forwarding table Each entry: MAC Address, Interface, Time Stamp v stale entries in table dropped (TTL can be 60 min) v r 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 CS 118/Spring 05
Switch example Suppose C sends a data frame to D switch 1 3 2 hub hub A address I B C F D E G A B E G C D interface 1 1 2 3 1 2 H r Switch receives from C v Add to forwarding table: C is on interface 1 v D is not in table: forwards to interfaces 2 and 3 r frame received by D When D replies back with a frame to C r Add to forwarding table: D is on interface 2 r Forward to C 5/31/05 5 CS 118/Spring 05
More on Switch r Traffic isolation: v same-LAN-segment frames (usually) not forwarded onto other LAN segments v segments become separate collision domains switch collision domain hub Collision domain hub collision domain hub r cut-through switching: frame forwarded from input to output port without first collecting entire frame r can combine 10/1000 Mbps interfaces 5/31/05 6 CS 118/Spring 05
Switches vs. Routers r both are store-and-forward devices v routers: network layer devices (examine network layer headers) v Switches: link layer devices r routers maintain routing tables, implement routing algorithms r switches maintain switch tables, implement self- learning algorithms Switch 5/31/05 7 CS 118/Spring 05
Switches: advantages and limitations r Transparent: no need for any change to hosts r Isolates collision domains v resulting in higher total max throughput r Can connect different types of Ethernet v because it is a store and forward device r Constrained topology: tree only v all inter-segment traffic concentrated on a single tree v (all multicast traffic forwarded to all LAN’s) 5/31/05 8 CS 118/Spring 05
Routers: advantages and limitations r Support arbitrary topologies r Efficient support for multicast routing v And can prevent broadcast storms r Require IP address configuration (not plug and play) r More complex data processing than switches r bridges do well in small setting (few hundred hosts), routers are used in large networks 5/31/05 9 CS 118/Spring 05
Point to Point Data Link Control r One sender, one receiver, one link v e. g. , dialup link, ISDN line r easier than broadcast link: v no Media Access Control v no need for explicit MAC addressing r popular point-to-point DLC protocols: v PPP (point-to-point protocol) v HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack! 5/31/05 10 CS 118/Spring 05
PPP Design Requirements [RFC 1661, 1662] r packet framing: encapsulation of network-layer datagram in data link frame carry data of any network layer protocol (not just IP) v ability to de-multiplex upwards v r bit transparency: must carry any bit pattern in data field r error detection r connection liveness: detect, signal link failure to network layer r network layer address negotiation: endpoint can learn/configure each other’s network address Non-requirements r no error correction/recovery r no flow control r out of order delivery OK 11 5/31/05 CS 118/Spring 05
PPP Data Frame r Flag: delimiter (framing) r Address: does nothing (only one option) r Control: does nothing; in the future possible multiple control fields r Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, IPCP, etc) r info: upper layer data being carried r check: cyclic redundancy check for error detection 5/31/05 12 CS 118/Spring 05
Byte Stuffing r “data transparency” requirement: data field must be allowed to include flag pattern <01111110> v Q: is received <01111110> data or flag? r Define the Control Escape octet as 01111101 r Sender: adds (“stuffs”) < 01111101> byte after each < 01111110> data byte r Receiver: < 01111101> followed by <01111110>: discard first byte, continue data reception v single 01111110: flag byte v 5/31/05 13 CS 118/Spring 05
Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data 5/31/05 14 CS 118/Spring 05
PPP Data Control Protocol Before exchanging networklayer data, data link peers must r configure PPP link (max. frame length, authentication) r learn/configure network layer information v for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address 5/31/05 15 CS 118/Spring 05
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