5/31/05CS118/Spring051 twisted pair hub 10BaseT, 100BaseT, hub r T= Twisted pair (copper wire) r Nodes connected to a hub, 100m max distance r 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

5/31/05CS118/Spring052 Interconnecting using hubs r Can use a backbone hub to interconnect LAN segments  Extends max distance between nodes r Create a single large collision domain r Can’t interconnect 10BaseT & 100BaseT hub

5/31/05CS118/Spring053 Ethernet Switch r Link layer device: stores and forwards Ethernet frames  forwards frame based on MAC dest address  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 hub switch 1 2 3

5/31/05CS118/Spring054 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  stale entries in table dropped (TTL can be 60 min) 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)

5/31/05CS118/Spring055 Switch example Suppose C sends a data frame  to D r Switch receives  from C  Add to forwarding table: C is on interface 1  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 hub switch A B C D E F G H I address interface ABEGABEG C 1 D 2

5/31/05CS118/Spring056 hub switch collision domain Collision domain collision domain More on Switch r Traffic isolation:  same-LAN-segment frames (usually) not forwarded onto other LAN segments  segments become separate collision domains r cut-through switching: frame forwarded from input to output port without first collecting entire frame r can combine 10/100/1000 Mbps interfaces

5/31/05CS118/Spring057 Switches vs. Routers r both are store-and-forward devices  routers: network layer devices (examine network layer headers)  Switches: link layer devices r routers maintain routing tables, implement routing algorithms r switches maintain switch tables, implement self- learning algorithms Switch

5/31/05CS118/Spring058 Switches: advantages and limitations r Transparent: no need for any change to hosts r Isolates collision domains  resulting in higher total max throughput r Can connect different types of Ethernet  because it is a store and forward device r Constrained topology: tree only  all inter-segment traffic concentrated on a single tree  (all multicast traffic forwarded to all LAN’s)

5/31/05CS118/Spring059 Routers: advantages and limitations r Support arbitrary topologies r Efficient support for multicast routing  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

5/31/05CS118/Spring0510 Point to Point Data Link Control r One sender, one receiver, one link  e.g., dialup link, ISDN line r easier than broadcast link:  no Media Access Control  no need for explicit MAC addressing r 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!

5/31/05CS118/Spring0511 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)  ability to de-multiplex upwards 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

5/31/05CS118/Spring0512 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, IP, IPCP, etc) r info: upper layer data being carried r check: cyclic redundancy check for error detection

5/31/05CS118/Spring0513 Byte Stuffing r “data transparency” requirement: data field must be allowed to include flag pattern  Q: is received data or flag? r Define the Control Escape octet as r Sender: adds (“stuffs”) byte after each data byte r Receiver:  followed by : discard first byte, continue data reception  single : flag byte

5/31/05CS118/Spring0514 Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data

5/31/05CS118/Spring0515 PPP Data Control Protocol Before exchanging network- layer data, data link peers must r configure PPP link (max. frame length, authentication) r learn/configure network layer information  for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address