quartz/content/notes/22-LAN-physical-layer.md

title	aliases	tags
22-LAN-physical-layer		cosc203 lecture

LAN vs WAN

Local area network

• a network connecting computers within a limited geographic area
• e.g., building, school, home etc
• wired or wireless

Wide Area network

• a network that extends over a large area such as town, country, region or the world
• used to connec LANs and other types of networks together
• often established with leased telecommunication circuits (e.g., chorus)

ethernet

dominant wired tech

• first used LAN tech
• simple cheap
• IEEE 802.3 standard
• kept up with speed race: 10 Mbps - 400 Gbps
• physical topology
  • bus: all nodes in the same collision domain can collide (old)
  • switched: active link-layer switch in center (new)

frame structure

• encapsulates IP datagram (or other network layer protocol packet) in ethernet frame
• 
• preamble: used to sync reciever and sender clock rates
  • 7 bytes of 10101010 followed by one byte of 10101011
• addresses: 6 bytes source, destination MAC addr
  • if adapter recieves frame with matching MAC destination address, it passes data in frame to network layer protocol, otherwise adapter discards frame
• type
  • most IP but other possible: e.g., Novell IPX, Appletalk
  • used to demultiplex up at reciever
• CRC: cyclic redundancy check at reciever
  • error detected: frame is dropped
  • it can correct but doesn't because just dropping and resending is faster

ARP

Address resolution protocol

arp table: each IP node on lan has table

• IP/MAC mappings for some LAN nodes
• mapping is forgotten after TTL is exceeded (usually 20min)

Switch vs Router

switch:

• link layer deivce
• takes an active role
• store and foward ethernet frames
• examine incoming frame's MAC addr, selectively forward frame to one-or-more outgoing links when frame is to be fowarded on segment, uses CSMA/CD
• transparent
  • hosts unaware of presence of switches
• plug and play, self-learning
  • switches do not need to be configured

multiple simultaneous transmissions

• hosts have direct connection to sw

All layers together