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

title	aliases	tags	sr-due	sr-interval	sr-ease
22-LAN-physical-layer		cosc203 lecture	2022-10-22	3	250

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 dedicated direct connection to switch
• switches can buffer packets
• ethernet protocol used on each incoming link so:
  • no collisions: full duplex
    • A to A' and B to B' can transmit simultaneously, without collisions
  • each link is its own collision domain
    • A to A' and C to A' cannot happen simultaneously
  • each swtich has a switch table, each entry:
    • mac of host, inferface, time stamp
    • looks like a routing table

self-learning

• learns which hosts can be reached through which interface
  • when frame is recieved, switch records the sender/location pair in table
• filtering/forwarding
  • if frame dest is unknown: flood
  • if known send just one link

vs routers

• both store-and-foward
  • R: network layer
  • S: link layer
• both have forwarding table
  • R: routing using IP and routing algorithms
  • S: learn forwarding table using flooding, learning, MAC

physical layer

• transmit and recieve bit streams over a physical transmission medium
• conversion betwen digital bits to analogue
• define characteristics linke, voltage, data rate etc

All layers together

• connect to internet:
  • need to get IP addr of:
    • itself
    • first hop router
    • DNS server
• uses DHCP: dynamic host configuration protocol
• DHCP is inside UDP inside IP inside Ethernet frame
• frame is broadcast on LAN with dest FFFFFFFFFFF, recieved at router running DHCP server
• DHCP creates a DHCP ACK with the info and sends it back to the client
  • can allocate temporaty IP address