---
title: "22-LAN-physical-layer"
aliases: 
tags: 
- cosc203
- lecture
sr-due: 2022-11-19
sr-interval: 18
sr-ease: 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
- ![ethernet frame diagram](https://i.imgur.com/ElTUCEi.png)
- 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)

![arp example](https://i.imgur.com/dnj9Fvi.png)

# 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