---
title: "16-networks"
aliases: 
tags: 
- cosc203
- lecture
sr-due: 2022-11-24
sr-interval: 23
sr-ease: 250
---

# Types of devices
- everything is connected
- ![](https://i.imgur.com/ECsSgfo.png)

- hosts - the end systems which run network apps

packet swtiches - forward packets (chunks of data)
communcation links - fiber, copper, radio, satellite
	- transmission rate: bandwidthc
networks -  collection of devices routers
- links managed by organisation


## internet structure
network edge
- hosts: clients and servers
	- servers (often in data centres)

access networkds, physical media
- wired inks
- wireless communcation links

network core
- "backbone"
- interconnected routers
- network of routers

internet is a network of networks

## edge devices
- sending function
	- takes app message
	- break into packers
	- transmit them
- recieving function
	- recieve packets
	- reasseble to create app message

# communication Medias
## physical media
- wired links
- twisted pair cable (TP)
		- twist to the distance between the noisy source is roughly uniform 
	- ![](https://i.imgur.com/NiL0cO7.png)
	- two insludate copper wires twisted together
	- one carries signal and the other is used as ground reference
	- reciever uses the different between the two
	- shielded vs unshielded
		- sheld can resits internal and external interference
		- more expensive but faster data rate
	- abt 10MBPS to 1- Gbps
	- max distance about 100 meters
	- telephone, DSL, ethernet LAN

Coaxial cable
- central inner conductor
- outer conductor as a shield
- special insulation
- long distance at high speed
- cable TV, ethernet LAN
- no grouding wire because the shield protects it and the signal is more stable
- ![](https://i.imgur.com/UI6vx1i.png)

Fibre-optic cable
- glass fibre carrying light pulses
- high speed 10 -100 GBps
- low error rate
- ![](https://i.imgur.com/fDMjJgs.png)

## wireless media
- wireless radio
	- signal carried in electro magnetic spectrum
	- broadcast, half-duplex
	- propagation environment effecs
		- reflection and obstruction
		- interference/noise
- radio link types
	- wireless LAN (WiFi)
	- wide area (e.g., 4g cellular)
	- 10's Mbps over ~10km
	- bluetooth  - short distance, limited rates
	- satellite - up to 45 Mbps per channel

# Packet and circuit switching
- network core
	- many routers connect to subnets
	
	Router
	- two functions
	- forwarding
		- aka switching
		- local action: move arriving packets from routers input link to appropriate output link
		- looks at fowarding table and sends to appropriate output link depending on the header
		- ![](https://i.imgur.com/4a6bTyz.png)
	- routing
		- global action: determine the source-destination paths taken by packets
		- routing algorithms
		- creates fowarding tables

packet switching
- host break application layer messages into packets
- network fowards packets from one router to the next, across links on path from source to destination
- store and foward: entire packet must arrive at trouter before it can be transmitted on next link
- packet queueing: occurs when work arrives faster than it can be serviced

circuit switching
- end-end resources allocated and reserved for "call" between source and destination
- one connection is set up - reserved resources are not shared.
- ![](https://i.imgur.com/HA7TWrq.png)
- circuit segment is idle if not used by call
- used for e.g., telephones

# network protocol
current network structure
- ![](https://i.imgur.com/JNzSZXa.png)
- composed of multiple subnets
- a network of networks
- not owned by one entity
- you need to connect using an ISP e.g., spark vodaphone, 2degress
- have their own network
- provice access to users
- not isolated from each other
	- connected using internet exchange point
	- or peering link?

types of ISP
- ![](https://i.imgur.com/aGTYf13.png)
- tier 1
	- potentially cover multiple countries
- content provider networks e.g., google have a private network
	- often bypass tier-1, regional isps


network protocol
- a set of rules that dictate how to format, transmit and recieve data so that network devices can communicate
- like a common language for computers
- specifies a format
	- what in withing the packet
	- reciever can understand based on the format

- complex layered design
	- explicit structure allows identification of systems pieces
	- modularization eases the maintenance
	- layer
		- each implements a service
		- via its on internal layer actions
		- relying on services provided by layer below


TCP/IP reference model
- application
	- exchanges message to implement a service using the services of transport layer
- transport
	- transfers from one process to another using services of network layer
	- encapsulates application layer message, M, with transport layer-layer header H1 to create a transport-layer segment
	- Ht used by transport layer protocol to implement its service
- network
	- transferse transport-layer segment from one host to another using link layer services
- link
	- transfers datagram from host to neighboring host using network layer services
- physical

TCP/IP protocol suite
- TCP - transmission control protocol
- IP - internet protocol
- ![](https://i.imgur.com/3BX3RRG.png)

OSI model
- two more layers
	- presentation
	- session
- ![](https://i.imgur.com/jBhk7Ox.png)

# network performance metrics
- bandwidth - maximum rate (bits/second) a link or network can theoretically transmit
- throughput - the actual reate a link or network is transmitting
- ![](https://i.imgur.com/XajLKbf.png)

- nodal delay - delay at a single router
	- propagation delay - time to send data along a link
	- transmission delay
	- processing - time to find route within router
	- queueing
	- dnodal = dproc + dqueue + dtrans + dprop
- end-to-end delay - total delay to send a packet from source to destination

- packet loss
	- queue (aka buffer) preceding link in buffer had finite capacity
	- packet arriving to full queue dropped
	- lost packet may be retransmitted by previous node, by source end system, or not at all
	- also lost due to corupted packets
	-