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Networking Chapter 1 - Introduction (Network core (Packet-switching: Hosts…
Networking Chapter 1 - Introduction
Some terminology
Hosts
: Servers/end systems, run
network apps
Communication links
: method of communication, fiber, copper, etc.
Have a
transmission rate/bandwidth
Packet switches
: Forward packets/chunks of data,
routers
and
switches
responsible for this
Internet
Network of networks, interconnected ISPs
Infrastructure provides services to applications
Provides programming interface to apps
Networking protocol
: defines format, order of messages sent and received among network entities, and actions taken on message transmission, receipt
Network core
ISP part, where all the action with the internet happens
Packet-switching
: Hosts break application-layer messages into packets
Forward packet from one router to next until destination reached
Each packet transmitted at full link capacity
Store and forward
: Router waits for contents of entire packet to arrive before sending to destination, src->router->dst
Total time = L/R (src->router) + L(R) (router->dst) =
end-end delay = 2L/R
Queuing loss/delay
Packets may have to wait on queue before link opens up to send them off
Finite length queue, if queue filled, packet loss occurs, dropping some of them
Two key functions
Routing
: Determines src->dst route taken by packets
Forwarding
: Move packets from router input to other router's output
Circuit Switching
: Resources for a communication path between src and dst are reserved for the duration of time required to do it, path is called the
circuit
Constant transmission rate of
total transmission capacity/(number of circuits per link)
, guaranteed to send
Multiplexing
Frequency Division Multiplexing (FDM)
Every connection gets a fraction of the total bandwidth, assigned a frequency range?
Time Division Multiplexing (TDM)
Each connection gets all the bandwidth periodically during brief fixed intervals of time
Transmission rate = (frames transmitted per second) x (number of bits per slot)
Packet switching vs Circuit switching
Packet switching allows more users to use network, using binomial probability distribution, can figure out probability of what percentage of total users would be online at once, really low
Also somewhat unreliable with packet delay/loss, and don't know how to guarantee circuit-like behaviour in networks
Network of networks
Different tiers of things
Access ISP
: Where your modem sends data to
Regional ISP
: Collection of access ISPs in one central place
IXP
: Link to connect two ISPs, internet exchange point
Tier 1 ISP/Big company
: Regional ISPs connect to these, as well as access ISPs. Big companies are
content provider networks
, which provide closer servers for faster connections
Network edge
Physical media
Unguided media
: Signals propagate freely, radio, etc.
Radio: Signal carried in electromagnetic spectrum, microwaves, LAN, satellite, cellular, etc.
Guided media
: Signals propagate in solid media, copper, fiber, etc.
coaxial and fiber optic: Two most common nowadays, fiber optic is hella fast and better insulated from electrical noise
Physical link
: What's between transmitter & receiver (the actual cable or radio receiver itself)
How to connect end systems to edge router?
Mobile access networks, school.company access networks, etc.
Network core
: Interconnected routers, network of networks kinda like the internet
Access networks
: Wired/wireless communication links
Wireless Access Networks
Wide-area wireless access
: Cellular data on phones, etc.
Wireless LAN
: 802.11 b/g/n/ac what's used in wifi, etc.
Enterprise Access Networks (Ethernet)
Very fast, what's used nowadays
Home Network
Router connected to cable/DSL modem, multiple devices connected to wireless access point on router or wired to router
Cable Network
FDM: Look into this
A bit faster than DSL, usable by today's standards
Makes use of cable TV infrastructure
Hybrid fiber coaxial
: Uses coaxial cable from home/cable modem to a neighbourhood junction, then from the junction to the cable headend @ TV provider a fiber optic cable is used
Digital Subscriber Line (
DSL
)
Relatively slow
Local computer -> DSL modem + telephone line -> central phone office's DSLAM -> telephone network for voice or ISP for data
Use existing phone line to get access to internet
Hosts
: clients and servers, servers often in data centers
Sending packets
Transmission delay = time needed to transmit L-bit packet into link = L/R
Transmits packet into access network at
transmission rate R
(link transmission rate/link capacity/link bandwidth)
Takes message, breaks into smaller chunks (
packets
) of length
L
bits
The part of the network that isn't the ISP/local part that's connected to the ISP
Network Layers
5 main layers
Application Layer
: Network applications/common protocols, HTTP and FTP built on this layer
Transport Layer
: Process to process data transfer, TCP and UDP here
Network Layer
: routing of datagrams from
source to destination, IP address and routing protocols
Link Layer
: data transfer between
neighboring network elements, WiFi, Ethernet
Physical Layer
: Actual bits on the wire
2 extra layers with ISO OSI model, between application and transport
Presentation
: Services for applications to interpret meaning of data, encryption/compression tools
Session
: Synchronization, recovery of data
Don't really need these layers on the network level though
Loss and Delays
Throughput
rate (bits/time unit) at which bits transferred between sender/receiver
bottleneck link
: link on end-end path that constrains end-end throughput, always the slowest link
average
: rate over longer period of time
instantaneous
: rate at given point in time
Other delays
Traceroute
: Program that sends packets to routers in between src and dst. Routers then send info about themselves back like domain name and time to reach router
When loss occurs
packet arrival rate to link (temporarily) exceeds output link
capacity, resulting in dropped packets/packet loss
4 sources of delay
Nodal delay
= nodal processing delay + queue delay + transmission delay + propogation delay
Queueing delay
: Time waiting at output link for transmission, variable, dependent on congestion level of router
Variables
We look at the composite
La/R
La/R > 1
: More work than can be serviced, average delay infinite
La/R ~= 1
: Average queuing delay large
La/R ~= 0
: Average queuing delay small or doesn't exist
a: average packet arrival rate
L: packet length (bits)
R: link bandwidth (bps)
Nodal processing delay
: Delay for the router to verify bits and determine the output link for a packet.
Typically < 1ms
Propogation delay = d/s
, d = length of physical link, s = propagation speed (2x10^8m/s) (delay is pretty much negligible). Delay for individual bits to physically move across wire.
Transmission delay = L/R
, L = packet length in bits, R = link bandwidth in bps, time it takes for the bits to get onto the wire? review this