Data Communications Flashcards
Network Edge?
hosts, access network, physical media
Network core?
packet/circuit switching, internet structure
mesh of interconnected routers
Performance?
loss, delay, throughput
The internet?
billions of connected computing devices
- hosts = end systems
- running network apps at internet’s “edge”
network of networks: interconnected ISPs
hosts?
end systems
packet switches?
forward packets (chunks of data)
- routers, switches
Communication links?
fiber, copper, radio, satellite
transmission rate: bandwidth
Networks
collection of devices, routers,
links: managed by an organization
protocols
define the format, order of messages sent and received among network entities, and actions taken on message transmission, receipt
how to connect end systems to edge router?
- residential access nets
- institutional access networks (school, company)
- mobile access networks (WiFi, 4G/5G)
frequency division multiplexing (FDM)
different channels transmitted in different frequency bands
HFC: Hybrid Fiber Coax
asymmetric: up to 40 Mbps - 1.2 Gbps downstream transmission rate, 30-100 Mbps upstream transmission rate
network of cable, fiber attaches homes to ISP router
homes share access network to cable headend
Access Networks: digital subscriber line (DSL)
- use existing telephone line to central office DSLAM
- data over DSL phone line goes to Internet
- voice over DSL phone line goes to telephone net
Wireless access networks
Shared wireless access network connects end system to router
- via base station AKA “access point”
Wireless local area networks (WLANs)
typically within or around building
Wide-area cellular access networks
provided by mobile, cellular network operator
Access Networks: enterpriser networks
companies, universities, etc.
mix of wired, wireless link technologies, connecting a mix of switches and routers
Access networks: data center networks
high-bandwidth links connect hundreds to thousands of servers together and to Internet
Host
sends packets of data
takes application message and breaks it into smaller chunks known as packets, of length L bits
transmits packet into access network at transmission rate R
- link transmission rate, aka link capacity aka link bandwidth
packet transmission delay equation?
PTD = time needed to transmit L-bit packet into link = L (bits) / R (bits/sec)
L/R
bit
propagates between transmitter/receiver pairs
physical link
what lies between transmitter and receiver
guided media
signals propagate in solid media: copper, fiber, coax
unguided media
signals to propagate freely
e.g. radio
Twisted Pair (TP)
two insulated copper wires
coaxial cable
two concentric copper conductors
bidirectional
broadband (multiple frequency channels on cable)
fiber optic cable
glass fiber carrying light pulses, each pulse a bit
high speed operation
- high speed point to point transmission
low error rate:
- repeaters spaced far apart
- immune to electromagnetic noise
Wireless Radio
signal carried in various “bands” in electromagnetic spectrum
no physical wire
broadcast “half-duplex” (sender to receiver)
propagation environment effects
- reflection
- obstruction by objects
- interference/noise
Radio Link Types?
- wireless LAN
- wide area
- bluetooth: cable replacement
- terrestrial microwave
- satellite
packet switching?
hosts break application layer messages into packets
- network forwards packets from one router to the next, across links on path from source to destination
Two key network-core functions
- forwarding
- routing
forwarding
aka “switching”
local action: move arriving packets from routers input link to appropriate router output link
routing
global action: determine source destination paths taken by packets
routing algorithms
packet transmission delay
takes L/R seconds to transmit L-bit packet into link at R bps
store and forward
entire packet must arrive at router before it can be transmitted on next link
queueing
occurs when work arrives faster than it can be serviced
packet queueing and loss
if arrival rate (bps) to link exceeds transmission rate (bps) of link for some period of time:
- packets will queue, waiting to be transmitted on output link
- packets can be dropped (lost) if memory (buffer) in router fills up
alternative to packet switching
circuit switching
FDM - Frequency Division Multiplexing
- optical, electromagnetic frequencies divided into (narrow) frequency bands
- each call allocated its own band, can transmit at max rate of that narrow band
Time Division Multiplexing
- time divided into slots
- each call allocated periodic slot(s) can transmit at max rate of (wider) frequency band (only) during its time slot(s)
