Data Communications

Question 1

Network Edge?

Answer 1

hosts, access network, physical media

Question 2

Network core?

Answer 2

packet/circuit switching, internet structure
mesh of interconnected routers

Question 3

Performance?

Answer 3

loss, delay, throughput

Question 4

The internet?

Answer 4

billions of connected computing devices
- hosts = end systems
- running network apps at internet’s “edge”
network of networks: interconnected ISPs

Question 5

hosts?

Answer 5

end systems

Question 6

packet switches?

Answer 6

forward packets (chunks of data)
- routers, switches

Question 7

Communication links?

Answer 7

fiber, copper, radio, satellite
transmission rate: bandwidth

Question 8

Networks

Answer 8

collection of devices, routers,
links: managed by an organization

Question 9

protocols

Answer 9

define the format, order of messages sent and received among network entities, and actions taken on message transmission, receipt

Question 10

how to connect end systems to edge router?

Answer 10

• residential access nets
• institutional access networks (school, company)
• mobile access networks (WiFi, 4G/5G)

Question 11

frequency division multiplexing (FDM)

Answer 11

different channels transmitted in different frequency bands

Question 12

HFC: Hybrid Fiber Coax

Answer 12

asymmetric: up to 40 Mbps - 1.2 Gbps downstream transmission rate, 30-100 Mbps upstream transmission rate

Question 13

network of cable, fiber attaches homes to ISP router

Answer 13

homes share access network to cable headend

Question 14

Access Networks: digital subscriber line (DSL)

Answer 14

• 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

Question 15

Wireless access networks

Answer 15

Shared wireless access network connects end system to router
- via base station AKA “access point”

Question 16

Wireless local area networks (WLANs)

Answer 16

typically within or around building

Question 17

Wide-area cellular access networks

Answer 17

provided by mobile, cellular network operator

Question 18

Access Networks: enterpriser networks

Answer 18

companies, universities, etc.
mix of wired, wireless link technologies, connecting a mix of switches and routers

Question 19

Access networks: data center networks

Answer 19

high-bandwidth links connect hundreds to thousands of servers together and to Internet

Question 20

Host

Answer 20

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

Question 21

packet transmission delay equation?

Answer 21

PTD = time needed to transmit L-bit packet into link = L (bits) / R (bits/sec)
L/R

Question 22

bit

Answer 22

propagates between transmitter/receiver pairs

Question 23

physical link

Answer 23

what lies between transmitter and receiver

Question 24

guided media

Answer 24

signals propagate in solid media: copper, fiber, coax

Question 25

unguided media

Answer 25

signals to propagate freely
e.g. radio

Question 26

Twisted Pair (TP)

Answer 26

two insulated copper wires

Question 27

coaxial cable

Answer 27

two concentric copper conductors
bidirectional
broadband (multiple frequency channels on cable)

Question 28

fiber optic cable

Answer 28

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

Question 29

Wireless Radio

Answer 29

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

Question 30

Radio Link Types?

Answer 30

• wireless LAN
• wide area
• bluetooth: cable replacement
• terrestrial microwave
• satellite

Question 31

packet switching?

Answer 31

hosts break application layer messages into packets
- network forwards packets from one router to the next, across links on path from source to destination

Question 32

Two key network-core functions

Answer 32

• forwarding
• routing

Question 33

forwarding

Answer 33

aka “switching”
local action: move arriving packets from routers input link to appropriate router output link

Question 34

routing

Answer 34

global action: determine source destination paths taken by packets
routing algorithms

Question 35

packet transmission delay

Answer 35

takes L/R seconds to transmit L-bit packet into link at R bps

Question 36

store and forward

Answer 36

entire packet must arrive at router before it can be transmitted on next link

Question 37

queueing

Answer 37

occurs when work arrives faster than it can be serviced

Question 38

packet queueing and loss

Answer 38

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

Question 39

alternative to packet switching

Answer 39

circuit switching

Question 40

FDM - Frequency Division Multiplexing

Answer 40

• optical, electromagnetic frequencies divided into (narrow) frequency bands
• each call allocated its own band, can transmit at max rate of that narrow band

Question 41

Time Division Multiplexing

Answer 41

• 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)