Flashcards in Unit 6: Introduction Deck (48):
Define bandwidth and unit it is measured in
Transmission rate of data - bits per second
What does a packet switch do?
Forwards packets (chunks) of data
What is the ‘nuts and bolts’ view of a network?
The internet is a network of networks i.e. consisting of interconnected ISPs. It consists of protocols that control sending and receiving of messages
What is the ‘service’ view of a network?
The internet is an infrastructure that provides services to applications and a programming interface to applications
Define a protocol
Protocols define the format and order of messages sent/received among network entities, and the actions taken on message transmission/receipt
What constitutes network edge?
The hosts - clients and servers
What constitutes access networks?
The wired and wireless communication links
What is the network core?
A mesh of interconnected routers
a) What does DSL stand for?
b) Describe a DSL access net (what is transmitted/where?)
a) DSL = Digital Subscriber Line
b) In DSL, voice and data are transmitted at different frequencies over a dedicated existing telephone line to a central office. The data goes to the internet, voice goes to the telephone net
What is frequency division multiplexing?
Where different channels are transmitted in different frequency bands
Describe a cable network access net
In a cable network a fibre cable attaches homes to an ISP router. Home share access networks to cable headend
State a difference between DSL and cable networks
Access networks are shared in cable networks, whereas DSL have dedicated access to the central office
What type of cable is used in cable networks?
Hybrid fiber coaxial (HFC) cables which are asymmetric (one direction)
Describe the modern home network access net and the different connection points
A wireless access point connects wireless devices to the router, in which wired devices are connected directly via wired ethernet. The router is connected to a cable or DSL modem.
Briefly describe wide-area wireless access
Cellular operators (e.g. Telco) provide 3G, 4G etc. by using technologies such as LTE
Define a bit in the context of physical media
What propagates between the transmitter and receiver pairs
Define a physical link in the context of physical media
What lies between a transmitter and receiver
Define guided media in the context of physical media
Signals that propagate in solid media e.g. copper
Define a unguided media in the context of physical media
Signals that propagate freely e.g. radio
Define a twisted pair in the context of physical media
Two insulated copper wires used in ethernet with high access speed
a) Define a coaxial cable in the context of physical media.
b) Where is it used?
a) Two concentric copper conductors which are
a) Define a fiber optic cable in the context of physical media
b) Give and explain a benefit of fiber optic cables
A high speed glass fiber that carries light pulses (each represent a bit). It has a low error rate as it is immune to electromagnetic noise and repeaters are spaced far apart
a) Describe radio signals in the context of physical media
b) Give and explain a drawback of radio signals
Signals are carried in the electromagnetic spectrum. It is bidirectional. Radio is susceptible to the environment because of reflection, obstruction and interference
Give 4 types of radio link
Terrestrial microwave, LAN (wifi), wide-area (cellular), satellite
Define packet switching
Hosts break application-layer messages into packets, forward them from one router to the next across links
Briefly describe the store and forward approach
The entire packet must arrive at the router before it can be transmitted on the next link
Give the equation for calculating how long it takes to transmit a packet using store-and-forward:
a) one way
b) 2 ways
L = packet size (bits), R = bandwidth (bps)
What causes queuing and loss in packet switching?
When arrival rate > transmission rate, packets will queue. If memory buffer fills up, packets can be dropped
What happens in circuit switching? Give an example where circuit switching is used
Dedicated rources are allocated to a 'call' between source and destination. There is no sharing of resources and the circuit is idle if not used a call. It is used in telephone networks
What is FDM?
Frequency Division Multiplexing - each user occupies a certain band of frequencies
What is TDM?
Time Division Multiplexing - time is divided into frames of a fixed duration which is divided into a fixed number of timeslots. Each connection occupies one timeslot
Given a 1Mb/s link and each user needing 100kb/s, how many users can be supported in circuit switching?
10 users by pre-allocating
Give 2 reasons why packet switching is good for 'bursty data'
1. It allows resource sharing
2. It is simpler, has no call setup
State a problem with packet switching
Excessive congestion is possible due to packet delay and loss meaning protocols are needed for rdt and congestion control
Which is the provider and which is the customer in the following:
1. Global ISP & Regional ISP
2. Regional ISP & Access ISP
1. Global ISP = provider Regional ISP = customer
2. Regional ISP = provider Access ISP = customer
What is tier-1 in an internet structure?
Commerical ISPs e.g. AT&T
What is a content provider network?
A private network that connects its data centers to the internet by often bypassing tier-1 and regional ISPs, e.g. Google
What is a message in the ___ layer called?
Why use layering as an architecture?
When dealing with complex systems, an explicit structure allows identification and relationship between the system's pieces. Modularisation eases maintenance and updating of the system
Give formula to calculate nodal processing delay
nodal processing delay = queueing delay + transmission delay (L/R) + propagation delay (length of link/propagation speed) + processing delay
Give the formula to calculate traffic intensity
L = packet length (bits), R = link bandwidth (bps), a = average packet arrival rate
If La/R ~ 0, what does this mean?
Answer the same for if La/R is 1 or > 1?
0 = average queueing delay is small
1 = average queueing delay is large
> 1 = more work arriving than can be serviced
What is throughput?
The rate (bps) at which bits are transferred between sender/receiver (either as an average or instantaneous measurement)
When comparing capacity, what does R(s) and R(c) represent?
R(s) = capacity of server link (bps)
R(c) = capacity of client link (bps)
Give the throughput and bottleneck link for the following:
a) R(s) < R(c)
a) throughput = R(s) (max capacity), bottleneck = R(s)
b) throughput = R(c), bottleneck = R(c)
When Rs and Rc for a total of 10 connections are connected to network core (R), how do you calculate the throughput?
Throughput = min(Rs, Rc, R/10)
Give two advantages of message segmentation
1. If a single bit error occurs, a whole large message doesn't have to be retransmitted
2. Huge packets would have to be sent into the network and smaller packets have to unfairly queue