Data Communications

Question 1

1. The physical layer

Answer 1

Defines all electrical and Physical specifications for devices
Layout of pins, voltages, and cable specifications, also hubs and
repeaters.
Primary roles of physial layer:
- establishment + termination of connections to communications medium
- Participation in load - sharing procedure
- Modulation/conversion between equipment & cabling

Question 2

2. Data-link layer

Answer 2

• Provides means to transfer data between network
  devices, and ID and correct errors in physical layer.
  (MAC addresses hard coded at manufacture)
• Switches and Bridges, LAN connections

Question 3

3. Network layer

Answer 3

Routes messages through complex layers
Simple networks: physical source & destination address (layer 2 MAC)
Complex networks: (internets) require more info (layer 3 IP processing)

Question 4

4. Transport layer

Answer 4

Assembles packets (layer 3) into proper sequence, checks for errors and passes them on to session layer (layer 5) .
Ensures reliable delivery from source to destination
Communications now between processes, not devices with network address
Transport layer ensures no errors, correct sequence, without loss or duplication.
↳ Reliable ≠ no loss or damage, only that it is detectable
Breaks large message strings into Packets (layer 3) to be handled by the network layer
Therefore controls flow of data, provides error recovery, reorders message units provides acknowledgement between communicating devices

Question 5

Bandwidth

Answer 5

The amount of data which can be transmitted over a network cable at one time, usually expressed in MBps, GBps, TBps.
1,000,000 MBps = 1,000 GBps = 1 TBps
If a cable can carry a wider ranged frequencies, it can transmit more data and thus has a higher Bandwidth

Question 6

Bandwidth distance product

Answer 6

Multiply bandwidth of a cable by the distance it can successfully operate at

Question 7

Attenuation

Answer 7

Gradual weakening of a signal as it passes through a wire or other medium
Greater distance/frequency = Greater attenuation
higher frequencies which give greater bandwidth attenuate first, this speed drops off with range

Question 8

Media types

Answer 8

UTP - Unshielded Twisted Pair
STP - Shielded Twisted Pair
Coaxial cable
Fibre optic

Wireless:
Infrared
Microwaves
Radio

Question 9

Unshielded twisted pair characteristics

Answer 9

Inexpensive, easy install
Usually RJ-45 jacks and plugs → 8 conductor connectors
(similar to RJ-11 on telephone cables)

Copper wire has high attenuation

YTP most susceptible to EMI and eavesdropping as no shield
Category 1 - telephone systems
category 3 - network minimum
Category 5e - high quality network cable

Question 10

Shielded twisted pair characteristics

Answer 10

Foil wire mesh between wire pairs
Screening: Foil between individual pairs
Shield is electrically grounded -reduces emmissions
and EMI susceptability

Question 11

STP advantages

Answer 11

Less susceptability to EMI
Less susceptability to crosstalk (with shielding)
Greater bandwidth

Question 12

STP Disadvantages

Answer 12

Greater cost
Greater weight
More difficult to install
Larger diameter
Special connectors required
More rigid

Question 13

Unbounded media examples and advantages

Answer 13

Infrared, Radio, Microwaves
Enable transmission and reception of EM signals without a bearer.
No cables therefore much more flexible

Question 14

Physical Network Topology definition

Answer 14

Layout/actual appearance of cabling (OSI Layer 1)

Question 15

Logical Network Topology characteristics

Answer 15

Data flow through the physical topology

Question 16

LAN Bus Network

Answer 16

All terminals wired to a common bus
Single cable supports entire network segment.
Bus cable also known as backbone, nodes attached at
various points = T-connectors or drop cable
Inexpensive, little cable required
Terminators at cable ends absorb signals so no bounce back
Any cable break takes network down -usually coax.
e. g. ADAWS combat system highway.

Question 17

LAN Star Network

Answer 17

All terminals wired to central server/hub
Each node connected with our cable (UTP) to hub → internal connections to other nodes
Ethernet 100 BaseT
More cable required, but more reliable, easier to expand
Large amount of wire needed, hub failure lethal
e.g CSS, DII

Question 18

LAN Ring Network

Answer 18

All terminals wired to a common ring
Each node recieves signals from upstream neighbour and passes to downstream neighbour
Node = Repeater, thus little degredation, but more expensive
Equal network access via tokens
Single node failure affects whole network, difficult to expand.

Question 19

LAN Mesh Network

Answer 19

Redundant rings
Each node connected to every otter node
High reliability, expensive to install and expand
Can use routers, often used by WAN.
Can use multiple routes to increase bandwidth.

Question 20

LAN Hybrid topology

Answer 20

Mix of all, common in large WANS
But devices on one topology cannot be placed onto another without specific hardware.

Question 21

Ring Logical Topology

Answer 21

Node-to-node transmission.
Does each node have separate recieve and transmit circuits?

Question 22

Bus Logical Topology

Answer 22

Transmissions (Frames) broadcast simultaneously to every point on transmission media.
Every network station checks intended recipient.
When signal reaches destination, it is removed from the network.
As transmission media are shared, only one station can transmit at a time.

Question 23

Ethernet notation

Answer 23

n signal m

n = data rate in MBps
Signal = signalling type (base for baseband, broad for
broadband)
M = T for UTP/STP networks, X for fibre networks

Question 24

Repeater characteristics

Answer 24

OSI layer 1 - Physical

Network cables have a maximum length, repeaters clean signal
Amplifier = boosts signal strength -analogue (broadband)
Repeaters = regenerate signals- digital (baseband)

GOOD: Reduce noise, increase signal strength, Can be used between UTP & Coax

BAD: Adds delay, cannot segment large inter networks, does not translate between network architectures e.g Ethernet and token ring.

Question 25

Hub Characteristcs

Answer 25

OSI Layer 1 - Physical

Connect nodes in star network
Also known as concentrators , fixed number of ports.
Passive hubs simply connect nodes

GOOD: regenerate signals, act as repeaters.

BAD: Active hubs require power

Question 26

Bridge characteristics

Answer 26

OSI Layer 2 - Data link

Connect 2 or more network segments
Monitor MAC address of packet, filter out packets thus
reducing network traffic.

GOOD: Can connect different transmission media, increase network bandwidth.

BAD: no translation, more expensive than repeaters, slight delay

Question 27

Switch characteristics

Answer 27

OSI Layer 2 - Data Link

Special hub to minimise traffic
Forward packet to appropriate port based on MAC address.

GOOD: Speed up crowded network

Ethernet network has switches instead of hubs = Switched ethernet.

Question 28

Router Characteristics

Answer 28

OSI Layer 3 - Network

Like bridges, Connect subnets and send packets to
appropriate destinations.
Use logical (network) address like IP, not physial address (MAC)
Static Routing: Fixed routing table entered by admin, lists destination network segment for each range of logical addresses.
Dynamic Routing: Routing table intelligently maintained, multiple dynamic routers use routing protocols to notify each other of routing table changes.

GOOD: can connect different network architectures, negligible delay.

BAD: Routers more sophisticated than bridges, more expensive + administration complex

Question 29

Gateway characteristics

Answer 29

OSI Layers 4-7 - Transport, Session, Presentation, Application

Translation e.g PC LAN to UNIX
Normally virtual , more powerful than router.