Data Communications Flashcards
- The physical layer
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
- Data-link layer
- 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
- Network layer
Routes messages through complex layers
Simple networks: physical source & destination address (layer 2 MAC)
Complex networks: (internets) require more info (layer 3 IP processing)
- Transport layer
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
Bandwidth
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
Bandwidth distance product
Multiply bandwidth of a cable by the distance it can successfully operate at
Attenuation
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
Media types
UTP - Unshielded Twisted Pair
STP - Shielded Twisted Pair
Coaxial cable
Fibre optic
Wireless:
Infrared
Microwaves
Radio
Unshielded twisted pair characteristics
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
Shielded twisted pair characteristics
Foil wire mesh between wire pairs
Screening: Foil between individual pairs
Shield is electrically grounded -reduces emmissions
and EMI susceptability
STP advantages
Less susceptability to EMI
Less susceptability to crosstalk (with shielding)
Greater bandwidth
STP Disadvantages
Greater cost
Greater weight
More difficult to install
Larger diameter
Special connectors required
More rigid
Unbounded media examples and advantages
Infrared, Radio, Microwaves
Enable transmission and reception of EM signals without a bearer.
No cables therefore much more flexible
Physical Network Topology definition
Layout/actual appearance of cabling (OSI Layer 1)
Logical Network Topology characteristics
Data flow through the physical topology
LAN Bus Network
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.
LAN Star Network
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
LAN Ring Network
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.
LAN Mesh Network
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.
LAN Hybrid topology
Mix of all, common in large WANS
But devices on one topology cannot be placed onto another without specific hardware.
Ring Logical Topology
Node-to-node transmission.
Does each node have separate recieve and transmit circuits?
Bus Logical Topology
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.
Ethernet notation
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
Repeater characteristics
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.
Hub Characteristcs
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
Bridge characteristics
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
Switch characteristics
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.
Router Characteristics
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
Gateway characteristics
OSI Layers 4-7 - Transport, Session, Presentation, Application
Translation e.g PC LAN to UNIX
Normally virtual , more powerful than router.
