Packet switching and network technologies Flashcards
What is circuit switching?
A communication mechanism that establishes a distinct path between the sender and receiver before sending data
What does circuit switching guarantee?
isolation from paths used by other senders and receivers
What technology is circuit switching usually associated with?
Analogue telephone technology
In regard to circuit switching, does each circuit correspond to a physical path?
No (virtual circuits or physical paths)
What are the 3 general properties of a circuit switched paradigm?
- Point to point communication
- separate steps for circuit creation, use and termination
- performance equivalent to an isolated physical path even though it isn’t
What is the main alternative for circuit switching? What does it form the basis of?
Packet switching
- forms the basis of the internet
Generally, what multiplexing method does packet switching use?
Statistical (time) multiplexing (communication from several sources competes for use of shared media)
What does a packet switching system require of the sender?
The sender needs to divide each ‘message’ into blocks of data (packets)
In packet switching - is packet size fixed?
The size of the packet varies depending on the packet switching tech
What are 3 general properties of packet switching?
- arbitrary, asynchronous communication
- no setup needed before communication starts
- performance varies due to statistical multiplexing among packets
What are two advantages of packet switching?
- Helps in detecting transmission errors
- Gives fair access for a shared connection between many computers
What type of multiplexing does packet switching use?
Time division multiplexing
What are pros and cons of shared communication channels?
- low cost
- not suitable for a wide area as delays inhibit coordination of sharing the network
When were shared LANs invented?
1960s
What do LAN, MAN, WAN stand for?
- local area network (single room or building
- Metropolitan area net.. (major city)
- Wide area.. (spans sites in multiple cities)
What are some examples of LAN topologies?
star (hub in centre)
ring (each connected to 2 others)
mesh (4 connected to all others)
bus (shared cable)
What are the pros and cons of a a star LAN topology?
More robust but may be a bottleneck
What are the pros and cons of a ring LAN topology?
Enables easy co-ordination but sensitive to a cable being cut
What are the pros and cons of a bus LAN topology?
Requires least wiring but is also sensitive to cable being cut
What is the main problem with mesh LAN topologies?
Poor scalability, 3 links for 3, 6 links for 4, etc etc
What LAN topology is a non switched ethernet cable an example of?
Bus network (single cable to which computers connect)
What structure is a mobile ad-hoc network an example of?
A mesh network
What are the features of a mobile ad-hoc network (MANET)?
-self configuring
- each node has to discover the network shape/topology
- announces it’s presence and listens for broadcasts
What distinguishes physical and logical topology?
Technology defines logical topology
Wiring scheme defines physical topology
What is the physical and logical topology of an twisted pair ethernet cable?
Logically - bus
Physically - star
Who specifies the scheme for hardware adressing?
The IEEE
What are the different terms for a hardware address?
- media access control address (MAC address)
- Ethernet address
- NOT IP address
How many bits is a hardware address?
48
What do different types of hardware have to define for packets?
A format/wrapping called a frame
What distinguishes packets and frames?
- Packet is logical
- Frame is actual
In a packet switched network, each frame corresponds to a…..
Packet
What are the two conceptual parts of a frame?
Header (metadata e.g. address)
Payload (contains actual data)
What does CRC stand for?
cyclic redundancy check
What does an ethernet frame consist of?
-fixed length header
- variable length payload
- fixed length crc
What is byte stuffing?
Special characters at the start and end of frames are reserved for control characters, the transmitter scans data and replaces control characters with specific characters, receiver inverse maps this
IEEE divides layer 2 (network interface) into 2 sub layers, what are they?
LLC (logical link control) and MAC (media access control)
What are the 3 subdivisions of Multi-Access protocols? (protocols that control access to a shared medium)
- Controlled Access protocols
- Random Access protocols
- Channelisation protocols
What are the 3 types of Controlled Access protocols covered in this module?
Polling, Reservation, Token passing
Briefly describe Controlled Access protocol Polling.
- A centralised network controller cycles through all stations on the network and gives an opportunity to transmit a packet (option to give stations higher priority)
Briefly describe Controlled Access protocol Reservation.
-Uses a centralised controller
- Each potential sender says whether they need to send a packet during the next round and controller sends a list of transmitting stations
- this list is used by stations to know when to transmit
Briefly describe Controlled Access protocol token passing.
A token is passed around continuously until a station needs to send a packet, it holds onto the token while doing this then sends it on again once finished.
What are the two Random access protocols covered in this module.
CSMA/CD and CSMA/CA
- Carrier Sense Multi-Access with Collision Detection (CD) or Collision Avoidance (CA)
When are CSMA/CD and CSMA/CA used?
CD - most widely used and basis of ethernet
CA - basis for wireless networks
What is a collision? What do other computers detect this as?
When multiple computers transmit at once, detected by other computers as a garbled transmission
What do computers normally do to recover after transmission collision?
Wait for a random delay period up to a specified max
- Doubling the max delay for each subsequent collision
How does offered traffic affect throughput?
More offered traffic means more collisions means more backing off (delay after collision) means more congestion means less overall throughput
What is the difference between bandwidth and throughput?
Bandwidth refers to capacity, while throughput details how much data actually transmits
Wireless LANs use _____ frequency and CSMA/C__ and are an example of passive ____ technology?
- Radio frequency
- CA (collision avoidance)
- passive bus technology
Why doesn’t CD (collision detection) work on wireless LAN?
Collision detection doesn’t work because a transmission by one computer may only be received by its immediate neighbours.
Wireless LANs can’t use CSMA/CD, what do they use instead?
CSMA/CA
How does CA (collision avoidance) work?
sender sends a small request to a receiver who responds with a clear to send message received by all adjacent computers.
In CSMA/CA how are collisions handled?
- control messages collide rather than real messages and no clear to send is sent
- a random back off is then applied and control messages are resent.
What are the 3 main channelisation protocols?
FDMA, TDMA, CDMA
Frequency, Time, Code (Division Multi Access)
What are FDMA and TDMA extensions of?
frequency and time division multiplexing respectively.
How does the channelisation protocol FDMA work?
Independent stations choose carrier frequencies that don’t conflict with other station’s carriers.
- sometimes a central controller tells stations which unused frequencies to use.
How does the channelisation protocol TDMA work?
Stations get a sequence number and transmit in numerical order
LAN access protocols are the….
mechanisms that allow computers to access a network for sending.
Why are there distance limitations in LANs?
- MAC protocols require time proportional to cable length
- Electrical signals weaken with distance
How does a fibre optic LAN extension work?
two fibre modems with a fibre optic cable between is used to extend normal cables that go between a computer and an ethernet hub
How do repeaters work (as LAN extenders)?
They join sections of maximum size ethernet cable together
- amplify the signal strength
- strengthens the signal but doesn’t help delay
- also amplifies noise.
Under the ethernet standard, how many repeaters can be used between 2 computers?
No more than four
How does a bridge work for LANs?
It connects two segments (two hubs) at frame level
- Receives all frames from computers
- Doesn’t forward erroneous frames e.g collisions and noise
What are the benefits of learning bridges/ frame filtering?
A frame is only forwarded when the destination is in the other segment or broadcast or multicast address is used
- The bridge learns which segment a computer is in when that computer sends a frame
What happens when a frame arrives at a bridge?
- Source address is extracted and knowledge is updated (where computers are)
- destination data is inspected to see if it needs forwarding to the other segment
Can communication occur simultaneously in both segments of a bridged network?
Yes
When planning a bridged network, where should two frequently communicating devices go?
In the same segment
What extension technique might be paired with a bridge to connect LANs in two separate buildings?
a fibre optic cable with two fibre modems between buildings and the bridge in one of the two
What extension technique might be paired with a bridge to connect LANs very far away from each other?
Two bridges each connected to emitters/receivers utilising a satellite
What is a switch?
A single electronic device that transfers frames between computers
A hub between computers simulates a shared medium, what does a switch simulate?
A bridged LAN where each computer has one segment
Describe the differences between a hub and a switch?
hub - operates as an analouge device forwarding signals between computers
switch - digital device forwarding frames between computers
What is the advantage of a switch?
Greater data transfer as both computers can transfer in parallel like a bridged LAN
Why can a switch potentially support multiple transmissions at a time?
It handles packets not signals and uses a fabric to provide parallel internal paths within the device.
Wireless networks can also be divided into LAN, WAN and MAN like wired networks, what extra category do they have?
PAN - personal area network (1-50m)
What are the three key parts of a wireless LAN?
- Access points
- Switch or router
- Set of wireless hosts
What are the two modes of a WIFI network?
- Infrastructure
- Ad hoc
What’s the difference between infrastructure and ad hoc modes of wifi?
infrastructure: the host only communicates with an access point (base station) and the access point relays all packets
ad hoc: the hosts communicate among themselves with no base station/access point
How does a wireless LAN (e.g. wifi) cover a larger area?
with several overlapping access points that clients are able to join (but can only associate with one)
How do wireless LAN access points co-ordinate hand offs?
Some will co-ordinate among themselves to hand over the client whereas some lower cost access points require the client to change their association manually
What are the two key multiplexing techniques used in Wi-Fi networks?
- Frequency division multiplexing
- Spread spectrum multiplexing
Describe spread spectrum multiplexing (for a Wi-Fi network).
- Sender spreads data across multiple frequencies and the receiver combines the information to rebuild the original data
What are some potential benefits / reasons for using different standards of wireless LAN?
Enhanced security, specific data rates, improved reliability when swapping access points
What are four examples of wireless PANs?
Bluetooth, InfraRed, ZigBee, ISM Wireless
What method of multiplexing does bluetooth use?
Spread spectrum frequency hopping
What is the network architecture of a bluetooth PAN?
Piconet where one master device connects to up to 7 active slaves
What type of network is a cellular communications system?
A wireless WAN
How does a cellular communications system work?
System consists of cells each containing a transmitter which are placed side by side to cover wide areas
What are all the Base stations of a cellular network connected to?
Mobile switching centre
How do the frequencies vary across a cellular WAN system of cells?
Adjacent cells use different frequencies to prevent interference
Give brief details of GPS satellites and GPS.
24 satellites orbiting 6 orbital planes
- use triangulation and is accurate to 20-2m ish
What are the problems associated with low earth orbit?
limited availability and complex to track
What does VSAT stand for (satellites)?
Very Small Aperture Terminal
What is VSAT satellite technology used for?
- entertainment and internet access
- businesses use to link stores
Why are satellites necessary to extend cellular networks?
Microwaves can’t bend round the earth so satellites are needed for long distance communication.
