Distributioned Systems & Networks Flashcards
What are the 5 network layers in Tanenbaums books model
Application
Transport
Network
Link
Physical
What headers does the Transport layer add
TCP or UDP headers
What headers does the Network layer add
IP Headers
What headers does the link layer add
It depends on the medium as it will add whatever headers are required for the physical medium
What is the purpose of the link layer, from a high level
To shield the upper layers from the specific connection type, and then to transmit bits in the form of frames
What addressing form is used at the link layer
MAC (Medium Access Control Address)
What IEEE standard defines transmission for WIFI
IEEE802.11
There are a large number of physical medium types, name some
Important ones:
+ Coaxial Cable
+ Twisted pair
+ Power line
+ Fibre optic
+ Wireless
Minor ones:
+ Laser
+ Sound
+ Ultrasonic
+ Pulses
+ Radar
What is the function of the link layer
The link layer:
+ Transmits frames over physical media, encapsulating IP Datagrams into link layer frames
+ Receives frames and parses the IP datagrams up the stack
+ Detects and handles transmission errors
There are many standards which have been defined and have evolved over the years
When being encapsulated by the link later, what do packets from higher levels become and get referred to as
Packets get encapsulated into FRAMES as PAYLOADS, they are prepended with headers and appended with trailers
What level of encapsulation differs based on the type of the physical layer
Data frames vary depending on the physical layer, for example, an ethernet frame will have a different form than a WiFi or a Fibre frame
What is the purpose of flow control
Flow control regulates the flow of data to avoid swamping slow receivers from fast senders
What are two major methods of flow control at the link layer
+ Messages sent to the sender saying more data can be sent
+ Rate based with an agreed speed
What are the three general link layer models, giving an example for each
+ Connectionless, no acknowledgements - i.e. wired ethernet
+ Acknowledged, connectionless service - i.e. Wifi (IEEE802.11)
Acknowledged, connection-oriented - i.e. satellite
What does it mean for a link layer model to be called Connectionless, no acknowledgement
Connectionless means that no signalling path is established in advance.
No acknowledgement means frames are sent and they may or may not be received by the destination.
What does it mean for a link layer model to be called an Acknowledged Connectionless service
This is used to allow frames to be sent without first setting up a connection, and then allows the acknowledgement of these frames
What does it mean for a link layer model to be called an acknowledged connection-oriented service
This is used to allow a connection to be established between two machines before frames are sent, these frames can then be acknowledged when they are received
What does ARQ Stand for?
Automatic repeat reQuest.
What is stop and wait ARQ
Stop and wait ARQ (Automatic Repeat reQuest) is a link layer ACK handling strategies which sends a frame and waits for an ACK before sending the next frame. If it does not receive an ACK then the frame is re-transmitted
At the link layer, when is an ACK not sent?
An ACK is not sent if the frame is lost or damaged (checksum doesn’t compute)
At the link layer, what is Go-Back-N ARQ
Go-Back-N ARQ is a method of handling acks which will send multiple frames (up to the window size) before it receives the first ACK, it uses sequence numbers on the frames to ensure it gets frames in the correct order.
If a frame is received out of order it gets discarded, and an ack is then sent for the last correct, in order frame and the sender re transmits from that point
What is selective repeat ARQ at the link layer
Selective repeat ARQ is similar to Go-Back-N ARQ, however it only re-transmits lost frames meaning that it is acceptable for frames to be received out of order and buffered
What is the job of error detection and correction at the link layer
To detect errors and provide a line “free of errors” to the network layer
What is parity bit
Parity bit is an error detection method where a bit marks “is the number of 1’s even or odd”, but this does not reveal all errors