Lecture 9: Packet Transmission Flashcards
Name five functions the data link layer is responsible for.
Access control, framing, addressing, flow control and error control.
What four pieces of information might the header and trailer of a frame include?
Source & destination address, error control (FCS), frame length, sequence numbers.
What is the MAC address?
The address of the network interface on an ethernet LAN.
What are three types of addresses?
Unicast - single source or destination.
Multicast - one source to many destination on LAN.
Broadcast - one source to all destinations on LAN.
What would you look at in an Ethernet MAC address to determine if it unicast or multicast? What would broadcast look like?
Final bit in first byte indicates unicast (0) or multicast (1). Broadcast all bits are 1s.
What is the Bit Error Rate (BER)?
Measure of how frequently an error is likely to occur.
Describe the difference between forward and backward error control.
Forward error control: can detect and correct errors.
Backward error control: can only detect errors. Uses retransmission scheme instead.
What happens if sender sends frames too fast for receiver?
It will overflow the receiver.
Name two ways flow control can occur in.
1) Sender sends frame and waits for acknowledgement from recipient to send next one.
2) Sender and recipient have agreed upon number frames that can be outstanding at a given time.
Name the three categories of Multiple Access Protocols:
Controlled Access, Random Access and Channelisation.
Name three types of controlled access protocols. Describe them.
Polling, token-passing and reservation.
Name four types of random access protocols. Describe them.
Multiple Access (MA) : any station can transmit any time. Waits for ACK and resends if doesn’t receive it.
Carrier Sense MA (CSMA): senses if transmission on network to avoid collisions before sending.
CSMA/Collision Detection (CSMA/CD): detects if collision occurs and resends if so.
CSMA/Collision Avoidance (CSMA/CA): waits long enough to reduce probability of avoiding collision. Largely used in WiFi.
Why would CSMA/CA be used for WiFi?
Collision detection is usually not possible with WiFi due to cost
Name four types of channelisation multiple access.
Frequency Division Multiplexing: splits bandwidth into several smaller channels using different carrier frequencies.
Wave Division Multiplexing: very similar but users different light wavelengths in optical fibre.
Time Divison Multiplexing: network is split into time slots and each station is allocated a slot to transmit at.
Code Divison Multiplexing: each station has special code to share single channel allowing all stations to transmit simultaneously using same frequency range.
