Review COMP90007 Internet Technologies
Lecturer: Semester 2, 2021
© University of Melbourne 2021
1
Medium Access Control (1)
MAC sub-layer is used to assist in resolving transmission conflicts
Static channel allocation
Time division multiplexing
Frequency division multiplexing
2
Medium Access Control (2)
Contention
ALOHA, Slotted ALOHA
Carrier Sense Multiple Access: 1-persistent, non- persistent, p-persistent, with collision detection
Collision Free: bit map, binary countdown
Limited Contention: adaptive tree walk
Wireless LANs: MACA
3
Data Link Layer
Reliable, efficient communication of “frames” between two adjacent machines.
Functions of the data link layer:
Provide a well-defined service interface to network
layer
Handling transmission errors Data flow regulation
4
Framing
Methods:
Character (Byte) count
Flag bytes with byte stuffing
Start and end flags with bit stuffing
Most data link protocols use a combination of character count and one other method
5
Error Control
Error Control
Detecting errors
Parity
Checksum
Cyclical Redundancy Check (CRC)
Correcting errors Hamming Code
6
Flow Control
The fast senders vs slow receivers problem requires a solution
Feedback based flow control: ack
Stop and wait
Sliding window: go-back-N, selective repeat
7
Go-Back-N vs. Selective Repeat
Trade-off between efficient use of bandwidth and data link layer buffer space
8
Physical Layer
The physical layer is concerned with the electrical, timing and mechanical interfaces of the network
Timing: data rate, latency
Mechanical: different media …
Electrical: modulations, voltage levels, signal strength …
9
Network Protocol Hierarchies (1)
Layers, protocols and interfaces
Consider the network as a stack of layers
Each layer offers services to layers above it through interface
Protocol is an agreement between the communicating parties on how communication is to proceed
10
Network Protocol Hierarchies (2)
Information flow supporting virtual communication in layer 5
11