- Functionality provided by the
Physical Layer:
- Transmission of
analog/digital data
using analog/digital signals
Example:
- Error detection
- Transmission of
analog/digital data
using analog/digital signals
- Framing:
- We can transmit of
bits
using analog/digital signals
Example:
64,000 question:
- How can you tell the start and end of a message ?
Example:
This problem is called:
- framing.
- We can transmit of
bits
using analog/digital signals
- Frame:
- A frame = a unit ("atom") of data transmitted and received by the Data Link layer
Therefore:
- The Data Link layer will:
- receive
one entire frame
(if the frame is
deemed error-free)
or
- reject one entire frame (if the frame is deemed contain bit errors)
- receive
one entire frame
(if the frame is
deemed error-free)
or
- The Data Link layer will never receive/reject a portion of a frame
- Functions
provided by the Data Link layer:
- Framing:
- An protocol (= agreement) on how to group a series of bits into logical unit (= frame)
- Error detection and recovery:
- Check if a
received frame is
correct
- If frame contains
bit errors:
- Recover the correct frame
Further conditions on correctness:
- Each frame
must be
received
exactly
once:
- Duplicate reception (that you cannot tell) of the same frame is in fact a communication error !!!
- Frames must be received in the same order as transmitted
- Check if a
received frame is
correct
- Flow control:
- Pace
the transmission rate
of the sender
in accordance
with the processing speed
of the receiver
Note:
- The flow control function is desirable but is not necessary in the Data Link layer
- Pace
the transmission rate
of the sender
in accordance
with the processing speed
of the receiver
- Framing:
- In this Data Link layer
section, we will
cover:
- Framing
- How to ensure reliable transfer (correct and exactly once)
- Framing
- Flow control:
- Flow control will discussed in the Transport Layer section