1. Lab 4 – Asynchronous communication (Physical Layer)
MPC8360 Computer Networks I
Lab 4 – Asynchronous communication (Physical Layer)
Semester B, 2010

2. The Open System Interconnection Reference Model (OSI)
Application 1 2 3 4 5 6 7
Presentation
Session
Transport
Network
Data Link
We Are Still Here
(For the last time)
Physical

3. Asynchronous Communication
Asynchronous transmission allows data to be transmitted without the sender having to send a clock signal to the receiver.
The sender and receiver must agree on timing parameters in advance and special bits are added to each word which are used to synchronize the sending and receiving units.

4. Asynchronous Serial Communication
Asynchronous serial communication uses a special structure called FRAME to transmit data (usually 5 to 8 bits of data)
FRAME
Start Bit
Data0
Data 1
Data2
Data3
Data4
Data5
Data6
Data7
Parity Bit
Stop Bit 1 1 1 1 1
Time
Voltage 5 -5
Line idle (No transmission)

5. Asynchronous Serial Communication
Asynchronous serial communication uses a special structure called FRAME to transmit data (usually 5 to 8 bits of data)
Start Bit
Data0
Data 1
Data2
Data3
Data4
Data5
Data6
Data7
Parity Bit
Stop Bit
Overhead information
Channel Utilization =
Data bits = 8
= 66%
Total bits transmitted
8+4

6. Asynchronous Serial Communication (Rx)
Receiver tries to sample the bit at the middle
0 V
-5 V
5 V
Sampling window in the middle of the bit

7. Asynchronous Communication – Pros & Cons
Simple, doesn't require synchronization of both communication sides Cheap, timing is not as critical as for synchronous transmission, therefore hardware can be made cheaper Set-up is very fast, so well suited for applications where messages are generated at irregular intervals, for example data entry from the keyboard Large relative overhead, a high proportion of the transmitted bits are uniquely for control purposes and thus carry no useful information

8. Synchronous Communication – Pros & Cons
Lower overhead and thus, greater throughput Slightly more complex Hardware is more expensive

9. Quick Overview