Lab 4 – Asynchronous communication (Physical Layer) MPC8360 Computer Networks I 371-1-0201 Lab 4 – Asynchronous communication (Physical Layer) Semester B, 2010

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

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.

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)

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

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

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

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

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