Synchronous Transmission

Synchronous transmission is a method of data communication that requires the source and destination to synchronize their clocks together. This synchronization of the clocks can occur externally to the data information or be incorporated with the data information.

The advantage to having the clocks synchronized is that longer blocks of data can be sent without loss of synchronization. Less overhead is required for the amount of data sent.

Synchronous Fields

Synchronous Transmission sends blocks of characters at a time. Each block of data is preceded by a Start Field which is used to tell the receiving station that a new packet of characters is arriving. The blocks of data also have End Fields to indicate the end of the data block. The packet can contain up to 64,000 bytes depending on the protocol. Both Start and End Fields have a special bit sequence that the receiving station recognizes to indicate the start and end of a data block. The Start and End Fields may be only 2 bytes each.


Synchronous transmission is more efficient than asynchronous (character transmission) as little as only 4 bytes (2 Start Framing Bytes and 2 Stop Framing bytes) are required to transmit up to 8K bytes. Extra bytes, like the Start and Stop Frame, that are not part of the data are called overhead. Overhead consists of control information used to control the communication.

In asynchronous transmission, there are 3 to 4 bits of overhead (start, stop, parity bits) sent with each character of data (7 to 8 bits). The start and stop bits were used to identify the beginning and end of transmission.

Efficiency example: An Ethernet frame has an overhead of 26 bytes including the "Start and Stop Frames", the maximum data size is 1500 bytes. The efficiency is calculated using the following formula:

The Ethernet frame's efficiency would be 98.3% efficient. Little bandwidth is wasted sending the overhead.

With synchronous transmission, blocks of data up to 64Kbytes in size can be sent without loss or corruption of data. A start field and end field indicate the beginning and end of transmission. Smaller overhead results in a more efficient delivery of data. Timing - Asynchronous vs. Synchronous Transmission explores the difference in efficiency between the two transmission methods.

There are two types of synchronous data transmission (or communication):

  • Externally clocked synchronous transmission
  • Internally encoded synchronous transmission

Externally clocked synchronous transmission

Externally clocked synchronous transmission has separate lines from the data lines for synchronizing the clock. For example. in addition to the normal handshaking control lines, the V.35 physical layer standard has two pairs of wires used for synchronizing the source and destination clocks.

V.35 34 pin connector pinouts

Signal Name

  Pin number

Chassis Ground


Signal Ground






















Transmit Data (A)


Receive Data (A)


Transmit Data (B)


Receive Data (B)


Terminal Timing (A)


Receive Timing (A)


Terminal Timing (B)


Receive Timing (B)


Transmit Timing (A)




Transmit Timing (B)


The Transmit Timing balanced pair is used to send out a timing clock to the destination, the Receive Timing balanced pair is a separate clock timing received from the destination. In theory, you could send data at one data transfer rate and receive data at a different rate. This however rarely occurs, 99.9% of the time the transmit and receive data rates are the same.

Since the clocks are now synchronized, blocks of data can be sent. Special sequence of bits called fields are required at the beginning and end of the block of data to inform the destination that new data is arriving.

Internally Clocked Synchronous Transmission

Internally clocked synchronous transmission is more difficult and expensive to implement than externally clocked synchronous transmission. The timing signal for synchronization between the source and destination is encoded within the data stream. Manchester encoding is an example of an internally clocked synchronous transmission code.

It is used with all higher transfer rates of communication: Ethernet, ArcNet, Token Ring etc... Internally clocked synchronous transmission is used in fast transfer rates 100 Kbps to 100 Mbps. Internally clocked synchronous transmission is often called self clocking as no external timing lines are required.

In the Manchester Code, there is a transition at the middle of each bit period. The mid-bit transition serves as a clocking mechanism and also as data: a low to high transition represents a 1 and a high to low transition represents a 0.

Note: The technical community is split almost right down the middle on the logic level that is indicated by the direction of the transition. Some references indicate that a low to high transition represents a logic 1 and others insist that it represents a logic 0. The point of confusion rises from where you measure the transition in the circuitry. There is an inverting line driver on the output of the manchester encoding circuitry. If you measure on the input side, you get one logic level, on the output the opposite logic level. We'll just acknowledge that there is contention in this area and keep it simple: low to high represents a logic 1.

Manchester Encoding has no DC component and there is always a transition available for synchronizing receive and transmit clocks. Because of the continuous presence of these transitions, Manchester Encoding is also called a self clocking code.

It has the added benefit of requiring the least amount of bandwidth compared to the other Line Codes (Unipolar, Polar, etc..). Manchester coding requires 2 frequencies: the base carrier and 2 x the carrier frequency. All other types of Line Coding require a range from 0 hertz to the maximum transfer rate frequency. In other words, Manchester Encoding requires a Narrow Bandwidth

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Copyright July 2013 Eugene Blanchard