Notes on Computer Networking

Durim Miziraj

2026-09-30

Transmission Modes

The set-out

How do two devices coordinate the delivery of a sequence of bits? Bits can be delivered sequentially or in paralell. It can be done synchronously, asynchronously or isochronously.

Fundamental categories of transmission modes

Paralell transmission

Multiple wires transmit bits that ultimately belong together. - High throughput. A transmission in N wires send N bits at the same time that a serial transmission sends 1 bit. - Matches underlying hardware that also are designed in paralell circuuits. Problems are caused by wires that differ in length. Even 1 millimeter in difference can cause issues.

Serial transmission

Everything is sent on one wire. This is the least complex mode, and is the most often used in computer networks.

Conversion between transmission modes

Most serial circuits need a chip that converts data in paralell mode to data in serial mode. This is called a UART (Universal Asynchronous Receiver and Transmitter).

Transmission order

A set of bytes and a set of bits can be sent in orders that are independent of eachother. A system that orders its data by its most significant bit first are called “big endian”, and “little endian” for data ordered by its least significant bit first.

Fig. 1: Visualisation of bit and byte order.

Byte order can be in little endian while bit order can be in big endian for the same piece of data. The important thing is that both communicating systems agree on how to order the data.

Timing in serial transmissions

Serial Asynchronous transmission

This timing mode is well suited for transmissions which can not be predicted to arrive at a certain time.

The downside is that the computer expends resources in waiting for input. This can be solved by sending bits that signal the start and end of a transmission.

+15v ┬   ┌─┐   ┌─┐   ┌─┐ ┌─┐   ┌─┐
  0v ┼   │ │   │ │   │ │ │ │   │ │
-15v ┴───┘ └───┘ └───┘ └─┘ └───┘ └──────
      1 1 0 1 1 0 1 1 0 1 0 1 1 0 ^ ^ 
      ^ ^                         │ │ 
   idle|start                  stop|idle

Serial synchronous transmission

This mode supposes a constant transmission.

+15v ┬┌─┐   ┌─┐   ┌─┐ ┌─┐   ┌─┐
  0v ┼│ │   │ │   │ │ │ │   │ │
-15v ┴┘ └───┘ └───┘ └─┘ └───┘ └...
       0 1 1 0 1 1 0 1 0 1 1 0 . . .

When comparing the two figures above, it is easily observable that the serial asynchronous mode carries more overhead.

The downside with serial asynchronous mode is that the sender must send data continuously. What happens when there is no more data to sendbut do not want the overhead of the serial asynchronous mode? The concept of framing needs to be introduced. The data is sent in a frame that can define idle gaps between the data.

Serial isochronous transmission

This transmission mode is well suited for transmitting live voice and video data. The constant stream of data, coming in exact intervals work well with the mediaplayers that must play the content at a certain bitrate.

Simplex, half-duplex and full-duplex transmissionor

These concepts explain the directionality of the data transmissions.

DCE and DTE equipment.

DCE and DTE are abbreviations for Data Communications Equipment and Data Terminal Equipment.

A service provider could ask a phone company to install DCE on their premises, and the let the consumer use an arbitrary device for comminicating through the DCE.

The first thing that comes to mind is the way that the early internet used phone lines for communication. Phone companies offered DCE’s (56k modems) to serviceproviders and consumers who then would attach an arbitrary DTE (computer) for communicating over already existing infrastructre.

Possible interfaces between a DCE and DTE are the following.