Serial Communication

Serial communication is the famous and broadly used technique to transfer data between processing equipment and peripherals. Serial communication is used to transfer data across the WANs. The sending router encapsulates the data using a communication protocol. The encapsulated data is sent on a physical medium to the WAN and receiving router de-encapsulates the encapsulated data back.

Serial communication uses Binary One for representing logic HIGH or 5 Volts and binary zero for representing a logic LOW or 0 Volts. It can take many forms depending on the type of transmission mode and data transfer. The transmission modes are Simplex, Half Duplex, Full Duplex, Synchronous and Asynchronous. The source is known as a sender and the destination is known is a receiver for each transmission mode.

Some well-known interfaces used for the data exchange are RS-232, RS-422, RS-423, V.35, HSSI, RS-485, etc. Each one is using a different signalling method. The three most important serial communication standards are affecting LAN-to-WAN connections. We should discuss these most important methods.

Asynchronous Data Transfer – In the Asynchronous Data Transfer the bits of data are not synchronized by a clock pulse. The clock pulse is a signal used for synchronization.

Synchronous Data Transfer – In the synchronous mode the bits of data are synchronized using a clock pulse.


RS-232 is referring to the Interface between data terminal equipment (DTE) and data communications equipment (DCE) using serial binary data exchange.  The data terminal equipment (DTE) is usually the user computer, while data communications equipment (DCE) is the modem. RS-232 was introduced in the 1960s and originally known as EIA recommended standard 232.  It is the oldest serial communication standard.

Most of the serial ports on personal computers are RS-232C or newer standards RS-422 and RS-423. All the standards use both 9-pin and 25-pin connectors. Modems, mice, keyboards and printers were uses serial port but now these types of peripheral devices have been replaced by new and faster standards such as USB and RJ-45 connectors.

RS-232 are commonly available with 4, 9 or 25-pin wiring. The 25-pin Rs232 known as DB-25 connects every pin while the 9-pin Rs232 known as DB-9 does not connect many of the uncommonly used connections.  The 4-pin Rs232 cables provide the minimum connections for data transferring.

Most RS-232 devices can operate with a minimum of 3 signal cables: Transmit (TX), Receive (RX) and Ground (GND). The TX cable of one device must be connected with the RX cable of other device and vice versa and the ground cable must be connected.

A 25 pin RS-232 port on a PC transmits on pin 2 and receives on pin 3, and Ground is pin 7.  In the 9-pin RS-232 port on a PC transmits on pin 3, receives on pin 2 and Ground is pin 5. We cannot connect simply two devices with a serial cable because the connectors fit. It is important to verify the functions of each pin on each device, as well as verify whether or not the cable is a straight-through or null-modem cable.


The V.35 interface was originally specified by CCITT as an interface for 48kbps line transmissions. It has been adopted for all line speeds above 20kbps and seems to have acquired a life of its own. It was discontinued by CCITT in 1988, and replaced by recommendations V.10 and V.11.

It is an International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T) specification standardized as an interface for 48 kbps line transmissions. The standard is used for modem-to-multiplexer communication. The standard is used for high-speed, synchronous data exchange combines the bandwidth of several telephone circuits.  The standard provides high data transfer rates (DTR) and connectivity between DCE and DTE over digital lines. In the U.S., this is used by most routers and DSUs that connect to T1 carriers.

The V.35 interface is working on layer 1 of the Open Systems Interconnection (OSI) model. V.35 interface is usually used for 56 kbps and 64 kbps data rates. It has balanced data and clock leads including handshake leads.  The V.35 has a 34 pin connector that combines the bandwidth of several telephone lines. The combined telephone lines provide a high-speed interface between DTE and channel service units.  Both balanced and unbalanced voltage signals on the same interface provide better speed and distance.  We can travel the V.35 cable up to 1200 meters with 100 Kbps speed.

The V.35 plug is standard with black plastic cover about 20mm by 70mm size, often with gold-plated contacts and built-in hold down and mating screws. The V.35 plug is 20 to 40 times the price of a DB25. Financially the RS-232 option is economical than the V.35 option.


HSSI is the abbreviation of High-Speed Serial Interface, which is used for short-distance communications. Usually, HSSI is used to interconnect routing and switching devices on local area networks (LANs) with the higher-speed lines of a wide area network (WAN) such as T3 lines.  It is a serial interface that supports data transmission up to 52 Mbps.  HSSI is developed by Cisco system for DTE and DCE. HSSI can transmit data up to a distance of 52 feet. HSSI is working at the physical layer of the OSI model. HSSI is used as a 50-pin connector. It is used ECL technology for transmission.