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The V35 interface and its alternatives

What is the function of the V35 interface between the modem and router?
When we connect network devices together, we need to have some sort of standardized interface that all manufacturers can agree on. Sometimes we use Ethernet to connect workstations, servers and routers together, sometimes we use ISDN to connect routers, and sometimes we use serial interfaces. V35 is a type of serial interface. Serial interfaces work best for short (say less than 20 meters), low speed and low cost applications. It has excellent reliability and propagation capabilities, supports asynchronous and synchronous communications, as well as full and half duplex connections.

So why V35? When using serial interfaces, the devices have to agree on how the serial connection is going to work. There are a number different serial interface types that have been used over the years; strangely, they were all a good idea at the time. As Radia Perlman says in her excellent book Interconnections 2nd Edition, "the purpose of the 802.1 committee was to standardise LAN protocols. So they standardised not one, but several protocols."

Everyone needs to agree on the same physical design, electrical design, data format, handshaking, etc. This is where standards bodies like the EIA/TIA, ITU, ANSI and others do their best work.

There are number of different standards for connecting to external serial devices. For example, V35, X21, V24, RS449 and RS530 are all currently used serial technologies. You don?t see RS530 or RS449 much, although RS449 is used a quite a bit in legacy IBM equipment and factory situations. V24 is used to communicate to asynchronous devices and is most popular for connecting 56K modems. V24 is the standard name for serial ports on the back of your PC. X21 is a short haul, medium rate serial interface that is widely used outside the United States, or for many new installations.

Many people like the smaller and easier to use connector. It will clock up to around 4 Mbits/s, has an easy to use DB15 connector, and gets a maximum distance of around 5 metres. V35 is very popular in the US; I suspect that a lot of people in the US don?t even know that there is an alternative. Personally, I don't like the big chunky V35 connector and use X21 wherever possible. But V35 connectors are idiot proof, they have big screws on the side that have to be screwed in for it to work, they clock up to 8 Mbits/s and can go up to 20 or 30 meters with good quality cable (I wouldn?t do that at high speed, that would be asking for trouble in my opinion).

So, a V35 interface is a serial connector specification. It transfers data from the router to an external device when you are using a serial interface. It is very popular with the older sections of the industry, and is generally recognized as the best technical solution. It is very similar to the connection used by the modem on your PC, but has better performance and supports synchronous and asynchronous applications.

This was last published in May 2001

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