Meaning/History of TTY and VTY lines

[vv0bbLeS]

Hello all,

I am curious as to the meaning/history of the TTY and VTY lines, in particular these 4 questions:

1. I have heard that VTY stands for Virtual Terminal Line, yet this doesn't seem to match the acronym of "VTY" and has left me wondering what the "Y" in VTY stands for?
2. What does TTY stand for?
3. Why are these terminal connections refered to as "lines" in Cisco? Does the term "line" refer to an actual physical connection (i.e. "line") or perhaps to the fact that it is a "line" in the config ?
4. Why are the vty lines 0-4 separated from the vty lines 5-15 ? I'm guessing it is because the 5-15 lines were added later, or perhaps they support different features than lines 0-4 ?

[Richard Burts · Accepted solution]

Here are my explanations of your questions:

1) and 2) Perhaps wikipedia is a place to start the explanation

https://en.wikipedia.org/wiki/TTY

which says that tty is an electromechanical typewriter paired with a communication channel (a physical connection). This provided one way to get input/output from early computers. A vty is sort of a virtual implementation of this - a way to get input/output from the network device that is not associated with a physical connection. 

When I think about the derivation of the term I start from TeleTYpe (physical) and think about it as virtual and substitute the first letter to get VTY. And the Y comes from type.

3) I assume that Cisco refers to them as lines because the original TTY did have a physical communications line and they were implementing the virtual version of that. It does not relate to being a line in the config. This is a "line" to permit communication with the network device but it is virtual and not physical.

4) is the easy one to answer. You are exactly correct that the early generations of Cisco routers and switches had vty 0 4 (5 virtual communications links). Later generations added the possibility of additional virtual communications links but the config parser separates them. The functionality is exactly the same.

[vv0bbLeS]

Ricahrd,

Excellent! That is exactly what I was looking for! I didn't catch the TeleTYpe for the "Y" part in TTY and VTY (virtual). And the "line" term makes sense in terms of coming from the physical links used in Teletypes.

And that is interesting that you mention the config parser separates the vty lines 0-4 and 5-15. I was just listening to the first half of a 40-minute podcast where Terry Slattery and Rob Widmer discuss their work in basically creating the modern IOS CLI back in the early 1990's I believe, and they discuss the parser and the various difficulties they encountered in their work. Very fascinating! Here is a link for anyone that is interested: https://networkcollective.com/2019/03/hon-cisco-cli/

Thanks again Richard!

[Richard Burts]

I am glad that my explanation was helpful. Thank you for marking the question as solved. I used to work for Terry Slattery and thank you for the link to his discussion.

[vv0bbLeS]

Of course! And that is so interesting that you used to work for him. I bet that was such a cool experience. Thanks again!