Terminology on data link, channel, VLAN

Question

I am new to networking and I'm following a CCNA course.

There are a few terms that I don't (properly) understand, and I've searched for them like a mad man, but with no success. I've only read 2 chapters so far, namely the introduction and how to configure an IOS, so please refrain from explaining things in terms of the OSI model, I am not that far yet.

1. As I interpret wikipedia, a channel is the media used for the connection. Then the definition of a data link is the means of connecting one location to another for the purpose of transmitting and receiving digital information. Does this mean they are the same thing?

2. What is a VLAN? As of now, I understand it as a "fake" LAN. So you have two LANs, in different geographical area's (e.g. seperate buildings), and you treat them as one. However, I don't see how it makes sense in the context of a switch console. In the CCNA course I'm following, I've navigated to (config-if)# by entering the the command interface vlan 1. Is this a different kind of VLAN?

3. Apparently theres many ways to configure a switch: CTY (console), VTY (telnet/SSH), AUX (based on a phone cable?), and TTY. After googling however, I still do not understand what TTY means. How do we use TTY? And are all these *TY terms examples of lines? Actually, what's a line? What am I saying when I use the command line console 0? Is a line just a port?

Sorry about the questions not being completely related, but I feel like these questions aren't worth asking seperately. You don't have to answer all 3 of the questions, any information is appreciated...

Answer 1

The first thing to remember is that all these terms came into use via the marketing departments of network equipment manufacturers. As companies developed technologies, they invented terms, created new meanings for existing terms, or borrowed terms from other disciplines with similar characteristics. The point is there are no hard and precise definitions, and sometimes they only mean what companies want them to mean at this moment in time -- they can change as new technologies are developed.

That said, here are some rough definitions:

1. A channel usually refers to a physical medium, while data link is usually a logical connection. Again, the terms can be fuzzy, and there are no doubt exceptions to what I just said, although I can't think of any at the moment.

2. A LAN can be thought of a set of devices that share a medium (e.g. Ethernet) with these two characteristics:

   a. Every device can directly address every other device via the media, creating a data link connection. In Ethernet, every device on a LAN can send traffic directly to every other device using the MAC address.

   b. Devices can send data to all devices on the medium at once (i.e., broadcast).

Back in the good old days, this was done via a physical medium: Ethernet cable, Token Ring, or Arcnet. On a modern network (Ethernet) switch, you can create groups of ports that have these two characteristics. For example, you can configure a 24 port switch so that only ports 1,2,3, and 10 can directly address each other and listen to broadcasts from these ports. You have created 'virtual' LAN or VLAN. It's virtual in the sense that it doesn't rely on physical media, but on the configuration of the switch.

You can create another group of ports (11,12,13) that also have the two characteristics -- you have created a different virtual LAN on the same switch. To answer your specific question, a VLAN interface is a logical layer 3 interface on a VLAN. It's the logical equivalent of connecting a router's interface to a VLAN. It is not the same thing as the VLAN itself.

3. This question refers specifically to Cisco devices. Other manufacturers use different terms (just to add to your confusion). There are a number of physical and logical "ports" you can use to configure a switch. The console and aux ports are physical ports on the device. Both use RS-232 as the physical medium and protocol (here the terms medium and data link get fuzzy).

TTY is an abbreviation of "teletype." Tt's a term that persists even though the original use is long obsolete (the "return" key is another term like that). Think of it as a logical connection to the administrative interface of the switch. You can get here via a physical connection (console or aux ports) or a logical connection using telnet/ssh, using a virtual tty port, or VTY.

Don't get too hung up on the specific definitions. VLANS and their importance will make more sense when you get to OSI.

Answer 2

Bandwidth (technically speaking), is the amount of data the physical media can carry. A physical media will have at least 1 channel, but can have more than 1 channel, for example 2 channels on 1 media (copper wire).

Each channel will have its own channel-bandwidth (throughput). The combined throughput of all the channels on that media is the media-bandwidth.

A VLAN is a layer 2 (Data-link layer) feature &/or function. Interface-VLAN is a layer 3 (Network layer) feature &/or function. There is a difference that most text books do not specifically/directly address.