# Are the channels mentioned here physical or logical in duplex and multiplex?

1. https://en.wikipedia.org/wiki/Duplex_(telecommunications)

In a half-duplex system, there are still two clearly defined paths/channels, and each party can communicate with the other but not simultaneously

Are the "two clearly defined paths/channels" physical channels?

2. https://en.wikipedia.org/wiki/Multiplexing

The multiplexed signal is transmitted over a communication channel, such as a cable. The multiplexing divides the capacity of the communication channel into several logical channels, one for each message signal or data stream to be transferred.

Is "a communication channel" a physical channel?

3. Is it correct that

• a physical channel can only allow at most one transmission at a time?
• a medium can have multiple physical channels? E.g. a duplex medium has two channels for opposite directional transmissions (see the quote in part 1)?

Thanks.

• Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide your own answer and accept it. Commented Aug 6, 2017 at 23:54

1. No, they do not have to be physical. They can be a logical, virtual, etc.

3a. No. A physical channel can be half or full duplex. It can also support multiple simultaneous transmissions. Actually, it depends more upon the capabilities of the endpoints.

3b. Again, they don't have to be physical.

Consider the "aether" (space-time) as a medium. You can transmit radio energy through it, and you can do it half duplex or full duplex. You can divide it up into "channels". Many transmitters can operate simultaneously. You can (but you don't have to) multiplex. Depending on how you want to define it, it can fit any of your definitions.

• thanks. 3a. If a physical channel can be half or full duplex., why "In a half-duplex system, there are still two clearly defined paths/channels, and each party can communicate with the other but not simultaneously"? How do you understand this quote from wikipedia?
– Tim
Commented Oct 10, 2015 at 0:26
• Because "channel" is really a conceptual idea. Also, Wikipedia is not the Authoritative Source. ;p Commented Oct 10, 2015 at 0:30
• Thanks. (1) Can you share your definitions of a medium, channel, link, path, and connection? What relations are between them? Even if you and Wikipedia call some concept by different names, it is helpful to point out the distinct concepts, rather than let different names confuse us. programmers.stackexchange.com/q/299392/699 (2) can you recommend some good references (textbook like)?
– Tim
Commented Oct 10, 2015 at 0:32
• I think Wikipedia definitions are good enough for a first approximation. I think you're getting too hung up on precise definitions of very abstract concepts. I don't have any book recommendations because frankly, more precise definitions have no practical value for me. Commented Oct 10, 2015 at 0:49