HCW - 5.06 - data comm - network
The networking layer talks about networking. Networking is the management of addressing of individual nodes and finding a path from the node that you are working on to the mode that you want to communicate with.
This is a very detailed and complex subject potentially, but, initially, it can be quite simple.
It is perhaps best to break it down into simpler parts. So we'll start with a distinction that is actually quite important in modern technology and communications. This is the distinction between circuit switched networks and packet switched networks.
To illustrate the circuit switched network we usually use the telephone system or, as it is very often described, the Plain Old Telephone System or POTS.
Circuit switched networks establish, through a series of switches, a circuit between the originating and the terminating devices. The circuit must be established and remain connected throughout the course of the communication. The circuit remains connected even if no communications is going on. Consider the example of a telephone conversation. If there is a pause in the conversation and nobody is actually saying anything you are still paying for the connection of the circuit. Therefore, circuit switched networks are not as efficient as packet switched networks.
Packet networks are more familiar to people dealing with data communications. It may seem strange to consider that a telephone call and email are both forms of communication and that, in many ways, they are equivalent. However, in modern communications technologies this is the case.
However, for the moment, all we need to consider is the case of an email or a long file that is being transmitted via a data network. In the case of the phone network there are many different possible connections, but one circuit is established and used for the duration of the call. In the case of data networks the packet networks in particular, the material and content does not need to be sent all contiguously, but can be broken up into packets. Not all packets need to take the same transmission route throughout the network. Therefore, as long as the packets are sequentially numbered and identified, they can be sent individually over whichever route is available at the time that they are sent, and then reassembled at the far end. This means that routing is much more efficient, and the entire network can be much more efficient.
This level of efficiency can be quite astounding. When I first started dealing with data communication, and providing seminars on data communication, most of the time I was dealing with people who were dealing with bulletin boards, rather than the Internet itself. The Internet, at that point, was a complete rarity and available only to a select few, mostly at either universities or research institutions.
When I was teaching about the difference between circuit and packet networks I would point out that when you got your bill from the telephone company, half of the money that you were paying did not go into the provision of actual telephone service. The other half of the money that you were paying was for the production of that bill itself. In one of the seminars someone stood up and identified himself as working for the telephone company, and said that I was incorrect. I said all right, you know better than I do, what is the actual figure? His reply was that it wasn't fifty percent, it was ninety percent.
An important aspect of networking and communications is that of network access. This is particularly important at the local segment level. Here, we don't necessarily have an awful lot of addressing or network addressing to do, but a number of devices may be connected to the same medium or piece of wire, and there has to be some means of determining who gets to talk.
The simplest form of network access, at least in terms of the complexity of the protocol, is polling. This simply has some device assigned as the controller, and the controller polls each device on the segment. This is essentially the same as the chairman of a committee asking each member of the committee in turn, do you have anything to say? The controller, of course, has to have some means of identifying all the devices that are on the network.
Next in order of simplicity, although somewhat restricted in terms of the technologies that it can be used with, is that of token passing. Token passing can be referred to as the First Nations style of network access protocol, after the "talking stick" idea. If you have the talking stick, you can talk. If you have the token, you can talk. If you don't have the token, you can't. In most networking technologies, the token is simply a single bit at the leading end of a packet, which constantly traverses the network. When a device wishes to speak it watches for the token and, when the token appears, flips the bit on the token, obtaining the token for itself, and therefore permission to broadcast. Once the device has finished its communication it waits for the next passage of the token, and flip the token back to an available status, thus leaving it available for the next device that wishes to transmit. One possible limitation of token passing is that, unless additional changes are made to the protocol, there is no upper bound on the amount of time that may elapse before your next opportunity to transmit anything.
A very widely used access method is carrier sense multiple access with collision detection. Although this sounds very complicated, it is otherwise known as the cocktail party protocol.
Just like at a cocktail party, everyone can have the opportunity to speak, but only when a space is available to speak. Everyone is listening to see if the person who is currently speaking falls silent. When the person currently speaking falls silent, then everyone waits a random amount of time, and then begins transmitting. The device that transmits first gets to speak. This is exactly like the conversational style at a cocktail party.
The multiple access part is that everyone wants to speak. The carrier sense part is that, if someone is transmitting at the moment, you can't speak. You have to wait for silence.
The collision detection part happens when two people start speaking at the same time, or nearly at the same time. When someone starts speaking, they start speaking with some meaningless leading conversation. When a device on a network does it, it fills this time with a tone signal. If two people start transmitting at the same time, then, by listening for a set amount of time before they start really transmitting what they wanted to say, they can hear if someone else is transmitting while they are transmitting. If two stations or devices try to transmit at the same time, that is a collision. The device that is transmitting is also listening, for a limited amount of time, and so if, within that specific amount of time, they hear someone else transmitting they know the collision has happened. When a collision happens, the two stations that are transmitting detect that a collision has happened, and both stop transmission. They then wait a random amount of time, before trying to transmit again. Once again, the first person to transmit gets the channel, and gets to transmit.
A variation on this is carrier sense multiple access with collision avoidance. This is the protocol that is used in wifi. On wifi, there may be a number of devices, all of which may wish to transmit at any given time. The protocol in this case is not that anyone can transmit at any time, and then detect a collision, but rather that the device wishing to transmit sends a request to communicate to the device that it wishes to communicate to. If this request, and the acknowledgment from the device with which it wishes to communicate transmits and is successfully received, then the right to transmit is granted to the original sender, and any chance of a collision is avoided.
Previous: https://fibrecookery.blogspot.com/2026/02/hcw-504-datacomm-physical.html
Introduction and ToC: https://fibrecookery.blogspot.com/2025/12/how-computers-work-from-ground-up.html
Next: TBA
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