Chapter |Shared|: Mediating Shared Access
Chapter |Intro| highlights the challenge of multiplexing multiple users on a shared network, but focuses on how that problem manifests on switches. They buffer packets that need to be sent on a given output port, and decide which of those packets to send next (typically the one at the head of the queue). When connected to a point-to-point link, there is no fear that someone else might try to use that link at the same time you do. (There is a node at the other end of the link, but traffic being transmitted in each direction is managed independently, without the risk of interference.)
Another possibility is to connect multiple nodes directly to a shared communication medium, a so-called multi-access network. Wireless networks are an obvious example, where the "shared medium" is the space carrying radio signals. There are also wired examples, with the original Ethernet being the most widely deployed. Just as with multiple people in a room trying to talk at the same time, multiple nodes simultaneously initiating a transmission will interfere with each other. Some strategy is needed to decide whose turn it is to transmit (or speak) next.
This chapter looks at three approaches, as adopted by three different network technologies. Wi-Fi and the Mobile Cellular Network[1] are two wireless examples; Passive Optical Networks (PON)—colloquially known as Fiber-to-the-Home—is the third. In addition to illustrating three different approaches to mediating access to shared resources, all three technologies can be categorized as access networks, indicating that they are the immediate network that edge hosts connect to, thereby being indirectly connected to the rest of the Internet. It is common for access networks to implement some form of authentication to ensure that the connecting host has permission to access its resources in the first place. Establishing identity is often the first step before resource allocation takes place.
| [1] | For simplicity, we refer to the mobile cellular network as 5G, except when we are making a point about issues that span generations of the technology. |
Before looking at our three example networks in detail, we first take a closer look at the problems they are trying to solve, and the design options for addressing those problems. We also include a short primer on the unique challenges of managing radio spectrum.