Point-to-point vs unicast, and multiaccess vs multicast/broadcast?

Question

1. From Tanenbaum's Computer Networks:

   Point-to-point links connect individual pairs of machines. To go from the source to the destination on a network made up of point-to-point links, short messages, called packets in certain contexts, may have to first visit one or more intermediate machines. Often multiple routes, of different lengths, are possible, so finding good ones is important in point-to-point networks.

   Point-to-point transmission with exactly one sender and exactly one receiver is sometimes called unicasting.

   What is the difference between point-to-point and unicasting?

   What does the last sentence mean? Doesn't a point-to-point transmission always have exactly one sender and exactly one receiver?

2. From Tanenbaum's book

   In the literature, broadcast channels are sometimes referred to as multiaccess channels or random access channels. The protocols used to determine who goes next on a multiaccess channel be- long to a sublayer of the data link layer called the MAC (Medium Access Con- trol) sublayer.

   From https://en.wikipedia.org/wiki/Media_access_control

   The MAC layer emulates a full-duplex logical communication channel in a multi-point network. This channel may provide unicast, multicast or broadcast communication service.

   Tanenbaum seems to suggest that multiaccess and broadcast are the same, while Wikipedia says multiaccess can be unicast, multicast or broadcast. What is the difference between multiaccess and broadcast?

Thanks.

Answer 1

When you discuss a point-to-point link, you are usually referring to a physical link that exists only between two devices.

A point-to-point transmission is a transmission from one device to a single other device. This may or may not cross point-to-point links, but that is not a requirement. A unicast is essentially the same idea in that it means the packet is addressed to a single recipient.

Contrast a broadcast or mutiaccess medium with the idea of a point-to-point link. A broadcast medium allows multiple devices on a single link. Each device looks at the destination address of a packet sent on the medium to see if it belongs to him. This doesn't directly relate to the 'cast designations since those are about how a packet is addressed rather than the medium on which it is sent.

The 'cast references refer to how the packet is addressed, not the medium on which the packet travels. A unicast is addressed to a single recipient, while a broadcast is addressed to every recipient. A multicast is addressed to every recipient which has subscribed to the multicast group (the destination address of the packet).