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?


1 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).

  • Thanks. (1) Is MAC sublayer about a single physical link which is broadcast or multiaccess? Or there can be multiple physical links? (2) Is multiaccess also referring to how a packet is addressed or the medium On which the packet travels? Does multiaccess mean the same as broadcast?
    – Tim
    Oct 5, 2015 at 13:17
  • 1. The MAC sublayer is about layer-2 addressing, not the actual medium on which packets travel. The medium could require the use of MAC addressing, but don't confuse the two. 2. Broadcast or multiaccess is referring to the medium, not packet addressing which can be confusing. There is a broadcast medium which has nothing to do with the addressing type called broadcast. You need to separate the ideas of the medium and packet addressing.
    – Ron Maupin
    Oct 5, 2015 at 13:51
  • The medium is an OSI layer-1 construct. Layers 2, 3, and 4 are where addressing comes into play. In the OSI model, each layer is independent of the others, which is why they are separate layers. Just don't get hung up on the idea that everything in the real world strictly follows the OSI model.
    – Ron Maupin
    Oct 5, 2015 at 13:55
  • Thanks. About 1, I know MAC sublayer is talking about stuffs above the physical layer. My question is: does MAC sublayer concern about layer-2 addressing over a physical link/medium, or a LAN? My question comes from the chapter for MAC sublayer in Tanenbaum's book, where he seems to talk about things over a physical link in the introduction of the chapter, but the MAC sublayer chapter also has sections for Wired LANs.
    – Tim
    Oct 6, 2015 at 2:35
  • My point is that the physical link is layer-1, and the MAC address is a layer-2 address, so it is independent of the layer-1 medium. The MAC address has the destination address of another device in the same layer-2 domain, regardless of the physical layer-1 medium. Wi-Fi uses MAC addresses in much the same way as wired ethernet.
    – Ron Maupin
    Oct 6, 2015 at 2:41

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