Unicast, Multicast and Broadcast

There are three primary methods of passing data between nodes within a communications network, one to one (Unicast), one to many (Multicast) and one to all (Broadcast). Each of these three basic methods has advantages and disadvantages which makes each suited for particular applications.

One to One - Unicast

Unicasting is a one-way link data transmission established between two nodes within a network, typically a server and client. The server transmits the data to the client either when scheduled or requested by the user. Although the unicast is one way it is highly likely that in a unicast architecture the recipient would have a return link. An example of unicasting would be video on demand, whereby a User requests a transmission when convenient and using a return channel to interact with the transmission in order to fast forward pause, stop and resume later etc. Internet via satellite is another application of unicasting whereby a User requests a URL via a suitable return link following which the requested information is unicast to the user. The main benefit of unicasting is that the user can control the data without affecting other Users. The main disadvantage is poor bandwidth efficiency as if the same data was required by ten Users the bandwidth required would be ten times that to support one User.

One to Many Users (Multicast)

Multicast is more efficient than unicast for one-to-many delivery of data. Unlike unicast, information is only transmitted once (or a pre-determined number of times) regardless of the number of recipients. What distinguishes multicasting from broadcasting is that multicast information is only processed by user systems that are configured for reception of that information. Essentially this is achieved by assigning users with a multicast address based on a pre-defined or dynamic set of attributes. Specific factors which make multicasting suitable for some Traffiq applications include the following:

• Users monitor the multicast until they identify data with a multicast address that matches the vehicle group(s) to which they are affiliated.
• Vehicle group membership can be either pre-assigned or dynamic
• Vehicles can join and leave groups at any time.
• Vehicle groups can be associated with:
  • Vehicle physical attributes (e.g. make, model, age)
  • Driver/passenger attributes (e.g. male, female, age)
  • Geographical attributes (e.g. all users within a specific area)
  • Other attributes (driver/passenger interests, pre-registered groups)
• There are no limitations to the size of a vehicle group.
• The only geographical constraints are the satellite coverage area.
• A vehicle can belong to more than one group simultaneously
• Membership of some vehicle groups will be determined by the system (geographically based groups) whilst others will be determined by the driver/passengers (driver passenger interest groups).

The main advantage of multicasting is bandwidth efficiency, particularly for large user groups. The main disadvantage is that the lack of control of the information being multicast by users. The best example of this would be a multicast video clip which was missed by one or a number of intended recipients. Typical solutions to solve these common problems include pre-arranged transmission times, repeat transmissions (with long and short repetition periods) and local storage by recipients. Multicast file transfer can be unreliable (due to the absence of a FTP) however techniques do exist which significantly improves multicast file transfer.

One to all recipients (Broadcast)

In a broadcast system all information is sent to all users at all times. There are no conditional attributes of the data stream that define intended recipients. A multicast system could be configured to broadcast information (a multicast to every recipient) however broadcasting is inherently more efficient as no overhead is required for routing, conditional access etc.

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