With the internet and digital technology blurring the boundaries between content, communication, and media types, these days you’re just as likely to tune into a concert, movie, or television show on your mobile phone, as you are to catch the same event on radio, at a cinema, or through your home entertainment system.
This transition may appear seamless, but behind the scenes, there are different modes of transmission at work, and different challenges which need to be met by the source, route, and receiver of each transmission. In this article, we’ll be comparing and contrasting the principal methods used in transmitting information and signals in the digital era of networking and communications.
What is a Unicast Transmission?
A unicast transmission is a one-to-one communication that passes from a single source to a single receiver or destination. One of the simplest everyday examples of unicast transmission would be a phone call between two people.
In computing terms, unicast transmission is the most common method of information transfer which takes place on networks. Traffic in the form of streams of data packets typically moves from a single host (such as a web server) to a single endpoint (such as a client app, computer, or browser).
Though a unicast transmission is point to point, the same information may be passed from the source node to any number of other nodes on the network, in a succession of one-to-one communications. A replica of each packet in the data stream goes to every host on the network that requests it.
More technically, unicast transmission employs Internet Protocol or IP provision techniques such as transmission control protocol (TCP) and user datagram protocol (UDP). These are session-based protocols which allow a communication to be set up, completed, and terminated as a single operation. A unicast transmission is sent to a single node on the network, which is identified by a unique 64-bit address.
Benefits of Unicast
Unicast transmission has been in use for a long time, with well-established protocols and easy to deploy techniques. Well-known and trusted applications such as http, smtp, ftp and telnet all use the unicast standard and employ the TCP transport protocol.
On a network, transmission takes place from host to host, which can reduce the traffic burden on a Local Area Network (LAN), as a whole.
Drawbacks of Unicast
If a network device is called upon to send a message to multiple nodes, it has to send multiple unicast messages, each addressed to a specific device. This first requires the sender to know the exact IP address of each destination device.
In addition, each unicast client that connects to the host server uses up some network bandwidth. If multiple clients are involved, this may introduce scaling issues as far as network and server resources are concerned. The problem becomes even more pronounced if many hosts are transmitting via unicast to many receivers, at the same time.
What is a Broadcast Transmission?
A broadcast transmission simultaneously transmits the same information to all nodes on a network. To ensure that the broadcast reaches all “corners”, the transmission may have to be refreshed or relayed at certain points. Television signals sent from a public network to viewers across the country or globe are a simple example of broadcast transmission.
For computer networks, broadcast transmission is generally used on LANs (Local Area Networks). This is primarily due to scale, as traffic has to stream from a single node to all possible endpoints within reach on the network. Broadcast information is sent from the source node only once – a copy of that data is then forwarded to all devices on the network.
LANs such as Ethernet networks support broadcast transmission, in which case the address resolution protocol (arp) may be used to send an address resolution query to all computers on the LAN.
Network layer protocols such as IPv4 also support a form of broadcast which allows the same data packet to be sent to every system in a logical network. The IPv4 address 255.255.255.255 is generally used as a broadcast address.
Benefits of Broadcast
Broadcast transmission is a “one-to-all” technique which ensures that all the nodes on a network receive a common signal or message. This can lend to economies of scale, if the equipment and resources needed for copying, refreshing, and relaying the broadcast signal to all points can be kept to within reasonable limits.
Even for cable TV systems, the source signal reaches all destinations (subscribers) – which is why some channels remain scrambled unless you have the right decoder.
Drawbacks of Broadcast
Broadcast transmission isn’t really a viable proposition for the public internet, as having every connected device receive all broadcast signals all the time would introduce unnecessary interference and tons of unwanted data.
On local networks, some configuration is needed, as switches are designed to forward broadcast traffic, while routers are designed to drop it. Routers won’t allow the broadcast of a message from one Local Area Network to another network segment. Their main function is to split up a large broadcast domain into many smaller ones.
We’ve already noted the limitations of unicast transmission in cases where multiple hosts on a network need to send a message to many receivers, at the same time.
Likewise, the “one message to all” nature of broadcast transmission introduces the likelihood of a network overload, when applied to a large-scale medium like the web.
There are also many “in between” scenarios, which neither unicast nor broadcast transmission are ideally suited for.
Not surprising then, that an alternate mode of transmission has also been used.
Multicast transmission is a “one-to-many” technique which sends information from a single source to as many destinations as express a specific interest in receiving it. So it’s an “on demand” system capable of catering for the likes of pay per view, or similar subscription-based services.
The system is capable of being scaled up to accommodate “many-to-many” type communications, with multiple hosts transmitting to multiple receivers. Traffic is spread across the network from source to destinations using multicast routers. These routers replicate data packets received at one input interface, then send the copied data out on multiple output interfaces.
The trick in managing multicast transmissions is to configure the multicast routers so that they can locate multicast sources on the network, send out copies of data packets via a number of interfaces, avoid loops, and keep the flow of un-called for data packets to a minimum.
Finding a Balance
None of the systems described above is perfect. For example, while multicast transmission can save on bandwidth and works well in corporate environments where all routers are multicast-enabled, it’s not so good for web servers, as only some sections of the internet support the multicast protocol.
Using the transmission method most appropriate for the operating environment and the network resources available is the ideal.
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