Duplexing defines the directionality of data within the connection between two nodes. Two modes are used in duplexing: half and full.

With half-duplexing, a device can either send or receive data–it can not do both simultaneously. Eg: Walkie-Talkie
With full-duplexing, the devices send and receive data simultaneously without any errors. Eg: Mobile phone

Pertaining to data networking, the device can either either send or receive frames in Ethernet if it’s in half-duplex mode. Here the Ethernet media is shared among many devices within the same collision domain. The technology implemented in such a case is CSMA/CD (Carrier Sense, Multiple Access / Collision Detection) to detect the collisions occurring in the shared media. The physical topology of the network includes hubs and 10Base2 or 10Base5 Coaxial cable media. We no longer design this kind of networks any more and are obsolete. You could still find them in some legacy networks.

Full-Duplex enables a device to send and receive frames simultaneously. This is essentially done by connecting the transmit circuit of one device with the receive circuit of the other and vice versa. Since there is a dedicated path exclusively for both transmission and reception of data, the communication is bi-directional. Full-duplex configuration requires what is known as “point-to-point” connections. In other words, the connection between the two devices is a dedicated one for both transmission and reception paths. Point-to-point connections require dedicated switch ports to connect the nodes. Each switch port is a collision domain and there’s only one device in it. PCs connected to switch or Router connecting to switch are common example of point-to-point connections, enabling full-duplex communication.

Following table summarizes the aspects of half-duplex and full-duplex data communication.

Three-way Handshake

If you have a 2Mbps E1/T1 WAN link, with half-duplex, the two devices have to share the link for both Tx and Rx and hence the throughput will be around 1Mbps on both ends. Whereas in full-duplex, the throughput will be more or less 2Mbps on each side, effectively 4Mbps of bandwidth.

(Bandwidth is the theoretical max. data rate measured in bps(bits per second), whereas throughput is the actual real-time data rate. Throughput is usually less than the half of the bandwidth.)

About Deepak Devanand

Seeker of knowledge
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