Full Duplex & Half Duplex Ethernet

Posted: August 26, 2010 in Ethernet

The differences between full-duplex and half-duplex:
1.Half-duplex mode adopts CSMA/CD(carrier sense multiple access with collision detection).The traditional shared LAN adopted Half-duplex mode ,resulting in collision on cable.
2.Full-duplex mode can be applied to point-to-point Ethernet connection and Fast Ethernet connection, because they use different lines of twisted pair cable, there will be no collision.

Half-duplex Ethernet operates in a shared collision domain.
Half-duplex Ethernet has lower effective throughput.

Half-duplex refers to that at any time of the communication process, data can be transmitted not only from A to B, but also from B to A, but transmission can be performed only in one direction. Delay will occur for data transmission in Half-duplex mode.
A good example for half-duplex would be single-plank bridge which allows one person to cross this bridge at each end , but only in one direction at a time.

Access method that Gigabit Ethernet uses is:

The Carrier Sense Multiple Access (CSMA) with Collision Detection (CD) protocol is used to control access to the shared Ethernet medium. A switched network (e.g. Fast Ethernet) may use a full duplex mode giving access to the full link speed when used between directly connected NICs, Switch to NIC cables, or Switch to Switch cables.

Full-duplex Ethernet can provide higher throughput than can half-duplex Ethernet of the same bandwidth.

Full-duplex mode: when data sending and receiving split stream, and transmit through two different transmission lines, both communication sides are able to send and receive at the same time, this kind of transmission is called full-duplex;
Half duplex manner: If a single transmission line is used both for sending and receiving, although the data can be transmitted in two directions, but the two sides can not simultaneously send and receive data, such transmission is half-duplex.
CSMA/CD is used to detect whether conflict protocol exists in half-duplex Ethernet. It is a half-duplex Ethernet work mode.
Full-duplex mode will use two links to distinguish between send and receive action, and thus avoid conflict domain.
To use full-duplex, the following requirements are required:
1. P2P Link, or point-to-point connection;
2. Both nodes support full-duplex;
3. Close conflict detection (CSMA/CD).

Replacing a hub with a switch affect CSMA/CD behaviour in an Ethernet network by effectively elimating collisions.

If all network segments in the Ethernet connect with repeaters, because they can not avoid conflict, they remain in the same conflict domain. Switches can be used effectively prevent conflict, but not HUB. Because switch can choose route using physical address, each of its port is a conflict domain. But HUB has no such ability, it will only send out the received data through broadcast, which will easily cause broadcasting storm. All of its ports are in a single conflict domain.

Q: Objectives of the Ethernet protocol using physical addressses is:
1.to uniquely identify devices at Layer 2
2.to allow communication between different devices on the same network.

Description: In computing, a physical address, also real address, or binary address, is the memory address that is electronically (in the form of binary number) presented on the computer address bus circuitry in order to enable the data bus to access a particular storage cell of main memory.
In a computer with virtual memory, the term physical address is used mostly to differentiate from a virtual address. In particular, in computers utilizing memory management unit (MMU) to translate memory addresses, the virtual and physical address refer to address before and after MMU translation, respectively.
In computer networking, physical address is sometimes a synonym of MAC address. The address is actually used on network’s data link layer, not on physical layer, as the name would suggest.


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