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2.2 Gigabit Ethernet Technology

  Gigabit Ethernet [GEA], also known as the IEEE Standard 802.3z, is the latest Ethernet technology. Like Ethernet, Gigabit Ethernet is a media access control (MAC) and physical-layer (PHY) technology. It offers one gigabit per second (1 Gbps) raw bandwidth which is 10 times faster than fast Ethernet and 100 times the speed of regular Ethernet. In order to achieve 1 Gbps, Gigabit Ethernet uses a modified version of the ANSI X3T11 Fibre Channel standard physical layer (FC-0). To remain backward compatible with existing Ethernet technologies, Gigabit Ethernet uses the same IEEE 802.3 Ethernet frame format, and a compatible full or half duplex carrier sense multiple access/ collision detection (CSMA/CD) scheme scaled to gigabit speeds.

Like its predecessor, Gigabit Ethernet operates in either half-duplex or full-duplex mode. In full-duplex mode, frames travel in both directions simultaneously over two channels on the same connection for an aggregate bandwidth of twice that of half-duplex mode. Full duplex networks are very efficient since data can be sent and received simultaneously. However, full-duplex transmission can be used for point-to-point connections only. Since full-duplex connections cannot be shared, collisions are eliminated. This setup eliminates most of the need for the CSMA/CD access control mechanism because there is no need to determine whether the connection is already being used.

When Gigabit Ethernet operates in full duplex mode, it uses buffers to store incoming and outgoing data frames until the MAC layer has time to pass them higher up the legacy protocol stacks. During heavy traffic transmissions, the buffers may fill up with data faster than the MAC layer can process them. When this occurs, the MAC layer prevents the upper layers from sending until the buffer has room to store more frames; otherwise, frames would be lost due to insufficient buffer space.

In the event that the receive buffers approach their maximum capacity, a high water mark interrupts the MAC control of the receiving node and sends a signal to the sending node instructing it to halt packet transmission for a specified period of time until the buffer can catch up. The sending node stops packet transmission until the time interval is past or until it receives a new packet from the receiving node with a time interval of zero. It then resumes packet transmission. The high water mark ensures that enough buffer capacity remains to give the MAC time to inform the other devices to shut down the flow of data before the buffer capacity overflows. Similarly, there is a low water mark to notify the MAC control when there is enough open capacity in the buffer to restart the flow of incoming data.

Full-duplex transmission can be deployed between ports on two switches, a workstation and a switch port, or between two workstations. Full-duplex connections cannot be used for shared-port connections, such as a repeater or hub port that connects multiple workstations. Gigabit Ethernet is most effective when running in the full-duplex, point-to-point mode where full bandwidth is dedicated between the two end-nodes. Full-duplex operation is ideal for backbones and high-speed server or router links.

For half-duplex operation, Gigabit Ethernet will use the enhanced CSMA/CD access method. With CSMA/CD, a channel can only transmit or receive at one time. A collision results when a frame sent from one end of the network collides with another frame. Timing becomes critical if and when a collision occurs. If a collision occurs during the transmission of a frame, the MAC layer will stop transmitting and retransmit the frame when the transmission medium is clear. If the collision occurs after a packet has been sent, then the packet is lost since the MAC layer has already discarded the frame and started to prepare for the next frame for transmission. In all cases, the rest of the network must wait for the collision to dissipate before any other devices can transmit.

In half-duplex mode, Gigabit Ethernet's performance is degraded. This is because Gigabit Ethernet uses the CSMA/CD protocol which is sensitive to frame length. The standard slot time for Ethernet frames is not long enough to run a 200-meter cable when passing 64-byte frames at gigabit speed. In order to accommodate the timing problems experienced with CSMA/CD when scaling half-duplex Ethernet to gigabit speed, slot time has been extended to guarantee at least a 512-byte slot time using a technique called carrier extension. The frame size is not changed; only the timing is extended.

Half-duplex operation is intended for shared multistation LANs, where two or more end nodes share a single port. Most switches enable users to select half-duplex or full-duplex operation on a port-by-port basis, allowing users to migrate from shared links to point-to-point, full-duplex links when they are ready.

Gigabit Ethernet will eventually operate over a variety of cabling types. Initially, the Gigabit Ethernet specification supports multi-mode and single-mode optical fiber, and short haul copper cabling. Fiber is ideal for connectivity between switches and between a switch and high-speed server because it can be extended to greater length than copper before signal attenuation becomes unacceptable and it is also more reliable than copper. In June 1999, the Gigabit Ethernet standard was extended to incorporate category 5 unshielded twisted-pair (UTP) copper media. The first switches and network NICs using category 5 UTP became available at the end of 1999.

next up previous
Next: 3 Testing Environment Up: 2 VIA and Gigabit Previous: 2.1 VIA Overview

Paul Farrell
Fri Aug 25 14:18:10 EDT 2000