Ethernet

Introduction

Ethernet History

Broadcast Network Operation

Ethernet Protocol

Ethernet at 10Mbps

Ethernet at 100Mbps

Ethernet at 1000Mbps and above

1000 Mbps and Above

With the huge rise in multimedia technologies, and the network-centric workplace, the demand for higher bandwidth has continued to rise. This has been met by a new Ethernet Standard, called Gigabit Ethernet, which runs at 1000MBps, but is still compatible with the standard Ethernet and Fast Ethernet Nodes.

Effect on Network Size

As we have already seen with Fast Ethernet, an increase in speed corresponds to a decrease in network size, due to collision detection, if the minimum frame size is kept the same. The speed increase to 1000MBps would have meant a maximum cable length of 10 metres, which is hardly practical. To deal with this, Gigabit Ethernet uses a bigger slot size of 512 bytes. To maintain compatibility with Ethernet, the minimum frame size is not increased, but the "carrier event" is extended. If the frame is shorter than 512 bytes, then it is padded with extension symbols. These are special symbols, which cannot occur in the payload. This process is called Carrier Extension.

Carrier Extension is a simple solution, but it wastes bandwidth. Up to 448 padding bytes may be sent for small packets. This results in low throughput. In fact, for a large number of small packets, the throughput is only marginally better than Fast Ethernet. Packet Bursting attempts to rectify this. Packet Bursting is "Carrier Extension plus a burst of packets". When a station has a number of packets to transmit, the first packet is padded to the slot time if necessary using carrier extension. Subsequent packets are transmitted back to back, with the minimum Inter-packet gap (IPG) until a burst timer (of 1500 bytes) expires.

Media Types

Gigabit Ethernet has defined four media types with which it operates:
  • 1000Base-LX - This long wavelength option supports duplex links of up to 550 m long length of 62.5-micron or 50-micron multimode fibre, or up to 5 km of 9-micron single-mode fibre. Wavelengths are in the range of 1270 to 1355 mm.
  • 1000Base-SX - This short-wavelength option supports duplex links of up to 275 m using 62.5-micron multimode or up to 550 m using 55 micron multimode fibre. Wavelengths are in the range of 770 to 860 nm.
  • 1000Base-CX - This option supports 1-Gbps links among devices located within a single room or equipment rack, using copper jumpers (specialized shielded twisted-pair cable that spans no more than 25 m). Each link is composed of separate shielded twisted-pair running in each direction.
  • 1000Base-T - This option makes use of four pairs of Category 5 unshielded twisted-pair copper wires to support devices over a range of up to 100 m.

Changes in Ethernet Stack

The Gigabit Ethernet layer stack is shown below. The GMII is an extension of the MII ( Media Independent Interface ) used in Fast Ethernet. It uses the same management interface as MII and can support 10, 100 and 1000 Mbps data rates.

Future of Ethernet

10 Gigabit Ethernet has already been ratified, and is being used for parts of the Internet backbone and for large scientific clusters. 1 Gigabit Ethernet is now a common Network interface upon new PC's, replacing Fast Ethernet. The next standard will be 40 Gigabit Ethernet, which has already been demonstrated by switching platforms.

<-Previous
 

Created by Ross McIlroy for the 2003/2004 NCT4 Ethernet Exercise