by Dinesh Thakur Category: Communication Networks

Gigabit Ethernet or GbE, is the penultimate evolution of Ethernet standard. Several improvements have been made for it to Fast Ethernet at 100 Mbit/s.

The new interface is called modified GMII (Gigabit Media Independent Interface). It includes a data path 8 bits, instead of 4 in the less powerful version. The transceivers operate with a clock operating at 125 MHz.

The adopted encryption products comes from Fibre Channel to reach the gigabit per second. Only one type of repeater is now accepted in this new version.
Different standard solutions are the following:
• 1000BaseCX, two twisted pairs of 150 Ω;
• 1000BaseLX to a pair of optical fiber of high wavelength;
• 1000BaseSX to a pair of optical fiber of shorter wavelength;
• 1000BaseT, four pair Category 5 UTP.
The access technique to the physical medium, the CSMA / CD, is also changed. To be compatible with other versions of Ethernet, which is a basic principle, the size of the transmitted frame should be between 64 and 1500 bytes. The 64 bytes, ie 512 bits correspond to a transmission time of 512 ns. This time of 512 ns is the maximum distance from the support for a broadcast station does not disconnect before receiving a possible collision signal. This represents 100 m for a round-trip. If no hub is installed on the network, the maximum length of the physical medium is 50 m. In fact, with a connecting hub and cable portions to the couplers, the maximum distance is reduced to a few meters. To avoid this too short, standard-setters artificially increased the length of the frame to bring it to 512 bytes. The module adds padding bytes that are then removed by the receiver module.
If there is a good solution to enlarge the Gigabit network, the throughput is very low if all the frames to be transmitted have a length of 64 bytes, one eighth of the bandwidth being used in this case.
The Gigabit Ethernet accept repeaters or hubs when there are several possible directions. In the latter case, an incoming message is copied to all the output lines. Figure shows a Gigabit repeater corresponding to the IEEE 802.3z standard. The different solutions of the Gigabit Ethernet can be interconnected via a repeater or a hub.

  Gigabit Ethernet Hub
Gigabit Ethernet also operates in switched mode, in a configuration full-duplex. It can, by this means, Gigabit Ethernet interconnect them or Fast Ethernet and conventional Ethernet.
Gigabit routers are also available when back up to layer network, for example the IP layer. In this case, it is necessary to recover the IP packet for routing to the Ethernet frame. Figure shows an interconnection of two networks by a switched Gigabit router.
The management of the Gigabit network, such as Ethernet older networks is ensured by conventional techniques, essentially SNMP (Simple Network Management Protocol). The MIB (Management Information Base) Gigabit Ethernet is described in the IEEE 802.3z standard.

  Connecting two Ethernet switched networks a router

MAC Sublayer Functions

MAC sub layer remains almost same in Gigabit Ethernet. There are two distinct approaches for medium access: half-duplex and full duplex.

In full duplex mode

(a) There is no collision and CSMNCD is not used.

(b) Each computer is connected to central switch or other switches as shown in fig.

(c) In this mode, each switch has buffers for each input port in which data are stored until they are transmitted.

(d) The maximum length of the cable is determined by the signal attenuation in the cable.

                   Gigabit Ethernet

In Half duplex mode

(a) Switch is not used rather a hub is used.

(b) All the collisions occur in this hub.

(c) To control this, CSMA/CD approach is used.

(d) The maximum length of the network in this approach is totally dependent on the minimum frame size.

Physical layer function


• In Gigabit Ethernet, two or more stations can be connected.

• Only two stations can be connected in point to point mode as shown in fig.

• Multiple stations (two or more) can be connected in a star topology with a switch or hub.

• The various possible implementations are shown in fig.

                                  Topologies of Gigabit Ethernet

                Topologies of Gigabit

Physical layer implementation

• Two different implementation of Gigabit Ethernet are two wires and a four Wire.

• Two wire implementation uses fiber optic cable. The various two Wire implementations are 1000Base-SX, 1000Base-LX, 1000Base-CX

• Four wire implementation uses category 5 twisted pair cable. It includes 100 Base-T.

               Types of Gigabit Ethernet

1000 Base-SX

(a) It uses multimode optical fiber with 2 wires.

(b) It uses short wave laser.

(c) The maximum length of segment supported by 1000Base-SX is 550 meters.

(d) It uses 8B/10B block encoding and NRZ line encoding as shown in Fig.

                           1000Base SX implementation

1000 Base-LX

• It uses multimode or single mode optical fiber (2 wires).

• It makes use of long wave laser.

• The maximum length of segment supported is 550 meters (in multimode) and 5000 meters (in single mode).

• It also uses 8B/IOB block encoding with NRZ line encoding.

1000 Base-CX

• It uses shielded twisted pair cable (2 wires) that carry electrical signal.

• The maximum length of segment supported by it is~5 meters.

• It also uses 8B/6B blocking encoding and NRZ line encoding.

1000 Base-T

• It uses category 5 UTP (4 wires).

• The maximum segment length supported is 100 meters.

• It makes use of 4D-P AM5 encoding to reduce the bandwidth.

• In 4D-PAM 5 encoding, are four wires are evolved input and output.

• Each wire carries 250 Mbps which is in the range for cat 5 UTP cable.

                             4D-PAM5 encoding/decoding

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Dinesh ThakurDinesh Thakur holds an B.SC (Computer Science), MCSE, MCDBA, CCNA, CCNP, A+, SCJP certifications. Dinesh authors the hugely popular blog. Where he writes how-to guides around Computer fundamental , computer software, Computer programming, and web apps. For any type of query or something that you think is missing, please feel free to Contact us.

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