Gigabit Ethernet Physical and MAC sub-layer

Physical Layer Physical Medium Specification

The Gigabit Ethernet standard 802.3z currently has the following proposed physical layer interface:
1000Base-SX - uses multimode optical fibre for short haul connection.
1000Base-LX - uses multimode or single-mode optical fibre for long haul connection.
1000Base-CX - uses specialised copper jumper for short interconnections within equipment rooms.

Gigabit Ethernet operating at 1.25Gb/s is too fast for LEDs and requires the use of lasers. Traditionally, laser-based data transmission has been used with single-mode fibre. The 1000Base-X standard has introduced laser-based transmission over multimode fibre and this new type of transmission has introduced new types of physical layer issues. 

The Gigabit Ethernet standard 802.3ab currently has the following proposed physical layer interface: 1000Base-T - uses Category 5 UTP and Category 5E UTP.

1000Base-T achieves the full duplex throughput of 1000Mb/s by transporting data over 4 pairs from both ends of each pair simultaneously. Each pair carries a full duplex 250Mb/s data signal encoded as 5-level Pulse Amplitude Modulation (PAM-5).


Physical Layer Data Encoding Scheme

1000Base-SX operates at 850nm wave length, maximum distance is 500 metres for 50/125mm cable and 275 metres for 62.5/125mm cable, both laser-based. 1000Base-LX operates at 1310nm wave length, maximum distance is 550 metres for both 50/125mm and 62.5/125mm cables, 5000 metres for 10/125mm cable, all laser-based.

1000Base-T is implemented with the Enhanced TX/T2 line code. TX/T2 was chosen because this signalling scheme has inherited the symbol rate and spectrum of 100Base-TX and is based on the line code used by 100Base-T2. 1000Base-T may be able to support a maximum distance of 100 metres over Category 5 UTP.

The 1000Base-T signal was made compatible with the 100Base-TX signal (125 Msymbols/s) so as to facilitate the development of a dual data rate 100/1000Base-T transceiver. One advantage of having equal symbol rates is that common clocking circuitary can be used for both data rates. Another advantage is that the spectra of both signals are similar with a null at 125MHz3, allowing for the use of common magnetics and emissions control circuitry.


MAC sub-layer

Allows half and full-duplex operation at speeds of 1000 Mbps. When operating in half-duplex mode, Gigabit Ethernet adopts the same fundamental CSMA/CD access method to resolve contention for the shared media. The Gigabit Ethernet CSMA/CD method was enhanced in order to maintain a 200-metre collision diameter at gigabit speeds. Without this enhancement, minimum-sized Ethernet frames could complete transmission before the transmitting station senses a collision.

To resolve this issue, both the minimum CSMA/CD carrier time and the Ethernet slot time have been extended from 64 bytes to 512 bytes. The minimum frame length of 64 bytes has not been affected but frames smaller than 512 bytes have been augmented with a new carrier extension field following the CRC field. Frames longer than 512 bytes are not extended. 

The small lost in performance for small frames is compensation by the introduction of packet bursting into the CSMA/CD algorithm. Packet bursting allows stations to send bursts of small packets in order to fully utilise available bandwidth. Devices that operate in full-duplex mode are not subject to the carrier extension, slot time extension or packet bursting changes.



Gigabit Ethernet Alliance
Gigabit Ethernet Whitepaper
Gigabit Ethernet References
Gigabit Ethernet: The Difference Is In the Details











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