The Internet Protocol Journal - Volume 2, No. 3

Gigabit Ethernet

Gigabit Ethernet
by William Stallings

In late 1995, the IEEE 802.3 committee formed a High-Speed Study Group to investigate means for conveying packets in Ethernet format at speeds in the gigabit-per-second range. A set of 1000-Mbps standards have now been issued.

The strategy for Gigabit Ethernet is the same as that for 100-Mbps Ethernet. While defining a new medium and transmission specification, Gigabit Ethernet retains the carrier sense multiple access collision detect (CSMA/CD) protocol and frame format of its 10 and 100-Mbps predecessors. So it is compatible with the slower Ethernets, providing a smooth migration path. As more organizations move to 100-Mbps Ethernet, putting huge traffic loads on backbone networks, demand for Gigabit Ethernet is intensifying.

Figure 1 shows a typical application of Gigabit Ethernet. A 1-Gbps LAN switch provides backbone connectivity for central servers and high speed workgroup switches. Each workgroup LAN switch supports both 1-Gbps links, to connect to the backbone LAN switch and to support high performance workgroup servers, and 100-Mbps links, to support high performance workstations, servers, and 100-Mbps LAN switches.

Figure 1: Example Gigabit Ethernet Configuration

*Note:Click above for larger view

Protocol Architecture Figure 2 shows the overall protocol architecture for Gigabit Ethernet. The Media Access Control (MAC) layer is an enhanced version of the basic 802.3 MAC algorithm. A separate gigabit medium independent interface (GMII) has been defined and is optional for all the medium options except unshielded twisted-pair (UTP).

The GMII defines independent 8-bit-parallel transmit and receive synchronous data interfaces. It is intended as a chip-to-chip interface that lets system vendors mix MAC and physical sublayer (PHY) components from different manufacturers.

Two signal encoding schemes are defined at the physical layer. The 8B/ 10B scheme is used for optical fiber and shielded copper media, and the pulse amplitude modulation (PAM) 5 is used for UTP.

Figure 2: Gigabit Ethernet Layers

*Note:Click above for larger view

Media Access Layer

The 1000-Mbps specification calls for the same CSMA/CD frame format and MAC protocol as used in the 10 and 100-Mbps versions of IEEE 802.3. For traditional Ethernet hub operation, in which only one station can transmit at a time (half-duplex), the basic CSMA/CD scheme has two enhancements:

  • Carrier extension: Carrier extension appends a set of special symbols to the end of short MAC frames so that the resulting block is at least 4096 bit-times in duration, up from the minimum 512 bit-times imposed at 10 and 100 Mbps. This extension makes the frame length of a transmission longer than the propagation time at 1 Gbps.
  • Frame bursting: This feature allows for multiple short frames to be transmitted consecutively, up to a limit, without relinquishing control for CSMA/CD between frames. Frame bursting avoids the overhead of carrier extension when a single station has a number of small frames ready to send. extension when a single station has numerous small frames ready to send.

With a LAN switch (full-duplex operation), which provides dedicated rather than shared access to the medium, the carrier extension and frame bursting features are not needed. They are unnecessary because data transmission and reception at a station can occur simultaneously without interference and with no contention for a shared medium. All the gigabit products on the market use a switching technique, and so do not implement the carrier extension and frame bursting.

With a switching technique, full-duplex operation is employed, and the CSMA/CD protocol is not needed. The gigabit specification expands on the pause protocol that is defined for 100-Mbps Ethernet by allowing asymmetric flow control. Using the autonegotiation protocol, a device may indicate that it may send pause frames to its link partner but will not respond to pause frames from its partner.

Physical Layer

The current 1-Gbps specification for IEEE 802.3 includes the following physical layer alternatives (Figure 3):

Figure 3: Gigabit Ethernet Media Options(log scale)

*Note:Click above for larger view

  • 1000Base-LX: This long wavelength option supports duplex links of up to 550 m of 62.5-mm or 50-mm multimode fiber or up to 5 km of 10-mm single-mode fiber. 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-mm multimode or up to 550 m using 55 mm multimode fiber. 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 a 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.

Digital Signal Encoding Techniques for Gigabit Ethernet The encoding scheme used for all the Gigabit Ethernet options except twisted-pair is 8B/10B. This scheme is also used in Fibre Channel. With 8B/10B, each 8 bits of data is converted into 10 bits for transmission. The 8B/10B scheme was developed and patented by IBM for use in its 200-megabaud ESCON interconnect system.

  • The developers of this code list the following advantages:
  • It can be implemented with relatively simple and reliable transceivers at low cost.
  • It is well balanced, with minimal deviation from the occurrence of an equal number of 1 and 0 bits across any sequence.
  • It provides good transition density for easier clock recovery.
  • It provides useful error detection capability.
The 8B/10B code is an example of the more general mBnB code, in which m binary source bits are mapped into n binary bits for transmission. Redundancy is built into the code to provide the desired transmission features by making n > m. Figure 4 illustrates the operation of this code. The 8B/10B code actually combines two other codes, a 5B/6B code and a 3B/4B code. The use of these two codes is simply an artifact that simplifies the definition of the mapping and the implementation; the mapping could have been defined directly as an 8B/10B code. In any case, a mapping is defined that maps each of the possible 8-bit source blocks into a 10-bit code block. There is also a function called disparity control. In essence, this function keeps track of the excess of zeros over ones or ones over zeros. An excess in either direction is referred to as a disparity. If there is a disparity, and if the current code block would add to that disparity, then the disparity control block complements the 10-bit code block. This complement has the effect of either eliminating the disparity or at least moving it in the opposite direction of the current disparity.

Figure 4: 8B/10B Encoding Parallel Data Byte Control

*Note:Click above for larger view

The encoding mechanism also includes a control line input, K, which indicates whether the lines A through H are data or control bits. In the latter case, a special nondata 10-bit block is generated. A total of 12 of these nondata blocks are defined as valid in the standard. These blocks are used for synchronization and other control purposes. For 1000Base-T, the encoding scheme used is PAM-5, over four twisted-pair links. Therefore, each link must provide a data rate of 250 Mbps. PAM-5 provides better bandwidth utilization than simple binary signaling by using five different signaling levels. Each signal element can represent two bits of information (using four signaling levels). In addition, a fifth signal level is used in a forward error correction scheme.


A good tutorial on Gigabit Ethernet is [1]. The Gigabit Ethernet Alliance is at

  • Frazier, H., and Johnson, H. "Gigabit Ethernet: From 100 to 1,000 Mbps." IEEE Internet Computing, January/February 1999.
  • Gigabit Ethernet: Technology and Applications for High-Speed LANs by Rich Seifert, ISBN 0-201-18553-9, Addison Wesley, 1998. (Reviewed in The Internet Protocol Journal, Volume 1, Number 2, September 1998.)

WILLIAM STALLINGS is a consultant, lecturer, and author of more than a dozen books on data communications and computer networking. He has a PhD in computer science from M.I.T. His latest book is Data and Computer Communications, Sixth Edition (Prentice Hall, 1999). His home in cyberspace is and he can be reached at