2-5 Gigabit Ethernet
COLIN MICK
Although most organizations are just beginning to adopt Fast Ethernet, a newer, faster Ethernet technology is already entering the marketplace and some users are already preparing to use it. Gigabit Ethernet may seem overkill to those who are still running with shared or switched 10M-bps networks, but more than 100 network component and systems vendors are betting that there is already a market for an Ethernet technology that offers a full two orders of magnitude improvement in data rate.
The initial market for Gigabit Ethernet will be primarily to upgrade network infrastructures by providing high bandwidth links for backbones and server connections. A second market consists of vertical markets with specialized applications that require high bandwidth to the end user station.
Gigabit Ethernet is a logical backbone technology to connect existing networks built with both Fast Ethernet and 10BASE-T. Modern network design practices, with their need for anywhere-to-anywhere connectivity, require high-bandwidth backbone support to ensure that users can reach servers anywhere in the organization. Today most networks use 100M-bps backbones based on ATM, Fast Ethernet, or FDDI technology. As requirements for bandwidth to end users increase, backbone capacity must be scaled up. Compared with other options, Gigabit Ethernet offers the simplest backbone upgrade for Ethernet-based networks; there is no translation, no fragmentation, and no frame encapsulation. New standards under development will augment Gigabit Ethernet backbones with Class of Service (COS) support.
Centralization of servers into server farms and the provision of anywhere-to-anywhere service both increase the demands on server performance. Gigabit Ethernet offers a simple upgrade to improve server performance. New PCI-bus servers are capable of delivering data bursts at more that 1G bps and continuous data well in excess of 100M bps. Today, most servers and server farms are connected to the network via Fast Ethernet links. Gigabit Ethernet offers a low-cost upgrade that can potentially double the throughput of each server; all this takes at the server end is installation of a Gigabit Ethernet NIC and driver. (You also will have to add a Gigabit Ethernet repeater or switch to connect your servers to your existing network.)
Graphics, image, multimedia, and video-based applications offer a ready market for Gigabit Ethernet now. Exhibit 2-5-1 shows the bandwidth requirements of a variety of applications.
Exhibit 2-5-1. Application Bandwidth Table
TECHNICAL OVERVIEW
Gigabit Ethernet extends the ISO/IEC 8802-3 Ethernet family of networking technologies beyond 100M bps to 1000M bps. The bit rate is faster and bit times are proportionately shorter, reflecting the 10× bit rate increase. In full-duplex operation, packet transmission time has been decreased by a factor of 10; in half-duplex operation the improvement is smaller because of changes made to ensure operation over a reasonable collision domain diameter. Cable delay budgets are similar to those seen in 100BASE-T and the achievable topologies for half-duplex operation are similar to those available for half-duplex 100BASE-T.
HOW IT WORKS
Gigabit Ethernet combines the tested and true Ethernet Media Access Control (MAC) and two different physical layer signaling technologies: 1000BASE-X and 1000BASE-T (see Exhibit 2-5-2.) It also takes advantages of full-duplex operation and flow control, new capabilities added to the Ethernet MAC by the recently completed 802.3x Media Access Control sublayer, partitioning, and relationship to the ISO Open Systems Interconnection (OSI) reference model.
Exhibit 2-5-2. Media Access Control Sublayer Partitioning and Relationship to the ISO Open Systems Interconnection (OSI) Reference Model
1000BASE-X Signaling Systems
The 1000BASE-X family couples the Ethernet MAC with hardware originally developed for Fibre Channel (ANSI X3.230). This approach follows the one used for developing 100BASE-X, where the Ethernet MAC was coupled with ANSI X3T12, encoding physical medium dependent specifications designed to carry FDDI over copper and fiber links.
Adoption of the Fibre Channel
The physical layer required modification of the interface to the Ethernet MAC, the use of 8B10B coding, and a 17% increase (from 1.0625M baud to 1.25 M baud) in the speed of PHY operation to accommodate the 1000M bps data rate.
The MAC interface change was accomplished by adding a Gigabit Media Independent Interface (GMII) between the MAC and PHY layers. Similar in operation to the Media Independent Interface (MII) found in 100BASE-T, the GMII is capable of supporting operation at 10Mbps, 100M bps, and 1000M bps through transmit and receive paths up to eight bits wide. A reconciliation sublayer maps signals between the MAC and the GMII.
The 1000BASE-X family consists of three physical layer signaling systems: short wavelength (1000BASE-SX) fiber, long wavelength (1000BASE-LX) fiber, and a short-haul copper system using twinaxial cable (1000BASE-CX). The signaling systems incorporate the Physical Coding Sublayer (PCS) and Physical Medium Attachment (PMA) sublayer.
1000BASE-SX specifies operation over a pair of optical fibers using short-wavelength (770-860 nm) transmission and either 62.5 micron or 50 micron multi-mode fiber. 1000BASE-LX specifies operation over a pair of optical fibers using long-wavelength (1270-1355 nm) transmission and either 62.5 micron or 50 micron multi-mode fiber or 10 micron single-mode fiber.
1000BASE-CX specifies operation over two pairs of 150-ohm shielded, balanced tw-style cabling. This system would typically be used for jumper cables to connect devices in an equipment rack or machine room. Cables use either shielded DB-9 subminiature connectors as specified by IEC 807-3 or the 8-pin shielded ANSI Fibre Channel connector (HSSDC) with the mechanical mating interface defined by IEC 61076-103.
1000BASE-T will use a DSP-based signaling system that supports operation over 100 meters of 4-pair Category 5 UTP cabling as specified in EIA/TIA 568-A and ISO/IEC 11801\emdash the same Category 5 links used for 100BASE-TX operation. This portion of the standard is being developed by a separate Task Force (802.3ab) and is planned for completion at the end of 1998\emdash some time after the completion of 802.3z.
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