Importance of Gigabit Over Copper
When a company chooses its cabling infrastructure, it is making a long-term investment that can last as long as ten to fifteen years. On average, almost half of the cabling infrastructure is in place for more than five years, according to Sage Research. Most of the cabling already installed in buildings today is Category 5.
Not surprisingly, organizations have been reluctant to tear out their existing Category 5 cabling and lose this investment in order to deploy high-speed networking. Prior to approval of 1000BASE-T, deployment of Gigabit Ethernet tended to be limited to areas where fiber cabling was required or desired. With the adoption of 1000BASE-T, however, widespread deployment of Gigabit is possible over the existing copper infrastructure.
The Cost-effective Choice
1000BASE-T is ideal when budgets are tight, because it increases performance by building on a company's current investment. Technicians are familiar with the technology, so re-training is not needed. Costly protocol, hardware and cabling changes can be avoided, and any disruption to the network is usually minimal. What's more, Gigabit over copper is the most economical cabling choice in terms of cost per Mbps, even more economical than Gigabit over fiber.
Another advantage is that Gigabit Ethernet employs all the same specifications defined by the original Ethernet standard, including the same Ethernet frame format and size. This is not true of other high-speed network technologies available. For example, ATM has a fixed-length data cell, while Ethernet frame lengths can vary greatly (64 to 1514 bytes) depending on the size of the protocol header, data and pad. As a result, connecting ATM to existing Ethernet or Fast Ethernet segments would require translating each ATM cell to an Ethernet frame.
Since it uses the same specs, 1000BASE-T is fully compatible with the large installed base of Ethernet and Fast Ethernet nodes, as well as V.90 modems. This backward compatibility makes it easy to scale. A variety of 10/100/1000 connectivity devices allow managers to turn up network speed to selected workgroups or segments now, while pre-enabling others for future migration to Gigabit. The migration path is consistent with the familiar path from 10 Mbps to 100 Mbps. A path from Gigabit to 10 Gigabit Ethernet is expected to be available soon.
Quality of Service
Gigabit Ethernet is also a good choice because it supports Quality of Service (QoS), which will be increasingly important for avoiding latency problems as voice, video and data share the network wire. Like Fast Ethernet, Gigabit Ethernet supports existing traffic management techniques that deliver Quality of Service over Ethernet, such as IEEE 802.1p traffic prioritization and Resource Reservation Protocol (RSVP).
RSVP is gaining industry acceptance as a preferred way to request and provide quality of network connections. 802.1p provides a means for priority "tagging" of Ethernet packets in a way that can be communicated to network devices. In intelligent L3/L4 switches, a network administrator can profile traffic by specific IP type (such as HTTP or FTP) and guarantee predictable delivery of this traffic.
Important Specifications
Cable Configuration
The 1000BASE-T physical layer standard provides 1 Gbps Ethernet signal transmission over four pairs of Category 5 UTP cable. It transmits at 125 Mbaud, the same symbol rate as Fast Ethernet. But by using more sophisticated five-level (PAM-5) coding along with four wire pairs, it is able to transmit much more data. To simplify, each wire pair sends and transmits simultaneously, for 250 Mbps per pair (125 Mbaud x 2 bits). Multiplying 250 Mbps by four pairs yields the nominal rate of 1000 Mbps.
Distances
The standard covers cabling distances of up to 100 meters, or networks with a diameter of 200 meters. The 100-meter cabling distance is the safe limit for reliable transmission, and the diameter assumes 100 meters in two directions from a switch.
Half-duplex and CSMA/CD
1000BASE-T supports full duplex and/or half-duplex operation. However, few if any products offered to date support half-duplex - presumably there s little demand, since the principal benefit of Gigabit Ethernet is to maximize performance. Full duplex offers virtually twice the bandwidth.
When running in half-duplex mode, Gigabit Ethernet uses the CSMA/CD protocol. Half-duplex collision domains should be the same as for 100BASE-TX, although each domain can support only one half-duplex repeater. In order to make CSMA/CD work for Gigabit Ethernet, it was necessary to alter the normal timing within a collision domain. At Gigabit speeds, smaller sized data packets could literally reach their destinations before the sending station could detect a collision signal. To overcome this problem, minimum-sized packets are padded with an extension field.
Full Duplex and Flow Control
In full duplex, Gigabit Ethernet relies on the same flow control method as full duplex Fast Ethernet. Nodes on a full duplex channel simultaneously send and receive packets. This makes CSMA/CD collision detection impractical. Instead, flow control methodology is used to avoid congestion and overloading. In simplified terms, each node periodically sends out a packet to indicate that it is transmitting and needs the other node to be in receive mode.
Reliability
Third-party tests have shown that nearly complete efficiency can be achieved in transmitting data with Gigabit Ethernet, and the bit error rate (less than one in ten billion) is the same as for Fast Ethernet. 1000BASE-T makes use of DSP signal equalization techniques to manage problems of noise, echo and other interferences, and to ensure the low bit error rate. It is a highly reliable technology, which can be deployed with confidence in mission-critical networks.
