by Joe Chang
A useful device in Ethernet networking is the cross-over cable. There are circumstances where it is necessary to network two computers together and nothing else. Examples include test environments and clustered systems. In such cases, it is desired to avoid the need for a dedicated network switch. On Ethernet and Fast Ethernet networks, this is accomplished with a cross-over cable. A patch cable, wired straight through, connects an adapter to a switch or hub port. The cross-over cable is wired with transmit and receive pairs crossed and allows two Ethernet or Fast Ethernet adapters to be connected directly. Gigabit Ethernet uses a patch cable to connect one adapter to either a switch port or directly to another adapter. This fact appears to be overlooked in much of the publicly available material.
Ethernet patch cable and cross-over cable usage.
Gigabit Ethernet is on the verge of becoming the economically viable de facto choice for networking servers and desktop computer systems. Gigabit Ethernet Adapters are now only somewhat more expensive than Fast Ethernet Adapters. A desktop Gigabit adapter for a 32-bit/33MHz PCI bus is less than $100. A server Gigabit adapter for a 64-bit/66MHz PCI-X bus is less than $200. Gigabit Ethernet Switches, on the other hand, are still moderately, although not excessively, expensive.
Consider the history of 100Mbit/sec Fast Ethernet. The first generation adapters were more expensive than 10Mbit/sec Ethernet. The second generation Fast Ethernet adapters were about the same cost as standard Ethernet adapters. Since a Fast Ethernet adapter could operate at either 10 or 100Mbit/sec, there was no economical reason not to deploy Fast Ethernet adapters regardless of whether the network infrastructure was 10 or 100Mbit/sec. Fast Ethernet switches on the other hand, took a few years longer to come down in price to the point where there was very little difference between a Fast Ethernet switch and a standard Ethernet switch.
Today, Gigabit Ethernet is at the intermediate stage where adapters are inexpensive relative their Fast Ethernet counterpart, but switches are not. It is expected that new single chip silicon devices will enable inexpensive Gigabit Ethernet switches in the near future. In the mean time, it can be desirable to directly connect two computer systems with Gigabit Ethernet adapters without a switch.
The IEEE 802.3ab 1000Base-T standard is designed to work with most Category 5 UTP cables. Obviously, the ability to work with Cat 5 cables is of enormous value because it is deployed so pervasively. Cat 5 specifies a cable that supports a 100MHz signal with certain attenuation and impedance characteristics (100§Ù) and other requirements.
The fact that signaling occurs at 100Mbaud does not necessarily imply that the data transmission rate is also 100Mbit/sec (Mbps). First, some form of encoding is necessary to ensure that long streams of zeros or ones do not cause the clocks on each device to fall out of synchronization. Ethernet uses Manchester encoding, which requires one extra bit for each bit of data, actually signals at 20Mbaud with a data transmission rate of 10Mbps. The 100Base-TX version of Fast Ethernet uses 4B/5B encoding and operates at 125Mbaud with a data transmission rate of 100Mbps.
Depending on the signal-to-noise ratio; advanced signaling techniques employing multiple voltage levels can achieve higher transmission rates than the signaling rate. The 1000Base-T implementation of Gigabit Ethernet employs a 5-level coding scheme that effectively contains 2-bits, excluding the encoding overhead, and 4 parallel signals to enable 1Gbit/sec data transmission rates on the same 125Mbaud signaling as 100Base-TX.
Despite the considerable effort made to enable 1000Base-T to operate on cables meeting the Cat 5 specifications, some additional specifications were required (Cat 5e). Fortunately, most Cat 5 cables manufactured actually meets the additional Cat5e specifications. So Gigabit Ethernet should work with most Cat 5 cables, but if new cables are being purchased, one should check that the cables meet the Cat 5e specification. Cables that exceed the Cat 5e specification rated for 350MHz may be only moderately more expensive than Cat 5, while Cat 6 cables that support 500MHz are more expensive.
Given that Gigabit Ethernet should be able to use Cat 5e, 350MHz and Cat 6 cables, one might ask whether a standard Ethernet or Fast Ethernet cross-over cable can be employed to connect two Gigabit Ethernet adapters directly. A quick look at the Cat 5e and 350MHz cross-over cable products available shows that only 10 and 100Mbps Ethernet modes are supported. So the question remains, what is necessary to connect two Gigabit Ethernet adapters directly? It turns out that the answer is standard Cat 5e or better patch cables!
The figure below shows the EIA/TIA 568B wiring standard (the EIA/TIA 568A standard have the wires with green and orange colors swapped). A Cat 5 cable has 8 wires in four pairs. Ethernet (10Base-T) and Fast Ethernet (100Base-TX) use only two pairs of the four pairs in a Cat 5 cable. One pair is used for transmit and a second pair is used for receive. Adapters use one specific pair to transmit, another pair to receive, while switch/hub ports use the opposite pairs arrangement.
TIA/EIA 568B wiring standard.
Ethernet (10Base-T) and Fast Ethernet (100Base-TX) on Cat 5.
Hence, an Ethernet and Fast Ethernet cross-over cable is implemented by crossing pins 1 to 3, 2 to 6, 3 to 1, and 6 to 2 as shown below.
Ethernet and Fast Ethernet cross-over cable.
The 1000Base-T Gigabit Ethernet standard uses all four pairs of the Cat 5e cable. Each pair is used to transmit and receive data simultaneously, known as Dual-Duplex transmission. Basically, this is a technique where it is possible to distinguish the direction a signal is traveling.
The figure below shows the difference between 100Base-TX and 1000Base-T signaling. The 10Base-T and 100Base-TX uses separate pairs for transmit and receive. Hence it is necessary to wire for a specific type of connection. The 1000Base-T transmits and receives simultaneously on each pair. Since there is no difference in the wires used to transmit and receive signals for any device, the standard patch cable, wired straight though, can connect a Gigabit Ethernet adapter to either a switch/hub port or another adapter.
Note that it still should be possible to use a Fast Ethernet cross-over cable to connect two Gigabit adapters if the adapter auto detects the best supported operating mode. But both the adapters should drop to 100Mbps.
Published with the express written permission of the author. Copyright 2002 Joe Chang. All Rights Reserved.