The standard LAN twist cable, as shown in the picture to the left, satisfies most computer-to-computer connections but there are occasions when this type of interconnection is known to cause network failures.
This is due to the cable not being terminated correctly by either or both LAN adapter interfaces resulting in the cable "ringing". Other known causes are high drive levels of some LAN adapters which then overload the receive circuitry of the adapter it is connected to.
The circuit shown below combats both these issues. It is nothing more than two 6dB (approx) attenuators (one for each leg or path). Attenuators exhibit two fundamental and valuable properties. They offer the cable a reasonable termination point as well as attenuating any reflections that may be present on the line.
If the problem is merely one of overloaded receive circuitry placing a 6dB attenuator in circuit reduces the voltage by 2 (power by 4). As the LAN specification calls for a transmit level of 2Vp-p and a receive sensitivity of greater than 350mVp-p the attenuator will not affect normal drive levels (6dB resulting in 1Vp-p in normal conditions). The attenuator now allows for a maximum of 4Vp-p before any overloading would occur.
If ringing or reflections are the problem, usually caused by a bad termination at the receive end, the attenuator, being resistors, damps any oscillations appearing on the cable. Any reflections caused at the receive end are reduced by 6db before it reaches the transmitter. When reflected back they are again reduced by further 6dB thus our 6db attenuator offers a total 12dB attenuation on any reflections.
The using of this circuit has resulted in the stabilising of a number of problematic LAN connections and implementation is recommended after all normal possible faults have been eliminated (bad connectors etc).
NOTE: Resistors are standard carbon film 1/8 watt or above and values are in ohms. Attenuators of other values or impedance can be calculated with the aid of the attenuator tables found at The diagram to the right shows the numbering of a RJ45 connector (from the left with cable entry to the bottom, pins to the top and facing you).
TIA/EIA 568A ............................ TIA/EIA 568B
1 - TX_D1+
2 - TX_D1-
3 - RX_D2+
4 - BI_D3+
5 - BI_D3-
6 - RX_D2-
7 - BI_D4+
8 - BI_D4-
Notes:
LAN cables are generically called UTP (Unshielded Twisted Pair) and are identified with a category rating. When installing new cable, unless there is a very good reason not to, you should be using category 5, 5e or 6 UTP which is rated for both 10 and 100mb LAN operation.Info on Shielded Twisted Pair (STP) cabling.
UTP comes in two forms SOLID or STRANDED. SOLID refers to the fact that each internal conductor is made up of a single (solid!) wire, STRANDED means that each conductor is made up of multiple smaller wires. Stranded cable (which is typically more expensive) has a smaller 'bend- radius' (you can squeeze the cable round tighter corners with lower loss) and due to its flexibility should be used where you plug and unplug the cable frequently. All other things being equal the performance of both types of cable is the same. In general, solid cable is used for backbone wiring and stranded for PC to wall plug (patch) cables. Beware: Each type of wire, solid or stranded, needs its own connector type.
There is NO excuse with all the choice of color cable and other techniques available to-day for not being able to visually spot the difference between at least a straight and a crossed cable before you spend 1 hour fitting the wrong cable into your network. For cheap-skates (which includes us) you can get heat-shrink colour tubing in a slew of colours which you fit on each end of the cable beside the connector to indicate the wiring type and standard instead of using different coloured cables. The advantage of this scheme is that when you change your wiring standard you can just change the sleeve colour - you don't have to rip out the cable.
Disadvantage: You have to remember to put the tubing on BEFORE the connector!
You CAN use 100base-TX wiring with a 10base-T network (but not always the other way round). In general ALWAYS use 100baseTX/T4 wiring standards.
If you are using category 5, 5e or 6 wiring EVERYWHERE you can use the 100base-TX standard (this only uses 2 pairs , 4 conductors). Most of the information below assumes you are using category 5, 5e or 6 cables.
If you are using category 3 or 4 cables with 100M LANs ANYWHERE you MUST use the 100Base-T4 standard and this has ADDITIONAL RESTRICTIONS documented throughout (it uses all 4 pairs, 8 conductors). LAN connections/pinouts are defined by IEEE 802.3u.
Maximum LAN cable runs are 100 meters (~300ft).
We provide a Cabling FAQ. which provides additional information or background.
We have added an article on mixing 100 MB LAN and Telephony. on a single category 5(e) or 5 cable. It can be done but you must be very cautious.
We have updated some of the material for 1000base-T (Gigabit Ethernet 802.3ab) which uses all 4 pairs (8 conductors) and added notes where relevant about Power-over-Ethernet (PoE 802.3af). A copper standard for 10GB Ethernet is being worked on (802.3an) but as of February 2005 the IEEE was still standardizing away.
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