illicit connection

Hi all,

I have read the following in a book

traditional Ethernet co-axial cables (10Base 2) can be tapped without cutting through it, but you need special tools and transceiver to make the illicit connection.

can any one give a brief explanation on how this is done ?

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Beware of google-groupers with books.

It certainly sounds like you're planning to do illicit things, no?

Find yourself a length of cable as used for 10Base2 (RG58), and look at how it is put together. Then it should become obvious how one'd go about making electrical contact with the core without disrupting continuity.

In fact, a procedure much like that was developed as the normal attachment method for 10Base5 (on different cable) with the idea that it'd allow for making attachments without disturbing the rest of the network.

So, which book were you reading? This little snipped sounds like the author didn't quite understand 10Base2 and 10Base5.

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I think I still have my 10base5 tap installer tool.

First there is a part that clamps onto the cable with a threaded hole. After clamping it on, one end of the tool drills a hole (by hand, it doesn't take long) through the shield, and almost through to the center conductor. Next, a part with a sharp point is threaded into the hole and tightened by hand. The sharp point goes into the center conductor. Also, the clamp part has two electrodes that cut through the outer layer and contact the shield (outer conductor) of the coax. Using two contacts to the shield allows more accurate sensing of collisions while transmitting.

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-- glen

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glen herrmannsfeldt

Can you please clarify this ? I never had the 'pleasure' of actually installing a vampire tap. But I am told that the cable was layed out in such a way, that on certain spots there was some kind of opening in the shielding. Spots that were marked on the outsite as 'vampire here'.

So, just to be sure: do you actually remove some of the shielding with some kind of drill or do you punch the tap at a spot where there is a pre-made void in the shielding?

Thanks in advance :-)

Reply to
Gerard Bok

Gerard Bok wrote: (snip)

There are marks on the cable, but they are unrelated to any openings in the shield. If too many taps were placed too close together, the effects of the capacitance of the transceiver input could cause problems with the signal.

To avoid that, the cable is marked every 2.5m where one is allowed to place a tap.

The tool is shown in the picture on the web site.

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It has what looks like an ordinary drill bit embedded in the plastic. It is just long enough such that the hole goes almost through to the center conductor. It is sharp enough, and the shield is soft enough, that it cuts and pulls the metal out of the hole. Stray pieces of shield could short the cable, but that never happened to me.

-- glen

Reply to
glen herrmannsfeldt

Slightly different - install the clamp, which has teeth to pierce the jacket, and make reliable contact with the outer shield. One next threads a cutting tool into the hole in the clamp. This tool cuts the circumference of the hole, like the spur of an auger bit wood drill (the old fashioned kind used with a 'hand brace' type of drill), as well as drilling a tiny hole PART WAY through the inner insulation. Remove any debris (strands of the outer shield, and anything of the inner shield), inspect the hole, and thread the transceiver into that hole. This act pokes the hole the rest of the way through the inner insulation to make contact with the center conductor.

Great fun.


-------------------- Mechanical Requirements

The cable used should be suitable for routing in various environments, including but not limited to, dropped ceilings, raised floors, and cable troughs. The jacket must provide insulation between the cable sheath and any building structural metal. Also the cable must be capable of accepting coaxial cable connectors, described in The cable must also conform to the following requirements:

- Center conductor must be 0.0855" +/- .0005" diameter solid copper.

- Core dielectric material must be foamed.

- Inside diameter of the innermost shield must be .242" minimum.

- Outermost shield must be greater than 90% coverage tinned copper braid, with an O.D. of 0.326" +/- .007"

- Jacket O.D. must be 0.365" minimum, 0.415" maximum.

- Cable concentricity must be such that the center of the center conductor is within 0.020" of its ideal concentric position with respect to the jacket.

The cable must also meet applicable flammability criteria and local codes for installed environment. Different (e.g. polyethylene and FEP dielectric_ types of cable sections may be interconnected while meeting the sectioning requirements of 7.6.1

[...] Jacket Marking

The cable jacket must be marked with annular rings in a color contrasting with the background color of the jacket. The rings must be spaced at 2.5 meters +/- 5 cm from a previous ring, regularly along the entire length of the cable. It is permissible for the 2.5 meter spacing to be interrupted at discontinuities between cable sections joined by connectors. (See 7.6.2 for transceiver placement rules which mandate cable markings.


That's from the XNSS-018211 document "The Ethernet" data link layer and physical layer specifications version 2.0 dated November 1982. Contrary to some published lies, the cable is not MIL-C-17, never mind RG-8/U or similar. The main difference is the specification of a solid center conductor, indirect mention of double shielding, and the jacket material.

As Glen mentions, and as required, the marks were to show transceiver placement _spacing_ rather than "here be a good spot".


The problem there would be accurately locating the tap with respect to the pre-made void. How do you control the machine that installs the jacket and/or marks the ring verses the location of the "hole" in the shielding?

Ethernet collision detection works by detecting distortion in the waveform on the wire. One thing that causes this distortion is two or more systems trying to transmit at the same time. ANOTHER thing that causes distortion is reflections, caused by discontinuities in the cable. While the cable was specified as 50 +/-2 Ohms, the terminators are 49.9 Ohm 1% resistors with a phase angle of the impedance not to exceed 5 degrees. Basically, if you understand VSWR, this is about a

1.05:1 which is pretty good.

Yet another thing that causes discontinuities is the connection of the transceiver. The transceiver is supposed to be a minimal influence (shunt capacitance < 2pf, input impedance > 100KOhm) but this mess has to be attached to the coax, and that implies what amounts to be a Tee type of connection. The specification only states that the length of this "tee" must be as short as possible:

Long (greater than 3 cm) connections between the coaxial cable and the input of the transceiver are not acceptable.

There is a further requirement that this stub be less than 2 pf (not including the < 2 pf of the transceiver). The spacing comes into play by minimizing the additions of the effects (spacing equates to phase shift - 2.5 meters _in_ the cable equates to about 0.11 wavelengths of the 10 MHz signal).

Old guy.

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