clements@BBNCCQ.ARPA (Bob Clements) (02/13/86)
I would appreciate knowing what, if anything, is the difference between a modular jack and a network interface, other than what's written on it. I seem to recall seeing ads for test gear that mentioned the "electrical signature" of a network interface. Is that right, or is my memory bad? I'm not talking about the lightning arrestor at the building entrance. Just the thing that looks like a RJ41 socket but says "Network Interface" and has legalese written on it. Is there anything in it besides the socket? Thanks, /Rcc
rdsnyder@MIT-CCC (02/23/86)
The network interface itself is a small (~1.5cm*~3cm) pc board with 3 components on it. (I haven't seen one for about 6 months, so I hope I remember everything correctly.) The components are wired in series. They consist of a capacitor (.1uF < value < 1uF), a resistor, and a device packaged in a WE case of the sort used for the 100A varistor found across a U1 receiver in a phone handset. This device was black and had a red band painted around the middle (sic), so it must be symmetrical. I would guess it is a varistor rated somewhere in the tens of volts. I would guess that an AC voltage higher than the varistor voltage is applied to the subscriber line and the impedance is measured. This test signal sounds a lot like a ringing voltage, but it might be of a higher frequency to prevent ringing the subscriber's ringer. The first network interface I saw was just the plain, unenclosed pc board connected across the protector block (lightning arrester), but I believe it is more popular to use the type where the pc board is installed inside a WE type 625 modular jack with the "Network Interface"+legalese label on it. Since the network interface is installed on the telco side of the "point of demarcation," I would suspect that its purpose is to allow the telco to test the continuity of the subscriber's cable pair upon receiving a trouble report from the subscriber so that they would not have to send a repairperson to the customer premises. Whether they would actually do this and lose the exorbitant fee of a premises visit is another question. I don't think that a network interface or a lack thereof is that critical at this time. Anyone who has heard the words "network interface" knows how to test his line at the protector and if you report trouble, the telco will probably send a repairperson anyhow. -Ross Snyder
JSOL@BUCS20.BUNET (Jon Solomon) (02/25/86)
A Network Interface is a box which has a jack on it just like a regular wall jack, but it also has a peice of electronics (Don't ask me what it does, I don't know offhand, but I heard a while ago that it simulated a telephone on the line for test purposes) within, which accounts for why it looks larger than a regular phone jack. Network Interfaces aren't meant to be used as jacks (although I use mine for that purpose), you are expected to plug a wire adaptor into them and wire your house from that, using standard jacks where you would like them. Hope that helps. --JSol