Crosstalk and Copper Wires

Electrical current flowing through any conductor (like the copper wires connecting his phone) will produce a surrounding electromagnetic field. If another conductor is within the surrounding field, an inductively coupled current will flow through the adjacent conductor.

Inductively Coupled Electromagnetic Flux

In the figure above current flowing through the conducting wire will produce an inductively coupled current in the adjacent wire. If the varying signal current represents a voice transmission the conversation can crossover from one line to another and voices can be heard on one line from another line conversation. Usually this is only an annoyance since crosstalk signal levels are typically low when compared with the signal levels of the conversation on the primary line. On the other hand digital data transmissions are extremely sensitive to crosstalk. Crosstalk can cause bit misinterpretation and will typically require a retransmission of the damaged data.

There are two types of crosstalk, near end and far end.

Near End Crosstalk (NEXT)

Near end crosstalk occurs between a transmitted signal and a received signal. Transmitted signals are typically stronger that a signal that is being received and interfere with the received signals.

Near End Crosstalk

Far End Crosstalk (FEXT)

Far end crosstalk occurs between two signals transmitted in the same direction. The adjacent conductors each produce a magnetic field and can interfer with each other.

Far End Crosstalk

The most common way to reduce crosstalk between adjacent wires is to twist the wires together in a way that cancels the crosstalk flux. That's why Unshielded Twisted Pair (UTP) cabling is used for high speed data cabling like Ethernet. In addition shielding, in the form of foil or metallic braid is also used in Shielded Twisted Pair (STP) cable.

Those Copper Wires Coming Into Your House - The Local Loop

It’s all going to be going away soon but, for most of us, our landline phones are still connected the way they were 80 years ago......

The analog Public Switched Telephone Network (PSTN) or Plain Old Telephone Service (POTS) local loop is defined as the twisted pair of copper wires many of us have coming into our home or business. This local loop is sometimes referred to as the “final three miles” or simply the “final mile”. The local loop has been “tuned” to our voice frequencies over the last 100 years and has a bandwidth of approximately 4000 Hz. This bandwidth includes two guardbands to prevent adjacent frequency interference. As can be seen in the figure below, bandwidth available to the local loop circuit for actual voice analog transmission is about 3000 Hz.

PSTN Bandwidth

The local loop wire pair consists of two wires and runs from a home or business to a Local Exchange Carrier (LEC) Central Office (CO) which is also referred to as the Central Exchange (CE).  The CO provides voltage (– 48V DC) for the telephone in our homes and businesses. The wires that make up a wire pair are identified as follows: The “tip” (red wire) is attached to the negative side of the CO 48 V battery and the “ring” (green wire) is attached to the positive side of the CO 48 V battery.

Local Loop Telephone Circuit

This diagram shows a basic local loop telephone circuit. Notice the CO provides the voltage for the telephone. This voltage is provided by batteries in the CO – we’ve all experienced power failures at one time or another and most realize telephones still work even when the power is out. Also notice the battery polarity is inverted and a –48 V DC is being provided to the phone. This is done for electrolytic corrosion reasons. In my next post we’ll look at the local loop in the form of a transmission line.

How Caller ID Works

Back in May I wrote a couple of posts using a book I published about ten years ago titled Introduction to Telecommunications Networks. As a follow-up to those basic telephony posts, here's something on Caller ID.

Caller ID, also referred to as Caller Identification Technology was introduced in New Jersey by Bell Atlantic in 1987 and became widely available in the United States with the implementation of a switching technology called Common Channel Signaling System 7 (SS7). I'll write about SS7 in a later post, for let's just say SS7 is necessary to provide Custom Local Area Signaling Services (CLASS). CLASS services include Caller ID, Call Return, Repeat Dialing, Priority Ringing, Select Call Forwarding, Call Trace and Call Blocking.

To transmit Caller ID information the SS7 system sends the telephone numbers of the caller and the recipient in the form of a signal to a transfer point, before the call arrives at the receiving end. The caller ID information, also referred to as the Calling Party Number (CPN) field is placed .5 seconds after the first ring between the first and second rings of a telephone call by the SS7 system in the transmitting central office (CO).

Caller ID or Calling Party Number (CPN) field

As the call travels over the caller to receiver route it may pass through network elements which have not yet been upgraded to SS7. If this happens the CPN information will disappear and the receiver will not be able to identify who is calling.

CPN information is delivered in frequency modulated digital data format. Simply put it’s a series of bits, or 1’s and 0’s, that are transmitted sequentially. The bits are organized into groups of 8, with each 8 bit group referred to as a Byte. Each 8 bit combination, or Byte, represents a letter of the alphabet, number or special character. In the CPN field a 1 is represented by a 1200 Hz tone and a 0 is represented by a 2200 Hz tone.

Let’s look at the layout of the CPN field in more detail.

Calling Party Number (CPN) field Detail

The message info fields include the type of information is included in the CPN field and how long (how many Bytes) the CPN field is. The variable length number field can include name and/or address information. Caller ID requires the use of telephone capable of displaying caller ID information or a display box attached to a non caller ID display phone. The receiver must wait for the time between the first and second ring to see caller ID information. If the receiver picks up the phone before the CPN information is received, there will be no caller ID information. This easy accessibility to caller numbers and other information concerns many people and has created a privacy debate.

The Constitutionality of caller ID has been repeatedly challenged in court. People have three major concerns: the right to be left alone, the right to be free from unreasonable searches or seizures and the right to not be subjected to unreasonable government intrusions. Call blocking has originated as a solution to this privacy problem. Call Blocking allows the caller to prevent their number from being displayed on the receiver’s caller ID unit. Caller ID can be blocked two ways. Per-Line Blocking blocks the callers ID from all connected telephones. Per-Call Blocking requires the caller to dial *67 prior to dialing any call for which the caller wishes to be anonymous. When a CPN is blocked, the telephone company replaces the CPN information with “Private Name Private Number” or something similar.

Broadband Growth In U.S. - 1.3 Million New Q3 2008 Customers

Leichtman Research Group, Inc. (LRG) has just published a short report that looks at the twenty largest telco and cable companies in the United States. These twenty companies represent 66.7 million (94% of U.S. market) customers. Here's a breakdown of some of the information in the report:

• Cable companies have 36.5 million broadband subscribers.
• Telephone companies have 30.2 million broadband subscribers.
• The top cable companies added over 870,000 subscribers, representing 67% of the net broadband additions for the quarter versus the top telephone companies.
  
• Overall, broadband additions in 3Q 2008 amounted to 61% of those in 3Q 2007 – with cable having 82% as many additions as a year ago, and Telcos 40.
  
• The top cable broadband providers have a 55% share of the overall market, with a 6.3 million subscriber advantage over the top telephone companies.

Bruce Leichtman, president and principal analyst for Leichtman is quoted in the report:

Over the past two quarters the top cable providers accounted for 71% of the net broadband additions, adding over 900,000 more broadband subscribers than the top telcos. Cable’s recent success compared to the telcos should not necessarily be interpreted as consumers suddenly choosing cable’s speed advantage over that of the telcos' DSL service. It is more a function of the telcos' shift in focus towards higher value subscribers while cable has been consistent in marketing broadband as part of its nearly ubiquitously available Triple Play bundles.

The report includes a very nice chart that breaks down subscriber numbers for all 20 of the companies. You can view the online version and download a PDF here.