The Basics

Serial Lines - this section deals with serial transmission lines.  They are also called "Point-to-Point", or "Private Lines".  They are also called DTS (Dedicated Transmission Service).  By "serial", we mean the data travels in single path, bit-by-bit.  The converse would be "parallel", where mutiple lines are used to transmit groups of bits at a time.  The Printer cable uses parallel transmission. 

Access Lines vs Private Lines - these are often thought to be completely different.  Access lines go from the customer originating site to a Wide Area Network POP.  Point-to-Point, or Private lines go from the originating customer site, across the provider's backbone, and terminate to another customer site.  They are different, but only as far as the terminating end goes.  Access lines terminate to a provider's POP.  Other than that they are identical.  Therefore, a T1 access line is fundamentally the same creature as a T1 private line.  For discussions within this section, think of access lines and private lines as the same - they both use a T1 or T3 for serial transmission of data from point A to point B.

Originating vs Terminating  -  since all dedicated transmission lines are bi-directional, this is a bit of a misnomer.  Data does not travel specifically from the orig to the term end  -  it travels both ways.  The provider will assign one end as the "Originating" and the other end as "terminating" simply as a means of recording which end is which in the various databases, such as FMS.  There, they can show "Drop1" as the orig and "Drop2" as the term end.

Plesiosynchronous  -  T1's are often referred to as "synchronous".  However, even though they are very close to synchronous, since the various T1 trunks run off of different clocks, they are not exactly in synch.  For example, a T1 in Wash DC may be clocked to the DC POP clock, while another T1 in San Fransisco may be clocked to the San Fran clock.  They may connect up in Denver, and one of them may be running slightly faster than the other.  This "almost synchronous" condition is known as "plesiosynchronous".  Where two T1 systems meet up (usually they have been multiplexed into T3's or higher by then), is called the "mid-span meet", and this is where the differing clocks must be addressed.  The method used it to insert occasional dummy bits, known as "stuff bits" to counteract the drift between the two T1 systems.  A marked byte of data must be used to flag the bit as a stuff bit, so that it is not taken as customer data.  The mark would tell the next system that the next byte will contain 9 bits instead of 8, which serves to synchronize the signals.  If the two signals are just "off" by one bit, then only one stuff bit is needed.  If one system is running faster that the other, then periodic stuff bits must be inserted at specific intervals to keep the two systems in synch.

Why V.35 ??  -  it is an old interface standard, and it typically comes with a huge rectangular plug called a "Winchester" - yet, it is so embedded in the telecom systems, that it is here to stay.   There is nothing wrong with the V.35 standard, which is only an electrical (not physical) standard  -  it is a serial data transmission standard, similar to the RS-232 modem standard, with send and receive DTE and DCE lines.  The T1 CSU/DSU's typically have the Winchester female on the back, which is often mistakenly called a "V.35 jack".  Most routers have designed other plugs instead of the awkward Winchester - but they use an adapter cable in order to connect to the CSU/DSU Winchester port.