Verizon T1 Timing
Summary - covers confusion typical with pt to pt circuits
Timing in Verizon is a function of the terminating equipment. If the
circuit is point-to-point where the customer terminates both ends,
the customer provides timing with the "one side internal one side
line" setting. DS1 (T1) is synchronous. The best way to have
synchronous service is to have only one timing reference, or
*perfectly* identical multiple timing sources. Nothing is perfect, so we want
to go with one timing reference.
When Verizon terminates one end of the circuit, like in ISDN PRI,
Frame Relay, ATM and etc. or channelized T1 service then the CPE
should be set for line time.
On Verizon designed point-to-point circuits there are only two
terminations, one at each end. The "middle" network elements are
always designed to be transparent.
So if only one of the terminating CSU's should provide the timing
reference, what does the other one do?
The line setting tells the CSU to check the 1st, 8th, 16th, and 24th
frame bits (on ESF framed circuits) and calculate or derive the
reference. It then makes adjustment to the transmit data returning
to the far end. There is no "clock pulse" per se on a payload DS1.
It really doesn't matter how many CO's your circuit is transported
across, the settings are the same because this configuration deals
with the "payload" level. We always clock the "transport" level.
An example would be your point-to-point is DS1 rate, you clock the
DS1 we clock the transport HDSL and DS3 or above rates (like thru
SONET).
Full T1 Timing Brief
The FCC-specified framed DS1 rate is 1544000 bps (1.544 Mbs) +/- 50 bps.
The payload or usable data rate is 1536000 bps (1.536 Mbs).
DS1 service is full duplex and synchronous. This means that transmit and
receive speeds are the same and that all DS1 network elements operate at
the same relative speed.
A timing or clock source actually generates the DS1 rate and this source
is a function of Data Circuit Terminating Equipment (DCTE or DCE) in the
Verizon network.
A more common name for customer-owned DCE is CSU/DSU or WIC.
On DS1 circuits where the customer terminates both ends of the circuit,
called a point-to-point configuration, the customer CSU is responsible for
providing the timing source. This source is built-in by the equipment
manufacturer and required by the FCC.
Commonly the hub or otherwise hi-end CSU is set for INTERNAL source and the
remote or otherwise lo-end CSU is set for LINE or LOOP source.
If both CSU's are comparable, then the CSU most convenient physically or
one with remote access is usually set to INTERNAL source and the least
convenient CSU is set for LINE or LOOP source.
If both CSU's are set for INTERNAL source, then errors will be noted sooner
rather than later in most circumstances, sometimes immediately.
If both CSU's are set for LINE or LOOP source, then the circuit may work
properly for years until some internal event (heat, cold, static, power
surge, etc) damages the CSU timing chip or external event (Verizon network
failure) causes the circuit to become unstable. The circuit may not
synchronize and become stable until the CSU source configurations are
corrected.
NOTE: Sometimes the INTERNAL source setting will become corrupted and the
CSU will behave as if set for LINE or LOOP. If the circuit is showing slips
or acting erratically it is always a good idea to refresh or reset the
INTERNAL source by setting it temporarily to line, loop, or external and
then BACK to INTERNAL.
On DS1 circuits where Verizon terminates one end of the circuit like ISDN,
Fast Packet, or channelized service originating in a Verizon switch, D4
channel bank, or 0-1 digital cross-connect, the CSU terminating the end
user side of the circuit should be set for LINE or LOOP source.
Where multiple DS1's terminate in a facility, the timing source
configuation may be more complicated.
If multiple DS1s terminate in a single device it is common for one DS1 to
be set as a PRIMARY source and the others as ALTERNATE sources. When a
circuit is designated as a PRIMARY source it is said to be a FACILITY
source.
Rules for FACILITY sources:
Rules for FACILITY sources:
1. Dont use a payload carrying DS1 as a source.
2. Dont use a DS1 transported via SONET as a source.
3. Source Verizon terminated circuits to a Verizon source.
4. Everybody ignores rules 1, 2, and 3 so experiment.
5. If experimentation fails to provide desired results, seek professional
help.
(Verizon can provide a non-payload DS1 for use as a facility source, please
refer to a sales engineer for more information)
Sometimes customers will be certain that Verizon provides timing on
point-to-point circuits. Verizon does, at the TRANSPORT level. Verizon does
not provide timing at the PAYLOAD level on point-to-point circuits.
An example of this payload/transport level issue on a DS1 point-to-point
circuit is the DS1 is payload or user data level and the HDSL, DS3, or STS1
etc. through the Verizon network is the transport level.
More about LINE and LOOP sourcing..
The CSU device set for LINE or LOOP source examines multiple frame bits in
the received signal and calculates the rate. It then takes this calculated
rate and compares it to the current transmit rate. If the result is outside
a pre-determined limit, a correction is calculated and applied.
This process takes time and can introduce error in some instances. On a
discrete or single DS1 this small error is usually not noticeable.
At installations with multiple DS1s where the customer has specified
PRIMARY and ALTERNATE facility sources, remember that the PRIMARY source is
actually LINE or LOOP sourced and that the rest of the DS1s probably are
EXTERNALLY sourced to the PRIMARY source.
Errors introduced by the LINE or LOOP timing process WILL BE amplified and
distributed by the PRIMARY FACILITY source to any circuits or other devices
using it as a timing reference.
A small error in the PRIMARY source or timing distribution system can cause
significant problems!
For example in an installation with 8 DS1s, and DS1 0 as primary source and
DS1 1 thru 7 as alternates a single small slip at the Primary DS1 0 may be
cascaded through the rest of the circuits with increasingly severe effects.
DS1 1 may show 10 slips, DS1 2 may show 20 slips, DS1 3 may show 50 slips,
and so on.
NOTE: DCE may not use these specific sourcing terms. INTERNAL refers to a
source internal to the DCE device. LINE or LOOP refers to the received DS1
signal. DCE may define EXTERNAL source as LINE or LOOP, or FACILITY.