T1 - what's that got to do with me, dude? Hey, I'm into video - got no time to
read stuff about T1's
Yeah - take a hike, Jack. Me, I'm into IVR - get to play with Java & Visual Basic.
Right on. I'm in charge of the Fax Server. Who needs a T1?? Get a life, dude.
This dude is a real clown, big time. We're putting in the latest and greatest Modem Pool and
my boss said get all the trunks you need. Spare no expense....
Get this dude outta here. I'm never goin' near T1's. We're putting in DS3's - fiber all-the-way, dude.
Whoa, am I reading your minds here or what?
Well, I've got what might be bad news for you but good news for phone guys
The good old T1 is alive and well.
Creative folks - that's why.
Heck, there are so many applications using T1 technology today that it leaves me
wondering where it is all going to end. It seems every time I turn around, someone
somewhere has dreamed up a new use for the Titan of Telecommunications.
So, what does this mean to you?
Well, if you can speak in terms of transmission rates, channels, trunking, D4 and clocking -
someone in the business might think you to be semi-intelligent.
Let's forget the application technology and focus on why and how the T1 became
such a sound Network Interface.
Okay, harking back maybe six or seven years ago - most companies were tied
to the Public Switched Telephone Network via single trunk lines.
Trunk lines could be engineered to carry In Only voice traffic, Out Only voice traffic or Bothway voice traffic.
Most data was usually mistakenly sent over expensive dedicated or point-to-point 56K circuits.
For some reason - Network Managers hated the word "Switched" as in Switched 56 Circuits.
Something to do with security and warm fuzzies.
Never quite understood the paragons of spend - spend - spend. (Few of them
were Democrats which was even more confusing.)
Now, let's do the math here.
You were a major company with a couple of thousand employees - 90% of your
voice traffic broke out to the Public Switch Telephone Network.
Imagine how many trunks were required to ensure Voice Quality service for
both inbound and outbound voice traffic. Busys?? Can we
really afford an Erlang Calculator??
Essentially, the recurring monthly costs were close to astronomical.
Guess what, maintenance costs for the Bell Operating Companies were equally prohibitive.
The number of hours spent tracing out circuits by phone guys
nationwide was probably
just a little more than the number of hours the world spends today while each
Windows 95 ported Desktop boots-up.
So, the market economy kicked in - in other words - self-interest (Economics 101).
What happened next was simply the biggest giveaway in telecommunications history.
The Bell Operating Companies offered free installation and migration from
old analog trunks onto the new digital voice path - the T1.
Whoa, what a deal. Smart Telco Managers jumped at this opportunity. Hey, get those
trunks outta here, dude. No more cluster you-know-whats. I'm finally going to get
a WAN infrastructure that I can understand.
But wait a mo' - the Bell Operating Companies forgot about just one crazy
little thing:- creativity.
Pretty soon, T1 Cards were being thrown into PBX's to support:
- Fax Servers
- Interactice Voice Response Units
- Modem Pools
- Video Conferencing
- PRI emulation between remote sites.
It is absolutely amazing what Telecom Managers can do with a
Hunt or Trunk Group.
So, after waiting on the list for a year or more - most major business were hooked-up to T1 Technology
along with Direct-Inward-Dial. (A block of numbers the last four digits of
which can be used as internal extensions)
What did they gain? Ever heard of a telephone bill?
Most Managers would agree that the savings were substantial especially if
you brought in a dedicated T1 from the Long Distance Carrier.
What else? Better voice quality.
The normal trunk voice path used 4 kbps. T1 offered close to 64 kbps per voice path.
Quite a leap. What else - just one little thing. Voice and Data over the same pipe.
What? Yip, you could engineer the circuit to accommodate Voice and Data transmission.
To me - that is still amazing. People knock the PSTN but hey, they don't
remember the rough days of single trunks and Switched 56 Circuits.
Okay, so let's get down to some technical stuff by dis-spelling a few myths:
So, what is a T1
- T1's cannot be used for internal trunking -False
- I'm going DS3 - can't use T1's on Fiber - False
- I can't send voicemail notification, caller id over point to point non-PRI T1 - False
- I can't send outbound and inbound traffic over the same T1 - False
- I can't send inbound 800 calls on the T1 along with other traffic - False
- I need a separate T1 for data transmission - False
- I can't configure analog OPX channels on a T1 - False
- You can't run Frame Relay over a T1 - False
- Each channel must be dedicated to a separate phone number - False
- The Smart Jack will still be in the loop if I migrate to a DS3 - False
Simply - a single four-wire (2 pair) copper circuit or a transmission path within a Fiber Pipe.
If offers 24 channels @ 64 kbps per channel for a total of 1.544 mbps transmission rate figured out like this:
(don't fall asleep on me - it's critical
that you grasp just a little of this stuff.)
The technology is based on the FAW - Frame Aligning Word. The FAW consists
of a single Bit added to each frame. The transmission alternates between a 1 and a 0.
A complete frame consists of 193 bit slots (24 Channels x 8 Bits) + 1.
Each channel is sampled at 8 KHz (8000 times per second - in other words - 8000 frames per second).
So we arrive at the transmission rate of 1,544,000 bits per second.
8000 frames per second times 193 bits per frame.
So to get to the sampling pulse - there's gotta be a clock, right?
You don't have to be brain-dead to figure out that it must be 1.544 MHz.
Okay - we've got over this hurdle - what's next.
Let's look at a copper-wire T1.
A copper-wire T1 consists of two-pair.
The transmission path is carried on Pins 1 & 2 and Pins 4 & 5 within an RJ48X modular plug.
It is dropped off to a Smart Jack (Westel) by the local phone company or to
a D4 Channel bank from a Long Distance Carrier (maybe). From the
Smart Jack/Channel Bank it is connected to a CSU (Channel Service Unit). From the CSU
to a T1 Interface Card within the PBX.
Of course, the same logic applies if you are creating internal T1 trunks
to an IVR Unit. However, you don't need a Smart Jack from the Phone Company or
a CSU. You can go directly to another T1 Card within a Server. (ain't cheap.)
A fiber T1 - let's say a DS3 or as it is commonly called - a T3.
If you ever get close to one - you might just see 28 RJ48 Ports on the back of the
of the Multiplexer. Yip, 28 T1's equal 1 DS3.
You still need the CSU to interface to the PBX's T1 Card but you don't need
the Smart Jack. One little perk. A lot of CSU's are becoming semi-intelligent.
You can monitor what's going on through an SNMP port. (A big deal, believe me.)
Let's talk about CSU's for a moment.
There are basically two types:
- Rack-mounted (nested)
Most companies set their CSU's up in an array (lets use the figure 12 - CSU 0 through 11)
The array can be monitored by a Controller via serial communications.
Controllers can be daisy-chained to support another array of CSU's.
A lot of folks blow-off the monitoring capabilities as something they
don't really want to know about.
Quite simply - that is a major mistake. You need to know when and how many
frame-drops/gain hits/bipolar violations occurred within a certain timeslice or do you?
Do you want your user's to trouble-shoot the circuits for you or do you
want to be proactive?
The new CSU rollouts include an SNMP Port that can be connected to your
Local Area Network. It's amazing what can be done on proactive management
In addition to the CSU there is also a fantastic device called the
I really believe that this little gizmo was the major reason why the
Internet took off like it did just a few years back.
I used to hear Network Managers gripe and complain about the PSTN
while at the same time free-loading down a 64 kbps pipe.
Guess what - they're still doing it today along with millions of others.
And, the griping hasn't stopped. Maybe, when the carriers drop-off ATM at
622 mbps - they'll change their tune. (It's not far off.)
So, I guess I'm trying to say - "Give the phone companies a little
Okay - the DSU - Data Service Unit. Not to throw a spanner-in-the-works here
but a Smart CSU can also be a Data Service Unit by simply dedicating it
to data transmission.
But the DSU/CSU is unique.
You can break up the 24 channels however you want. Let's say - send 12 to
the PBX and 12 to a V.35 serial port on a Cisco Router.
Pretty slick? And good value for the money. It's called Drop and Insert
I get a sense that I may be overwhelming you here. Think it's maybe time to
go on to another topic.
Ok - let's get into Signalling.
This is kind of fun.
So we know that a T1 is actually a DS1 - Digital Signal Level 1. It consists of 24 channels
or 24 DS0's - Digital Signal Level 0.
There are 28 T1's on a T3 or DS3 - Digital Signal Level 3.
The signaling/transmission process is common to all configurations.
- B8ZS - Binary Eight Zero Substitution
- ESF - Extended SuperFrame
- ZCS - Zero Code Suppression
Whoa. Wait a minute - what the heck does this mean?
What we're talking about here is - if you order a T1 - go for B8ZS.
Why? Because it offer's clear channel signalling and supports voice & data
transmission at a full 64 kbps per channel. (Also, returns Bipolar violation
If you're running OPX's to an internal Interactive Voice Response voice card - then you probably
want to configure the T1 Card in the PBX to support ZCS.
And so, that's almost it on T1's.
I don't know if this page will help or hinder you. Hopefully, the former.
If you are pursuing Telco as a career choice - I suggest you buy a good book
on Communications at your local College Bookstore. You won't regret it or have
to depend on sites like this. I write this stuff on the fly based
on experience only. Sometimes I'm right on - sometimes I'm way off. So cover your
bases and read as much as you can.
Here's a few tips that might prove useful:
Pick up as much information as you can on TDM - Time Division Multiplexing.
The day of the Tip & Ringer is in it's twilight years. Think of the
T1 as the Gateway to both the Wide Area Network and the Local Area Network.
Tip and Ring will still be around but phone guys like us may soon have a hard time finding a phone line
or telephone for that matter.
Remember, applications usually drive events. Be open to all. Learn to type and point-and-click.
Some techie stuff:
Okay, so you know that a RJ48X pin-out equals Pin 1 - 2 and 4 - 5 but do
you know the male pin-out on the DB15/RJ48X Adapter going into the back of the CSU?
Here's the short version:
- Pin 1 - Green
- Pin 9 - Red
- Pin 3 - Orange
- Pin 11 - Blue
Here's the long version:
- Pin 1 - Green
- Pin 2 - Brown
- Pin 3 - Orange
- Pin 4 - White
- Pin 7 - Black
- Pin 8 - Yellow
- Pin 9 - Red
- Pin 11 - Blue
How do you know which pin is which on the DB15? Each insert on the DB15 is numbered. You might
need a microscope to see them but they're there.
If you are connecting an internal PBX T1 Card to a Interactive Voice Response Unit Card within
a Server and cannot get a link. Roll the pairs. Jumper them across a double-gang
station jack - (Internal C4's) - Blue to Orange - Orange to Blue.
A loopback plug???
Grab an unused RJ45 Modular Plug and four or five inches of white/blue - blue/white
cross-connect. (Any color-code will do)
Loop the white/blue across Pins 1 and 4
Loop the blue/white across Pins 2 and 5
Crimp. Done. A loopback plug..
Plug it into the far-end of the circuit and send a local loop from the PBX.
That's it - I'm done.... So, think before you ignore this great Titan of
..as always - remember that Telecommunications is fun.