SUMMARY: Advice on T1 circuit utilization...

From: Dan Penrod (
Date: Tue Oct 07 1997 - 15:21:06 CDT

Hey Managers:

Thanks for the terrific response.

My original query was asking for advice on the nature of a T1
network connection, how do you know when it's maxing out
on utilization, and when should you consider upgrading or
adding a new T1 to your network.

Here is a list of 12 fascinating facts I learned about T1s;
(I'm paraphrasing others in each case)

1. Unlike ethernet, there's no such thing as collisions
and generally no such thing as network saturation.

2. T1 circuits use 4 wires, 2 for traffic in each direction,
point to point, therefore there is only one sender and one
receiver on the circuit and T1s are full duplex.

3. It's a serial interface and as such behaves according to
rules of serial communications (DTE and DCE)

4. Data can flow through it anywhere in the range of 0 - 100%
utilization of the 1.54Mbps throughput. Theoretically there is
little or no observable degradation as utilization increases.

5. A T1 circuit can only be considered "saturated" at 100%
(unlike an ethernet which begins to saturate about 20 - 30%).

6. In terms of cost-effectiveness, if you want to use the full
capacity of your T1, you can wait until you are using it at 100%.
There is technically no good reason not to wait for that point.

7. In terms of planning ahead... you may want to watch the
growth rate of your T1 utilization so that you can predict well
ahead of reaching 100% utilization. This is important because
it can take up to 3 months to get your ISP to hook you up a new
T1. You don't want to be sitting at 100% for 3 months! So,
maybe you should shop for a new T1 at about 70 - 80%.

8. If you max out a T1 at 100% utilization the following can occur:
    * High CPU at both ends as the routers try to buffer packets
    * Dynamic routing protocol failure, since route announcements
    have no bandwith to advertise.
    * Packets begin to get dropped as queues overflow, requiring
    retransmissions, further bogging down throughput.

9. A T1 packet is similar to a TCP packet but the data portion of
a T1 packet contains data for up to 24 different communication units
in 1/8000th of a second slices, transmitting the bits 1.5 million
Said differently... 193 bit packets x 8000 samples/sec = 1.544Mbps

10. With the "simple" T1 protocol...
    A T1 circuit can be referred to as a DS-1.
    A T1 is composed of 24 channels called DS-0 elements.
    There are other T1 protocols (SF-D4, SLC-96, ESF) where
    you can multiplex the signals (interleave them) and get better

11. While you may not be saturating your T1 with existing traffic...
you may be saturating your router! A router can easily get 20 - 30
packets all at once from multiple 10mbps ethernets, which queue up.
So even though average loads may be 40%... instantaneous loads
overflow the buffers and packets are lost, requring retransmissions.
One user says that queuing theroy suggests that this form of
degradation can begin to occur at 40%, but that a T1 can be expected
to sustain loads of 80 - 90%. Measurements of instantaneous loads
and lost packets are of greater interest than T1 utilization.

12. Rich Kulawiec thinks this information is unrelated to Sun
network management and inappropriate for this list.

A million thanks to all the managers who responded. This
very useful and interesting to me.
Thanks to;

Doug Carman <>
Somkit Khemmanivanh <>
Glenn Satchell - Uniq Professional Services <>
Jason Marshall <>
Rich Casto <> (Russell David)
David Wolfskill <>
Jim Harmon <>
Daniel R. Falconer" <>
hawk@UU.NET (Michael Hawk)
Rich Kulawiec <>
Nickolai Zeldovich <>

| Dan Penrod - Network Administrator                  |
| Paradyne / Networking Services                      |
| 8545 126th Ave. N., Mailstop LG130, Largo, FL 8597  |
| vox:813/530-8597 fax:530-2480 |

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