What is ESF and D4?
Dave
know whether I'd like "ESF" or "D4". Could someone please explain
what these terms mean?
TIA,
Dave Z dave...@netcom.com
William Wood
> When ordering a T1 line for data, the local fiber company wants to
> know whether I'd like "ESF" or "D4". Could someone please explain
> what these terms mean?
Dave, lots of folks will try to explain this to you, and most won't
know they don't have a clue what they're talking about. To fully
understand the differences between ESF and SF (D4) is a long
explanation. I ve tried to put some of it into several of my postings
with limited success. My suggestion to you is to order a book called
The Guide to T-1 Networking. It is available from the Telecom Library
Inc, 12 West 21 Street, New York, NY 10010. Call 212-691-8215 or FAX
212-691-1191. It is a paper back 8.5 x 11 size, 269 pages, ISBN
0-936648-26-0. It is well written, covers what you want to know, and
requires no propeller on your beanie to understand.
I have ordered many books from the Telecom Library folks over the
years (you can also get Harry Newton's dictionary from them --
everyone should have this book) and they respond very quickly. After
reading it, if you're still confused, you might want to check out our
two day on-site only seminar Learning to Talk the Talk and Walk the
Walk of Telecommunications. Much of what most people need to know
about this industry can be easily understood if a person has the
fundamental concepts correct and in logical order. Most do not. It is
the single overriding factor I have seen in thousands of people I have
addressed in my introductory seminars. Dazed and Confused is not just
a movie title, it seems to describe many of the workers in our
industry too.
WE Wood Technotranslater
Techtrans Animatics Group
Techish to English Translations
Michael Jennings
> When ordering a T1 line for data, the local fiber company wants to
> know whether I'd like "ESF" or "D4". Could someone please explain
> what these terms mean?
The following is a brief and by no means exhaustive explanation of D4
versus ESF.
D4 was the original AT&T (Western Electric) product used by the Bell
System for digital multiplexing of voice and data circuits at 1.544 Mb/s
over copper transmission lines.
When asked the way it was of you, the fiber company is now referring to
the time slot (channel) framing format that you would like your T1 line
to have. "D4" refers to the original specification which is also
referred to as the Superframe Format based upon the way framing bits are
used to define groups of the 24 channels multiplexed onto a T1 line.
"ESF" refers to a newer framing format called Extended Superframe Format
(hence, ESF).
ESF provides improved false framing protection and network maintenance
capabilities (performance monitoring.) In addition, ESF typically is
associated with "clear channel capability" or, the ability for the user
to take advantage of the full 64 kb/s data rate of any of the 24
channels on the T1 line.
Typically the "D4" type of framing requires that there be a minimum
number of logical "1's" being transmitted over the T1 line. This is
necessary because various types of transmission gear along the T1 line
(like repeaters) need sufficient transmitted energy to be able to
extract timing from the signal. Hence, users were typically restricted
from putting any data on a 64 kb/s channel that contained too many
zeroes. In essence, the channel was not fully "clear" for the user.
Alternative methods for maintaining a minimum 1's density were
introduced about the time ESF was developed and included Bipolar with
8-Zero Substitution (B8ZS) and Zero Byte Time Slot Interchange
(ZBTSI). Each of these permit the user of the data channel full
freedom over the data stream that they send over it. That is, the
channel is "clear" for the full 64 kb/s bandwith.
B8ZS is the more common method and simply substitutes a special code
whenever 8 consecutive zeros are encountered which is decoded at the
other end. While associated often with ESF it does not require ESF to
operate.
The ZBTSI method requires the ESF format because it utilizes the 2 kb/s
overhead data link inherent in the ESF format. However, ZBTSI is not
often employed by many carriers today.
I don't know if this will help you determine which type of framing
format you want its at least a little backround on what they mean.
Dr. R. Levine
in the framing bit position of the T-1 bit stream. The repetition
of the framing bit pattern occurs after 24 frames rather than 12
in "plain vanilla" D4 T-1 framing. Some of the framing bit values
in ESF are not fixed and can be used for operations, administration
and maintenance (OA&M) messages, and others are always used for
an error detecting code (CRC6 code). ESF is also a requirement
to install ZBTSI clear channel line coding, but this is only
used by USWest and PacTel, not by the other local/regional
telcos.
To determine if you want/need ESF, first examine the extra cost of
the hardware/software in your terminal equipment (your channel
bank or PBX). Then examine the extra cost (if any) for the service
from the carrier. What you get (primarily) with ESF that you don't
get with D4 is the ability to automatically and continuously
monitor the T-1 link for bit errors while all the 24 voice
channels are in full-time service. With D4, if you suspect problems,
you need to arrange to take one or more channels out of service
and perform a manual test involving co-ordination with the telco
in most cases. Try to make a cost comparison mainly between the
presence vs. absence of the automatic test capability. A good
customer equipment software system should warn you of even small
bit error rates which may predict more serious problems and allow
preventive maintenance. What is this worth? Does it justify the
extra initial and/or monthly cost for ESF vs. D4 service?
Chip Sharp
line (even in individual DS0s), then I would recommend an ESF line,
since HDLC Flags, when used as idle code, can simulate Yellow Alarm on
a D4 line. There are ways around it, but it must be taken into
account.
Hascall H. ("Chip") Sharp Teleos Communications, Inc.
Sr. Systems Engineer 2 Meridian Road
voice: +1 908 544 6424 Eatontown, NJ 07724 USA
fax: +1 908 544 9890 email: h...@teleoscom.com
Mike Schomburg
> When ordering a T1 line for data, the local fiber company wants to
> know whether I'd like "ESF" or "D4". Could someone please explain
> what these terms mean?
You may already be aware that a T-1 circuit is a bi-directional serial
connection, sending and receiving at 1.544 Mbps (million bits per
second). Each second, 8000 data frames are exchanged (in both
directions, send and receive). Each frame consists of 193 bits: 24
eight bit channels plus a framing bit.
ESF and D4 refer to the "framing format" used on any particular T-1.
When a T-1 starts up, or "frames", it picks a bit going past and
assumes it is the framing bit. Every 193rd bit after is examined and
compared to an expected pattern. As soon as the pattern is violated,
the next bit in sequence is chosen as the framing bit and the
comparison begins again. This process is continued until the actual
framing bit is found and the line established, or too many errors are
encountered and the line fails to start. Once framing is established,
it becomes possible to locate the 24 data (or voice) channels and
exchange information.
D4 is probably still the most common framing format (lets not start
that again), but ESF is coming up fast. D4 simply allows the terminal
gear to locate the payload channels, plus a very crude ability to
signal the far end that problems have interrupted the line (when
appropriate).
ESF (Extended Super Frame) also "frames" the data, but additionally
includes tha ability to send operational messages end-to-end, and
provide a level of line quality reporting. This is done by organizing
groups of 12 frames into "super" frames. With processing power, the
line can stay framed while half of the framing bits are borrowed to
form a message channel. A CRC scheme is used to calculate line
quality, detecting about 93% of error conditions.
There are at least two common flavors of ESF, a proprietary AT&T
scheme and an ANSI scheme, T1.403 I believe. Be sure you know which
one your terminal gear supports. If you are running data, you may also
want to check on the "line coding" format, AMI (Alternate Mark
Inversion) or B8ZS (binary 8 zero superssion). In the past, all T
spans were AMI, which limited the ability to send strings of zeros.
All "ones" were sent as alternating polarity pulses (to keep the DC
component low) on the line, and "zeros" were sent as the absence of
pulses. Obviously, too many zeros and the line would quickly lose
synch. B8ZS is a scheme to avoid the line dropping while sending
unlimited zeros. For reasons that I have not gone into, if you want to
be able to use the full 64kbps capacity of each channel (instead of
just 56kbps) then you want B8ZS. Hope this helps.
Mike D. Schomburg Network Manager, Continental Cablevision
708 834 4239 Chicago region
scho...@continental.com "linux - the choice of a GNU generation"