- 1000BASE-SX
(short wavelength: 850 nm)
MM
Fiber 62.5 m m up to 220 m
MM Fiber 50 m m up to 300 m
- 1000BASE-CS
Short
haul copper (25 m)
The short haul copper solution uses (IBM) triax
cable, and is intended only for backbone applications in an
equipment room --interconnecting of hubs or other networking
electronics in an equipment room. It is definitively not considered
part of a generic cabling solution. It is expected that these
shorthaul copper cables will be factory produced in fixed
lengths.
This
portion of One-Gbps Ethernet was approved June 1998. The fiber
standards development encountered some remaining issues with
modal bandwidth resulting in excessive jitter on multimode
fiber. This resulted in the definition of the maximum distance
on MM fiber as shown above. The modal dispersion and resulting
jitter is a function of the diameter of the core and the wavelength
(and spectrum) of the light source.
IEEE
802.3ab is now fully devoted to One-Gbps Ethernet on category
5 twisted pair cabling. All 4 wire pairs in the standard four-pair
cable are used, and transmission is full duplex on all 4 wire
pairs. NEXT cancellation techniques are also implemented.
This technique was first developed (but never implemented)
for the proposed 100BASE-T2. The latter was defined as a two
wire-pair solution on category 3 for Fast Ethernet (100 Mbps
data rate). A five level encoding system was adopted; it is
called PAM-5, more about this later. The initial goal of the
IEEE 802.3 committee was to obtain a completed standard by
late 1998, delays over Return Loss caused a delay. It was
however resolved an agreed in August 1999.
The
IEEE 802.3ab working group requested assistance from the TIA
TR41.8.1 UTP task group to fill in requirements needed for
One-Gbps operation over category 5 cabling. (Note that in
December 1998 the name of this TIA group changed to TR.42.)
This
task group has adopted a "fast" track project to do so, and
the goal was to match the timeline for 1000BASE-T. Both projects
have "slid" together. It is emphasized in every possible way
that it is expected that the existing -- and currently installed
-- category 5 cabling should normally meet the additional
requirements, which were previously left unspecified. As a
result, the TIA will still call the newly compliant cable
"category 5", and not anything like "category 5a" or "category
6". The Cat 5 specifications have been amended with a recommended
performance level for the new test parameters (FEXT-related
measurements and Return Loss). The recommendations are specified
in a Telecommunications Systems Bulletin (TSB95). TSBs don't
have the weight of a "standard"; they are recommendations.
(TSB67 is actually an exception; it has the normative weight
of a standard.)
We
are saying that the ultimate measure of success in data transmission
is the fact that frames are successfully transmitted. There
are no bit errors (no FCS errors) and no re-transmissions.
The physical layer plays a critical role in achieving error
free transmission on the data link layer. The bandwidth characteristics
of the physical layer must match the requirement of the physical
signal encoding used by the network.
(1)
We need to explain the basic ground rules for all of the "frequency"
plots we will be using during the discussions of the standards
and especially to describe the performance of parameters that
vary with the frequency such as NEXT and attenuation. In the
frequency domain, we plot frequency along the horizontal axis
and we show "something" about a signal with that frequency
in the vertical axis. The simple example below represents
at the left side how a pure sinusoidal frequency signal varies
in time. If we assume that the period is 1 microsecond, the
signal will repeat one million times per second or is called
one megahertz (MHz). In the time domain plot on the right,
we represent the amplitude of that signal.
The beginnings of the Fourier analysis.
(2)
We have a second goal. To lay the groundwork to explain that
digital signaling contains a multitude of frequencies and
that the transmission medium needs to do an "adequate job"
-- defined by a standard -- for all the frequencies of interest.
Lastly,
this set of drawings may be used to introduce the digital
test technique. The DSP Series testers from Fluke send pulses
that contain many frequencies.
Add
two sinusoidal signals to get the time domain signal depicted
in the left-hand side plot. We have added to the 1 MHz signal
of the previous slide a signal of 3 MHz with an amplitude
equal to 1/3 of the 1 MHz signal. The frequency domain picture
above shows the two frequencies each with its amplitude value.
We
now have added 4 signals together. The signals with higher
frequencies, called harmonics, have successively smaller amplitudes:
1/3, 1/5, 1/7, etc.. You can see that the time domain
picture is "approaching" digital signaling, i.e. two distinct
voltage levels.
Finally,
we are ready to flip the whole thing into the other direction.
In theory, we are transmitting the digital signal shown in
the time domain picture, a perfect square wave. The frequency
domain shows that such a digital signal contains a number
of frequencies. As a matter of fact, every frequency between
0 and some upper value is represented. For a two-level digital
signal, the upper value is the frequency equal to the data
rate.
Example:
using the NRZ encoding for ATM 155, this null point is at
155 MHz. Shouldn't we test to 155 MHz? The signal created
by the transmitter does not exhibit the perfect rise and fall
times that you see in the theoretical model. Changes from
one voltage level to another require a finite amount of time
(measured as the rise and fall times). The frequency spectrum
of the "real" ATM NRZ signal is such that the "tail" in the
frequency domain picture drops dramatically. It has been debated
by several people as to how much energy is really present
above 100 MHz. The second issue to remember is that the receiver
may not need or expect any frequencies above 100MHz to properly
decode the digital signal that is transmitted.
Megahertz
(MHz) is not equal to Megabits per second (Mbps)
- MHz:
A unit of frequency, describes electrical signals. Pertains
to the Physical Medium
- Mbps:
A data rate, describes throughput achieved by the system
(electronics, software and medium)
Time for a story:-
Once
upon a time, I was very happy if I could get my modem to work
reliably at 4800 bps, as a matter of fact, I was ecstatic
if got connected at 9600 or 9.6 kbps. Now I am using a 56
kbps modem that seems to do just fine (although you never
get connected at exactly 56k). The phone line to my house
hasn't changed; it still the same copper wire. The signal
encoding (standard V.90) combined with error correcting codes
and compression has made this faster data transfer possible
and even more reliable. A similar scenario is unfolding for
Gigabit Ethernet over Cat 5.
Digital
Signal Encoding
"Man"
in the second line designates "Manchester" encoding which
is used for standard Ethernet. The bottom line depicts "Differential
Manchester" encoding which is very similar (but different,
as you can see) and is used by Token Ring. In both Manchester
systems, the signal goes through a transition from high to
low or the opposite direction in the middle of each bit time
slot. This transition guarantees good synchronization between
sender and receiver. Therefore, people sometimes state that
10BASE-T runs over "barb wire". Indeed it uses a very robust
signal encoding technique. But also note that the Manchester
signal encoding goes through roughly twice as many level changes
per time as the NRZ signal above. Therefore, Manchester encoding
is very inefficient as far as bandwidth requirements. To transmit
10 Mbps you need at least a 10MHz bandwidth for the signal
on the cable. (That is a very bare minimum. Fortunately, Cat
3 behaves pretty well up to 16 MHz.)
Obviously,
to get higher data rates over twisted pair cabling, we had
to find other signal encoding systems that could still provide
for reliable synchronization. One such system is the 4 bit-
5 bit encoding. Every four bits of data are translated into
a sequence of 5 bits for transmission. Five bits provide 32
different combinations. Out of these 32 combinations only
16 (half) have to be selected for data encoding. We can select
those 5-bit sequences that provide the maximum number of "transitions"
for good synchronization. For example 00000 and 11111 will
be excluded, for sure. Some additional advantages are listed:
we can utilise the remaining 16 codes for delimiters or idle
patterns, and if an "illegal" pattern appears, we have detected
that the cable transmitted something in error. The data stream
has grown 25% though. To transmit 100 million bits of data,
we need to transmit 125 million signal on the cable and signal
level is valid for 8 nsec. To contain the bandwidth requirement
for this signaling rate. The signaling uses a "pseudo-ternary"
encoding. This is not a tri-level logic signal but instead,
we will chose 0 volt for a signal that represents a logical
0. The logical 1 signal will "toggle" between +1V and -1V.
See below. It will appear intuitive that fewer signal transitions
are required per unit of time. There is also a mathematical
proof for the signal bandwidth requirements.
100BASE-TX
Signal Encoding
We
will explain a four-level signal encoding. Gigabit Ethernet
actually uses PAM-5, a five level encoding scheme. The "fifth"
level is used for additional synchronization as well as error
detection/error correction. Note that the signal timing is
8 nsec which is exactly the same value as we encountered in
Fast Ethernet's 4B-5B encoding.
The
signals on the cable can take five different levels while
the total voltage swing from min to max is still the same
2V swing (from -1V to +1V). The signal levels are no longer
separated by 2V, but by 0.5 V. The direct result of this separation
is that if a noise spike of 0.25V hits the cable, the receiver
will most likely not be able to determine which signal level
had been transmitted. This situation is somewhat alleviated
by the error detection/error correction encoding level.
Four-level
Signal Encoding
This
is an example of what a four-level encoding scheme might look
like. Remember this illustrates the type of signal encoding
used in 1000BASE-T. The real encoding system is called PAM-5,
which is a five-level system.
Nyquist
theorem for a noise-free channel
To
throw some theory into the picture. You may have heard of
the Nyquist frequency. Here is the explanation in short. Shannon's
law applies to predict how much bandwidth needs to be available
above the Nyquist minimum based on the expected signal-to-noise
ratios.
Limitation
determined by signal bandwidth
R=2Wlog2M
Where R is the rate of data transmission, W
is the maximum frequency and M is the number of levels of
encoding
Example
1: 10BASET
This
is a two level encoding so M=2,
Therefore
the bandwidth (W) = R / log22 * 2 which gives 10MHz
(Remember that throughput of 10BASET is 20Mbits)
Example
2: 1000BASET
This
is a four level encoding so M=4 (5th Level is for
syncronisation only)
Therefore
the bandwidth (W) = R / log24 * 2 which gives 62.5MHz
(R = 250Mbits/s)
This
is theory, and in real life the protocol for 1000BaseT needs
a little more typically 80MHz, so the IEEE specifies cable
testing on all pairs up to 100MHz.
| Type
|
Data
Rate |
Pairs
Used |
Frequency
|
| 10BaseT
|
10Mbps
|
2
|
10MHz
|
| 100BaseT4
|
100Mbps
|
4
|
15MHz
|
| 100BaseTX
|
100Mbps
|
2
|
80MHz
|
| 100VG-AnyLAN
|
100Mps
|
4
|
15MHz
|
| ATM155
|
155Mbps
|
2
|
100MHz
|
| 1000BaseT
|
1000Mbps
|
4
|
100MHz
|
Transmission
performance for Cat 6 components and installations needs to
be verified to 250 MHz. Using the ACR model of bandwidth,
the installation is predicted to have a positive margin similar
in the size to the margin of a Cat 5 installation at 100 MHz.
At 250 MHz the installation will have a negative ACR margin.
The IEEE has been the instigator to encourage testing to 250
MHz with an eye on the possibility that the continued development
of DSP technology will allow transmission beyond the ACR bandwidth.
Recall that this technology had initially been developed for
100BASE-T2, which never was implemented. The 1000BASE-T standard
relies heavily on these DSP techniques to guarantee reliable
transmission over Cat 5.
I
hope that this paper has been of use.
Reproduced
with kind permission Adrian Young
Please
visit Adrian Young's excellent website www.cablemeter.com