&" '&
A physical layer has been developed for demand priority local area
networks that accommodates different cable types by means of different
physical medium dependent (PMD) sublayers. The major goal was to
provide 100-Mbit/s transmission on existing cables, including Category 3,
4, and 5 UTP, STP, and multimode optical fiber.
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(continued on page 20)
MAC
PHY
MAC
PMI
PMD
PHY
60.0
Field Strength (dB mV/m)
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50.0
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40.0
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FTZ-1046 VDE-B
30.0
EN 55 022 Class B
PMI
PMD
Link
20.0
10.0
31.6
100.0
316.2
1000.0
Frequency (MHz)
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Cross talk in UTP cables is caused by capacitive coupling between pairs. Signals
on pair A cause noise signals on pair B, and often the cross talk noise proves to
be the limiting factor in the link performance. Cross talk occurs in two ways. Nearend cross talk (NEXT) happens when a signal from a transmitter at one end of a
cable interferes with a receiver at the same end of the cable. Far-end cross talk
(FEXT) occurs when a signal interferes with a receiver at the opposite end of the
cable from the transmitter.
Near-End Cross Talk (NEXT)
Near-end cross talk loss is defined as:
NEXT = 20 logV nV i,
where Vn and Vi are shown in Fig. 1a. The minimum NEXT loss between pairs in a
cable tends to follow a smooth curve, as shown in Fig. 1b, decreasing at a rate of
15 dB per decade. However, the actual NEXT between two particular pairs deviates
significantly from this curve because of resonances in the twisted-pair. Typical
measurements of the NEXT loss between some pairs in a 25-pair cable are also
shown in Fig. 1b.
Far-End Cross Talk (FEXT)
Far-end cross talk loss is defined as:
with NEXT loss, the actual FEXT loss between two particular pairs deviates from
this curve. Typical measurements of the FEXT loss between some pairs in a
25-pair cable are shown in Fig. 2b.
Cross Talk Measurements
Our analysis of cross talk required a database of accurate and detailed measurements of cross talk between pairs in 25-pair cables. A measurement system was
constructed to measure NEXT and FEXT losses of all pair combinations in 25-pair
cables (see Fig. 3, next page).
Individual pairs were routed to the stimulus and response ports of a network analyzer via a computer-controlled switch. This allowed the automatic selection of 300
different pair combinations for NEXT measurements and 600 pair combinations for
FEXT measurements. Any pair not being measured was terminated in 100 ohms via
a balun and a 50-ohm termination internal to the switch. The network analyzer
measured the cross talk loss (phase and magnitude) to 40 MHz, and this was downloaded to a computer database. Using this system, the NEXT and FEXT losses
were measured for many thousands of pair combinations in a selection of 25-pair
cables of varying manufacturer and age. The database was used to input NEXT
and FEXT loss characteristics to the computation of cross talk noise described in
“Cross Talk Analysis” on page 22.
(continued on next page)
FEXT = 20 logV fV i,
where Vf and Vi are shown in Fig. 2a. The minimum FEXT loss also decreases
with frequency following a smooth curve, but at a rate of 20 dB per decade. As
Pair A
Pair A
Vi
Vo
Vi
NEXT
FEXT
Pair B
Vn
Vf
Pair B
Noise
Noise
(a)
(a)
80.0
100.0
70.0
80.0
Loss (dB)
Loss (dB)
60.0
50.0
40.0
60.0
40.0
30.0
Minimum FEXT Loss
Minimum NEXT Loss
20.0
20.0
10.0
0.0
0.0
10.0
20.0
30.0
0.0
Frequency (MHz)
(b)
Fig. 1. (a) Near-end cross talk (NEXT). (b) Minimum theoretical NEXT loss and actual measurements.
10.0
20.0
30.0
Frequency (MHz)
(b)
Fig. 2. (a) Far-end cross talk (FEXT). (b) Minimum theoretical FEXT loss and actual measurements.
(continued from page 19)
Controller
Network
Analyzer
Controller
Network
Analyzer
251 Switch
251 Switch
100-Ohm
Termination
252 Switch
25-Pair Cable
25-Pair Cable
For FEXT Loss
For NEXT Loss
(continued from page 18)
Given these constraints, the design goals for the PHY wereĆ
met by developing a physical medium dependent (PMD) subĆ
layer for each media type (UTP, STP, and multimode optical
fiber) and a physical medium independent (PMI) sublayer
that contains all the functions common to all media types.
These two sublayers together form the demand priority PHY.
In the remainder of this article we will discuss the design
choices made for each of the three demand priority PMDs.
!# The first PMD to be developed was to support UTP cabling,
since this addresses the large 10BaseĆT upgrade market.
10BaseĆT uses fullĆduplex signaling at 10 Mbit/s on UTP
cabling.2 One twisted pair is used to transmit and one to
receive data. The dc content of the signal is minimized by
Manchester coding the data before transmission on the
twistedĆpair channel. Manchester coding is a very simple
form of block coding:
0
1
10
01
This is a 1B/2B block code, and results in a 100% bandwidth
expansion. Manchester coding is spectrally inefficient relative
to other block codes, but does provide a guaranteed transiĆ
tion for every two symbols, has very low dc content, and is
very simple to implement.
The transmission rate in 10BaseĆT is 20 megabaud, giving a
data rate of 10 Mbits/s (1 baud is one symbol per second).
This means that the transmitted signal can be lowĆpass filĆ
tered with a cutoff frequency somewhat less than 20 MHz.
This helps minimize radiated emissions above 30 MHz, the
lower bound of stringent EMC regulations.
# #"
The progression from 10BaseĆT to 100VGĆAnyLAN was made
in three simple steps.
First, it was recognized that since fullĆduplex transmission
was not absolutely necessary in a hubĆbased network, both
twisted pairs used for 10BaseĆT could be used simultaneously
August 1995 HewlettĆPackard Journal
Fig. 3. Measurement system for NEXT and
FEXT loss.
for transmission in one direction or reception in the other.
This immediately doubles the bit rate achieved on existing
10BaseĆT networks.
Second, the spectrally inefficient Manchester code was replaced
with a more efficient 5B/6B block code (5 bits of data are
coded to 6 transmitted binary symbols). This reduced the bandĆ
width overhead from 100% with Manchester coding to 20%. As
a result, the data rate on each pair could be increased to 25
Mbits/s (that is, a symbol rate of 30 megabaud after 5B/6B
coding) while the main lobe of the data spectrum remained
below 30 MHz, the lower limit of EMC regulations. As with
10BaseĆT, lowĆpass filtering with a cutoff below 30 MHz can
then be applied to minimize the risk of excessive radiated
emissions. The 5B/6B code chosen also has very low dc conĆ
tent, which avoids distortion from the coupling transformers.
See the article on page 27 for more details.
Third, the UTP PHY takes advantage of the two unused pairs
available in every fourĆpair cable. Surveys of customer cable
plants revealed that a large proportion of these customers adĆ
hered to structured cabling recommendations3 when installing
cable, and connected fourĆpair cable to each wall outlet. Two
of these four pairs currently lie unused. By transmitting 5B/6B
coded data at 30 megabaud on all four pairs, it is possible to
provide a total signaling rate of 120 megabaud (100 Mbits/s)
over UTP cable.
! The fourĆpair transmission scheme, called quartet signalĆ
ing, uses binary transmission, that is, only two voltage
levels are used as symbols. Other approaches to the UTP
PMD were examined. One was multilevel (mĆary) signalĆ
ing (see Multilevel Signaling" on page 21). In a multilevel
scheme, n data bits are mapped to one of m = 2n symbols,
and each symbol is a unique voltage level. For example, in
a quaternary scheme, two data bits may be mapped to one
of four voltage levels. In this way the number of symbols
transmitted, and hence the transmission rate required, is
reduced by a factor of n. However, for a fixed power supĆ
ply voltage, the voltage separation between symbols is
reduced by a factor of 1/(m1) from the binary case. The
binary quartet signaling scheme maximizes the voltage
separation between symbols, which provides greater imĆ
munity to noise at the receiver.
In the four-level scheme, groups of two data bits are mapped to one of four symbols. Only one symbol need be transmitted for each pair of data bits, so the symbol rate is half the bit rate. The drawback of the multilevel scheme is that symbols
are separated by a smaller voltage than in the binary scheme. This means that
when noise is added to the data signal (cross talk or impulse), the probability of
the noise changing one symbol to another is increased. The symbol separation
could be increased to that of the binary scheme by increasing the peak-to-peak
transmitted voltage by a factor of (m – 1) for an m-level scheme, but this is generally not possible given fixed power supply voltages, and in any case it increases
the power required for a transmitter.
0
0
1
0
1
1
0
The susceptibility of a scheme to errors caused by noise is measured by the ratio
of signal separation to noise. Fig. 2 shows the signal-to-NEXT-noise ratio plotted
against the transmission bandwidth for several multilevel schemes and for multipair schemes for a bit rate of 100 Mbits/s. A 16-level scheme reduces the bandwidth to 25% of the bit rate, but the S/NEXT ratio is 13 dB (a factor of 4.5) worse
than for a four-pair scheme with 25 Mbits/s per pair, which is the scheme used in
the 100VG-AnyLAN standard.
20.0
n=4
n=3
15.0
S/NEXT (Normalized) (dB)
Multilevel signaling is often used as a means of compressing the bandwidth required to transmit data at a given bit rate. In a simple binary scheme, two single
symbols, usually two voltage levels, are used to represent a 1 and a 0. The symbol rate is therefore equal to the bit rate. The principle of multilevel signaling is to
use a larger alphabet of m symbols to represent data, so that each symbol can
represent more than one bit of data. As a result, the number of symbols that needs
to be transmitted is less than the number of bits (that is, the symbol rate is less
than the bit rate), and hence the bandwidth is compressed. The alphabet of symbols may be constructed from a number of different voltage levels. Fig. 1 shows an
example for a four-level scheme.
n=2
10.0
Multipair Signaling
1
m=5
m=8
m=4
m = 12
5.0
m=2
m=6
m=3
m = 16
V
Multilevel Signaling
00
10
m=4
11
01
V/3
0.0
20
40
60
80
Transmission Bandwidth (MHz)
m=2
n=1
100
Fig. 1. Two-level and four-level signaling.
Fig. 2. Susceptibility to errors from noise.
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Many previous analyses of twisted-pair transmission systems have assumed that
the distribution of cross talk noise is Gaussian. These have yielded reasonably
accurate predictions of system bit error rates. However, in applying the error rate
analysis, there is an implicit assumption that the cross talk noise is independent of
the data on the disturbed system. This is often the case in telecommunication
systems, but is not always the case in LANs, where the disturbing links are those
on which the disturbed data is being retransmitted. For example, the NEXT interfering with data received at one port of a hub is a result of the retransmission of
earlier bits of the same data on other ports. If the cross talk noise is of sufficient
amplitude to cause an error in the received data, this error is extremely likely to be
repeated every time the same data is transmitted. If a packet is errored by cross
talk, that particular packet is likely always to be errored.
To guarantee the error-free transmission of any packet, the worst-case peak cross
talk voltage must be found. The peak cross talk noise from multiple disturbers can
be calculated directly from a knowledge of the cross talk channels and the disturbing data source. The NEXT and FEXT cross talk channel frequency responses
can be calculated for any pair combination using measurements of the pair-to-pair
NEXT or FEXT loss. Once the cross talk channel frequency response is known, it
is possible to find the impulse response of this channel by inverse Fourier transform. We define the impulse response of the NEXT channel as:
pk,NEXT
maximum FEXTnoise voltage due to pair i disturbing pair k as vi,k
.
pk,FEXT
Multiple-Disturber Cross Talk
The worst-case cross talk environment for UTP PMDs operating over 25-pair
bundles consists of three far-end disturbers and four near-end disturbers. Model
ing this environment is relatively straightforward given the distributions of v i,k
pk,NEXT
and v i,k
pk,FEXT
. For each choice of disturbed pair (k), four NEXT disturbers (a,b,c,d)
and three FEXT disturbers (p,q,r), the total noise voltage for multiple disturbers is:
v pk,total +
ȍ
i,k
v pk,NEXT )
i+a,b,c,d
ȍ
j,k
v pk,FEXT.
j+p,q,r
By calculating the multiple disturber noise voltage in this way we assume, pessimistically, that the maximum noise voltages for the worst-case disturbing patterns
from each disturbing source occur at the same time and with the same polarity on
the disturbed pair k. This is obviously a worst-case scenario.
and the impulse response of the FEXT channel as:
g f(t) + F *1ǒH f(f)Ǔ
To find the cross talk noise voltage at the receiver decision point, n(t), caused by
any data pattern f(t), the impulse response is convolved with f(t):
n(t) = f(t) g(t),
(1)
where g(t) represents either gf(t) or gn(t) as appropriate. Our goal is to find the
worst-case cross talk for any data pattern, so n(t) must be calculated for all values
of f(t). It is therefore useful to apply some limit to the duration of f(t) to shorten the
computation time, and this can be done by taking into consideration the finite duration of the cross talk channel impulse response. A typical impulse response of a
NEXT channel is shown in Fig. 1. The duration of the cross talk impulse response
is typically less than 1400 ns, which is equivalent to 42 symbol periods for the
30-megabaud transmission rate used in quartet signaling. Therefore, the cross talk
waveform during the last six symbols of a pattern f(t) can be predicted accurately if
the duration of f(t) is restricted to 1600 ns.
The cross talk for any f(t) is calculated as follows. The disturbing data source is
assumed to be the output of a 5B/6B block coding function and consists of eight
sequential six-bit codewords, each chosen from an alphabet of 32 codewords. The
total number of permutations that f(t) can take is therefore 328. For each permutation, n(t) is computed according to equation 1. The peak cross talk noise voltage
generated by any value of f(t) can then be found by searching each resultant n(t).
0.040
0.030
0.020
0.010
A Monte-Carlo approach has been used to choose combinations (k,a,b,c,d,p,q,r)
randomly from the 25 pairs of a cable. For each choice, vpk,total was calculated.
The resulting distribution of vpk,total is shown in Fig. 2. The maximum multiple-disturber cross talk noise expected on any pair of the cable for any choice of disturbing pairs can be estimated from the higher extreme of this distribution (such as the
first percentile). This number represents the noise voltage for the worst-case
choice of disturbing and disturbed pairs, with the maximum noise contributions
from all disturbing pairs occurring simultaneously on the disturbed pair. For the
example shown, the first percentile of the total peak noise distribution is 47% of
the signal. This allows a substantial margin for error-free signal detection.
1.0
0.8
Probability (Peak Noise Voltage > x)
g n(t) + F *1ǒH n(f)Ǔ
Amplitude (V)
The peak cross talk noise voltage, which represents the maximum noise generated by a worst-case data pattern, can be calculated in this way for each pair
combination in a 25-pair cable by repeating the search described above using the
NEXT or FEXT loss particular to that pair combination. For each pair combination,
the maximum cross talk noise voltage is recorded. The distribution for a typical
cable is shown in Fig. 2 for NEXT and FEXT. (The cross talk noise voltage is
normalized to the signal amplitude at the receiver decision point.) We denote the
maximum NEXT noise voltage for pair i disturbing pair k as v i,k
, and the
0.6
vpk,NEXT
vpk, total
0.4
0.000
0.2
–0.010
Vpk, NEXT
–0.020
–0.030
0.0
–0.040
0.0
0.0
1000.0
2000.0
Time (nanoseconds)
Fig. 1. Typical impulse response of a NEXT channel.
0.1
0.2
0.3
x (Volts)
3000.0
i,k
i,k
0.4
0.5
0.6
Fig. 2. Distribution of v pk, FEXT, v pk, NEXT, and v pk, total for a typical 25-pair cable.
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%; 5(75$160,77,1* 7+( 3$&.(7 72 6(9(5$/ '(67,1$7,216 6+$5,1*
7+( 6$0( =3$,5 &$%/( $6 7+( 6285&( :28/' 5(68/7 ,1 (552=
1(286 5(&(37,21 $7+(5 7+( 3$&.(7 ,6 6725(' 817,/ 5(&(37,21
,6 &203/(7( $1' ,6 7+(1 )25:$5'(' 72 $// '(67,1$7,216 6,08/7$=
1(286/; 1 7+,6 :$; 12 " 2&&856 '85,1* 5(&(37,21 +,6
6725(=$1'=)25:$5' 7(&+1,48( ,6 21/; ,03/(0(17(' :+(1
=3$,5 &$%/(6 $5( $77$&+(' 72 $ +8% ) 21/; )285=3$,5 &$%/(6
$5( 86(' $// 3$&.(76 6,1*/( $1' 08/7,3/( $''5(66(6 $5( )25=
:$5'(' ,00(',$7(/; 6,1&( 7+(5( ,6 12 " %(7:((1 7+(
,1',9,'8$/ )285=3$,5 &$%/(6
,* 6+2:6 $ %/2&. ',$*5$0 2) 7+( =1; ,03/(=
0(17$7,21 2) 48$57(7 6,*1$/,1* +( $1' 68%/$;(5
)81&7,216 $5( &216,'(5(' 6(3$5$7(/; :+(1 75$160,77,1* 25
5(&(,9,1* '$7$
PMI Transmitting. +( 63/,76 7+( '$7$ ,172 )285 675($06
($&+ 2) :+,&+ ,6 6&5$0%/(' +,6 5(029(6 3$77(516 7+$7
:28/' 5(68/7 ,1 $ 5(3(7,7,21 2) 7+( 6$0( &2'(:25' ,1 7+(
PMI Transmitting
4 × 25 Mbits/s
100 MBits/s
4 × 30 Mbits/s
Scrambler and
5B/6B Encoder
Preamble and Delimiter
Insertion
MII
4 × 30 Mbits/s
NRZ Encoder
PMD
Transmitting
4 × 30 MBaud
Transmit Filter
287387 2) 7+( &2'(5 +,6 +(/36 $92,' 63(&75$/ 3($.6
7+$7 0,*+7 9,2/$7( 5(*8/$7,216 :+(1 7+( &2'(' '$7$ ,6
75$160,77('
;3,&$//; 75$)),& &217$,16 '$7$ 3$77(516 7+$7 $5( 6,03/;
5(3(7,7,9( 6 25 6 ) /()7 816&5$0%/(' :+(1 63/,7 ,172 48,1=
7(76 ),9( %,76 $7 7+( 7+( ',675,%87,21 2) 48,17(76 ,6
+($9,/; %,$6(' 72:$5'6 $// 6 48,17(7 9$/8( 2) 25 $// 6
48,17(7 9$/8( 2) +,6 ,6 &21),50(' %; ,* :+,&+
6+2:6 7+( ',675,%87,21 2) 48,17(76 2%7$,1(' )520 5($/ 75$)),& &5$0%/,1* 5(029(6 7+,6 %,$6 3529,',1* $ 025( 5$1=
'20 ',675,%87,21 2) 48,17(76 $7 7+( ,1387 72 7+( &2'(5
)7(5 6&5$0%/,1* 7+( 3(5)2506 7+( &2',1* 7 7+(1
$''6 67$57 $1' (1' '(/,0,7(56 72 7+( )285 675($06 $1' $
35($0%/( 6(48(1&( $ … 3$77(51 72 7+( 67$57 2) ($&+
675($0 +( )285 3$5$//(/ 675($06 2) &2'(' '$7$ %,766
3(5 675($0 $5( 7+(1 3$66(' 72 7+( PMD Transmitting. +( &219(576 7+( )285 3$5$//(/ '$7$
675($06 72 7+( %,1$5; 6,*1$/,1* /(9(/6 ± 21 ($&+ 2) 7+(
)285 7:,67(' 3$,56 +( '$7$ ,6 1215(7851=72=<(52 #
&2'(' $1' /2:=3$66 ),/7(5(' +( /2:=3$66 ),/7(5 +$6 $ &872))
$7 < $1' ,6 86(' 72 $77(18$7( 63(&75$/ &20321(176 7+$7
:28/' &$86( 81'(6,5$%/( (0,66,216 $%29( < 7;3,&$/
(;( ',$*5$0 $7 7+( 287387 2) 7+( ,6 6+2:1 ,1 ,* PMD Receiving. !+(1 5(&(,9,1* '$7$ 7+( /2:=3$66 ),/7(5 ,1 7+(
5(-(&76 287=2)=%$1' 12,6( 21 7+( 7:,67(' 3$,5 +( 6,*=
1$/6 21 7+( )285 &+$11(/6 $5( 7+(1 (48$/,<(' +,6 &203(1=
6$7(6 )25 7+( $77(18$7,21 2) 7+( &$%/( $1' 0,1,0,<(6 7+( ,1=
7(56;0%2/ ,17(5)(5(1&( $7 7+( 6$03/,1* 32,17 2) 7+( '$7$ 2
3(5)250 7+,6 )81&7,21 )25 $1; &$%/( /(1*7+ %(7:((1 $1'
0 7+( (48$/,<(5 0867 %( $'$37,9( +( 35272&2/ 3529,'(6
$ 75$,1,1* 6(48(1&( '85,1* :+,&+ 7+( (48$/,<(5 75$,16
,76 5(63216( 72 &203(16$7( )25 7+( /(1*7+ 2) &$%/( 35(6(17 ,1
7+( /,1. 7;3,&$/ (;( ',$*5$0 $7 7+( 287387 2) 7+( 5(&(,9(5
Four Twisted Pairs
30000000
Receive Filter
(55 M Quintets)
4 × 30 MBaud
Unscrambled
Scrambled
PMD
Receiving
Equalizer
Number of Quintets
20000000
4 × 30 Mbits/s
NRZ Decoder
MII
100 MBits/s
5B/6B Decoder
Descrambler
4 × 25 Mbits/s
10000000
Preamble and Delimiter
Removal
4 × 30 Mbits/s
0
0
PMI Receiving
03/(0(17$7,21 %/2&. ',$*5$0 +( ,6 7+( 0(',80 ,1'(=
3(1'(17 ,17(5)$&(
10
20
Quintet Value
30
,675,%87,21 2) 48,17(76 2%7$,1(' )520 5($/ 75$)),&
8*867 (:/(77=$&.$5' 2851$/
%7)?" ;%7 1%-28%-2)( ))4-2+ 8,) &%9( 6%8) 03; 1%/)7
-8 )%7= 83 -140)1)28 -2 % 78%2(%6( ! 463')77 8,)6)&=
1))8-2+ 8,) 03;?'378 6)59-6)1)28 ",) -2'6)%7)( (%8% 6%8) -7
1%-20= %886-&98%&0) 83 136) )**-'-)28 &03'/ '3(-2+ %2( &)88)6
97) 3* 8,) %:%-0%&0) 8;-78)(?4%-6 ',%22)07
",) 46383'30 97)7 '328630 7-+2%07 83 86%27*)6 6)59)787 %'?
/23;0)(+1)287 %2( 86%-2-2+ 7-+2%07 &)8;))2 8,) ,9& %2(
8,) )2( 23()7 ",)7) '328630 7-+2%07 1978 &) '328-293970=
%:%-0%&0) 32 %00 2)8;36/ 0-2/7 ;,)2):)6 (%8% -7 238 &)-2+
86%27*)66)( ",)= 1978 &) *900?(940)< %2( (-78-2'8 *631 %2=
(%8% 4%88)62
"6%271-77-32 )=) (-%+6%1
)59%0->)6 -7 7,3;2 -2 -+ 2 8,-7 '%7) 8,) '%&0) 0)2+8,
;%7 1 ;,-', %4463<-1%8)7 % ;3678?'%7) ?1 '%&0)
*8)6 )59%0->%8-32 8,) 6)')-:)( 7-+2%07 %6) 7%140)( ",)
6)'3:)67 % '03'/ *631 8,) 6)')-:)( 7-+2%07 &= 97-2+ 8,)
46)%1&0) 4%88)62 +)2)6%8)( &= 8,) 86%271-88-2+ ",-7
'03'/ -7 97)( 83 7%140) 8,) 6)')-:)( (%8% ",) *396 ',%22)07
%6) 3*8)2 1-7%0-+2)( -2 8-1) &= 94 83 8;3 &-8 4)6-3(7 ;-8,
6)74)'8 83 )%', 38,)6 %*8)6 86%:)0-2+ 3:)6 % 0)2+8, 3* #"
'%&0) ",-7 -7 &)'%97) 8,) 8;-78 6%8) %2( ,)2') 8,) 4634%+%?
8-32 ()0%= :%6-)7 *631 32) 8;-78)( 4%-6 83 %238,)6 ",) 6)%0-+27 8,) 6)')-:)( 7-+2%07 83 % '31132 '03'/-2+ 43-28
&)*36) 4%77-2+ 8,) *396 4%6%00)0 786)%17 3* (%8% 83 8,) PMI Receiving. ",) *968,)6 6)%0-+27 8,) 6)')-:)( (%8% 83
6)13:) %2= 7/); &)8;))2 8,) 78%68 ()0-1-8)67 32 )%',
786)%1 ",) (%8% 32 )%', 786)%1 -7 8,)2 ()'3()( 92?
7'6%1&0)( %2( 6)*361%88)( -283 % 7-2+0) (%8% 786)%1 ;,-',
-7 4%77)( 83 8,) 6)')-:-2+ ",) %073 4)6*3617 %
291&)6 3* )6636?',)'/-2+ *92'8-327 ",)7) 463:-() )<86%
4638)'8-32 %+%-278 )6636)( 4%'/)87 &)-2+ %'')48)( %7 :%0-(
&= 8,) %&3:) 8,) 4638)'8-32 3**)6)( &= 8,) *6%1) ',)'/
7)59)2') ",) *-678 ',)'/7 8,%8 8,) 78%68 ()0-1-8)67 32
)%', 786)%1 %6) %00 :%0-( %2( 3''96 ;-8, 8,) '366)'8 8-1)
6)0%8-327,-4 83 )%', 38,)6 ",)2 ;,-0) ()'3(-2+ -8 ',)'/7
8,%8 320= :%0-( ?&-8 '3();36(7 %6) 6)')-:)( 8 7-+2%07 83 8,)
-* %2 )6636 -7 ()8)'8)( -2 8,) 6)')-:)( (%8%
9%68)8 7-+2%0-2+ 1))87 %00 8,) ()7-+2 +3%07 -()28-*-)( *36 8,)
$ %4%68 *631 '328630 7-+2%0-2+ ()7'6-&)( 2)<8 = 6)8%-2?
-2+ % &-2%6= 7-+2%0-2+ 7',)1) 8,) 7-140-'-8= %2( 63&9782)77 3*
!-2') 8,) '328630 7-+2%07 %6) 6)59-6)( 83 34)6%8) '328-293970=
7-+2%0-2+ %8 8,) (%8% 6%8) 3* 1)+%&%9( ;%7 6).)'8)( &)?
'%97) 3* 8,) 0%6+) 0):)07 3* '6377 8%0/ 8,%8 1-+,8 &) +)2)6%8)(
;,)2 ?4%-6 '%&0)7 ;)6) 97)( 278)%( % 03;?*6)59)2'=
7-+2%0-2+ 7',)1) ,%7 &))2 ():)034)( 8,%8 %003;7 %00 *-:)
'328630 78%8)7 83 &) 86%271-88)( 97-2+ 320= 7-+2%07 ;-8, *92?
(%1)28%0 *6)59)2'-)7 0)77 8,%2 > 3 :%0-( (%8% 4%88)62
'328%-27 '31432)287 ;-8, 79', 03; *6)59)2'-)7 463:-(-2+
(-78-2'8 -()28-8= *36 8,) '328630 7-+2%07 900?(940)< 34)6%8-32
-7 %',-):)( &= 97-2+ 8;3 4%-67 -2 )%', (-6)'8-32 83 '%66=
'328630 7-+2%07
";3 '328630 7-+2%07 %6) ()*-2)( ! -7 % ?> 759%6)
;%:) *361)( &= 6)4)%8-2+ % 4%88)62 3* 7-<8))2 7 *3003;)(
&= 7-<8))2 7 %2( ! -7 % ?> 759%6) ;%:) *361)(
&= 6)4)%8-2+ )-+,8 7 *3003;)( &= )-+,8 7 396 3* 8,) *-:)
'328630 78%8)7 %6) 6)46)7)28)( &= '31&-2%8-327 3* ! %2(
! 32 8;3 4%-67 %7 7,3;2 -2 "%&0) †
",) *-*8, '328630 78%8) -7 6)46)7)28)( &= 7-0)2') 23 )2)6+=
32 8,) 8;3 4%-67 !-0)2') -7 97)( &= % ,9& 83 -2(-'%8) 8,%8 %
6)59)78 83 86%271-8 (%8% ,%7 &))2 +6%28)( 2 6)')-:-2+
7-0)2') %2 )2( 23() '%2 ')%7) '328630 7-+2%0-2+ %2( &)+-2
86%271-77-32 -11)(-%8)0= 7-2') 8,) '%&0) -7 %06)%(= 7-0)28
",-7 %003;7 6%4-( 8962%6392( 3* 8,) ,%0*?(940)< 0-2/
",) 8;3 759%6) ;%:)7 ,%:) &))2 ',37)2 83 ,%:) % 0%6+)
7)4%6%8-32 -2 *6)59)2'= 73 8,%8 8,) 6)')-:)6 -7 %&0) 83 (-78-2?
+9-7, 8,)1 ;-8,398 ,%:-2+ % '03'/ 8,%8 -7 46)'-7)0= 4,%7)?
03'/)( ",) '328630 7-+2%07 %6) +)2)6%8)( %2( 6)'3:)6)(
;-8,-2 8,) #" ",) !" ;%7 ():)034)( 83 7944368 )<-78-2+ 83/)2?6-2+
2)8;36/ '%&0-2+ ",)7) '327-78 3* '%&0)7 94 83 1 -2
0)2+8, ;-8, 8;3 7,-)0()( 8;-78)( 4%-67 ",) 348-'%0?*-&)6
;%7 ():)034)( 83 463:-() % 1)%27 3* '322)'8-2+ ,9&7
%2( )2( 23()7 3:)6 032+)6 (-78%2')7 8,%2 8,) 1 463?
:-()( &= 8,) !" %2( #" 7 ",-7 )<86% (-78%2') -7 4%6?
8-'90%60= -14368%28 ;,)2 '%7'%()( 2)8;36/7 %6) &9-08 %2(
,9&7 %6) (-786-&98)( 3:)6 % '%1497 %6)% ",) 348-'%0?*-&)6
%003;7 7 83 ,%:) 94 83 ?/1 (-%1)8)6
† The control signals shown in Table I are for the 100VG-AnyLAN implementation of the IEEE
802.12 standard. The control signal definitions in the standard have different names.
)')48-32 )=) (-%+6%1
9+978 );0)88?%'/%6( 3962%0
Table I
Control Signaling in 100VG-AnyLAN
Tone Wires
Transmitted from
Received by
1
2
End Node
Root Hub
Another Hub
End Node
Root Hub
Another Hub
CS1
CS1
IDLE
IDLE
IDLE
IDLE
IDLE
IDLE
CS1
CS2
REQ_N
INCOMING
REQ_N
INCOMING
REQ_N
INCOMING
CS2
CS1
REQ_H
ENABLE_HIGH_ONLY
REQ_H
Reserved
REQ_H
ENABLE_HIGH_ONLY
CS2
CS2
REQ_T
REQ_T
REQ_T
REQ_T
REQ_T
REQ_T
REQ_N = Normal-priority request
REQ_H = High-priority request
REQ_T = Link training request
INCOMING = A packet is about to be transmitted
ENABLE_HIGH_ONLY = Put normal round-robin sequence on hold while a high-priority request is serviced
STP has less attenuation and greater NEXT loss than even
Category 5, dataĆgrade UTP, and is already used for data
transmission at 100 Mbits/s. For example, the SDDI specificaĆ
tion4 provides for transmission of FDDI traffic at 100Mbits/s
over STP.
The STP physical layer uses the same modulation and codĆ
ing techniques as SDDI, that is, binary signaling at a rate of
100 Mbits/s (before coding). The low attenuation of the
cable makes it possible to transmit 100Mbits/s on a single
pair, so one pair is dedicated to transmitting and one pair to
receiving at each end of a link. The cable shield reduces
radiated emissions to satisfactory levels.
Multimode opticalĆfiber links have also been in use for many
years at data rates of over 100 Mbits/s (such as FDDI).5 The
opticalĆfiber PMD uses components that have recently been
developed for lowĆcost FDDI implementations. Examples are
the HFBR 5106 and HFBR 5107 developed by HP's Optical
Communications Division. These use LED transmitters with
wavelengths of either 850 nm (for 500Ćm links) or 1300 nm
(for 2Ćkm links). The transmitter forms part of a small module
that also includes an optical receiver. This module forms a
simple interface between the optical fiber and a transceiver
chip, which can be identical to the STP transceiver (see
OpticalĆFiber Links for 100VGĆAnyLAN" on page 26).
For the demand priority STP and opticalĆfiber PHYs, the
block code and NRZI coding specified for SDDI have been
replaced with an alternative scheme based on the same
5B/6B block code that is used for the UTP PHY. This apĆ
proach allows the 5B/6B block coder to be placed in the
PMI sublayer. The amount of logic that is common to all
physical layers is thereby increased, resulting in a lowerĆcost
PHY device.
The PMI provides four 30ĆMbit/s channels of scrambled and
5B/6B block coded data at the MII. These four channels are
multiplexed, codeword by codeword, by the STP and fiber
PMDs (see article, page 27). Even after codeword multiplexĆ
ing, the 5B/6B code retains its advantageous properties. The
serialized data stream is NRZ coded and transmitted at 120
megabaud on one pair using symbol levels of 0.25V for
STP media or passed to the optical module mentioned above
for opticalĆfiber cables.
The twoĆpair, twoĆtone control signaling developed for the
UTP PMD is not suitable for the STP and opticalĆfiber PMD
because only one pair per direction is available for control
signaling. Because cross talk is not an issue with STP or optiĆ
calĆfiber links, we were able to explore the use of higherĆfreĆ
quency signals. Square waves were again attractive because
they can easily be chosen to be distinct from valid data patĆ
terns.
One concern was that the control signals not produce harĆ
monics that, when transmitted on STP, might cause radiated
emissions that violate regulations. It was decided that the
control signal spectra should always fall below the random
data spectrum, since it was known that the data transmission
met EMC regulations. Calculations showed that square waves
with frequencies less than 4 MHz met this requirement. A
lower bound on the control signal frequency was set by the
need to avoid distortion of the square wave resulting from
transmission through the transformers used to couple to
twistedĆpair.
The final choice of control signals is five square waves with
frequencies between 1.875 and 3 MHz. Again, the control
signals are separated in frequency sufficiently to allow detecĆ
tion without a phaseĆlocked clock at the receiver.
The physical layer developed for 100VGĆAnyLAN local area
networks accommodates different cable types by means of
three different physical medium dependent (PMD) sublayers.
The major goal was to provide 100ĆMbit/s transmission on
the existing cable plant. The three PMDs allow LANs to opĆ
erate across the vast majority of LAN media installed today:
UTP (categories 3, 4, and 5), STP, and multimode fiber. As a
result, customer investment in structured cabling is protected
while at the same time an upgrade path to highĆspeed LANs
is created. All three PMDs are designed to be simple and
costĆeffective. Customers can now benefit from a factor of
up to eight improvement in the cost/performance ratio of
their LANs without the significant cost penalty of replacing
their cable plant.
August 1995 HewlettĆPackard Journal
! ! As data rates increase, low-cost optical-fiber links play an increasingly significant
role in LANs for extending the length of links beyond what can be achieved with
copper media, while meeting the full range of electromagnetic emission and susceptibility requirements for networks.
The 100VG-AnyLAN standard defines a serialized interface with a 120-megabaud
signaling rate for STP and multimode optical-fiber links. The standard defines two
optical-fiber link length specifications, which allow the use of low-cost 850-nm
technology for 500-m building backbones (this is the same technology used in the
existing IEEE 802 standard CSMA/CD 10Base-F and 802.5J token-ring links) and
1300-nm technology for 2-km campus backbone links.
Fig. 1 shows the new Hewlett-Packard low-cost industry-standard optical transceiver package. This small package is 1 inch wide and 1.5 inches long and has a
duplex SC optical connector on the front and a 1-by-9 row of electrical pins at the
rear. HP transceivers HFBR 5106/5107 meet the two 100VG-AnyLAN link length
standards and allow interchanging link technology in the same printed circuit board
footprint. The transceivers are also available with AT&T ST optical connectors to
address the large installed base having ST connectors for building and campus
backbones.
Del Hanson
Principal Engineer, Fiber-Optic
Networks and Standards
Optical Communications Division
Fig. 1. HP optical transceiver.
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