BEL AN12012

Bel Stewart
May 1
Network Connectors Interfaces and
Differential to Common Mode Conversion
Bel Stewart Connector 11118 Susquehanna Trail South Glen Rock, PA 17327
Tel: 717.235.7512
Page 1
Network Connector Interfaces and Differential to Common Mode
The paper describes standard connector interfaces used in the networking
including category 6a RJ45 and category 7a ARJ45. It discusses the differences
between connectors within the networking equipment and premise wiring channels
that share the same interface. The paper provides data for the Transverse
Conversion Loss contribution of the interfaces within the twisted transmission media
up to 2000 MHz.
The effects of the Differential to Common Mode Conversion (DCMC) on the
network performance are discussed . The TCL can be reduced by utilizing a
balanced RJ45 interface and further improved by utilizing a higher level category 7a
interface. The Ethernet systems from 1 to 40 GbE shall benefit from the reduction of
the TCL and corresponding common mode noise
Internet over Copper Wire Channels
Close to 1 billion users are connected to the Internet today. Many of them are
connected through copper-wire channels either leading to a final user or a part of
the infrastructure. The Internet is built upon the standardized Ethernet protocols
described in the IEEE 802.3 standards. The move toward the 40 and 100 GbE
requires better and faster connectivity and lower noise.
The Ethernet signals utilize differential mode transmission. Each differential
channel consists of two conductors, and requires a balance within the transmitter
or receiving pair, or, in electrical terms, the characteristic impedance of each
conductor within the channel shall be equal, typically 50 Ohm. If the balance is
violated that results in transformation of a portion of the original signal energy
into the parasitic common mode noise. The common mode conversion worsens
the transmission by reducing the signal strength and adding to the noise.
In order to focus on the relation between the interfaces and DCMC we limit this
discussion to the application spectra above 500 MHz.
Interfaces and Connectors
The interface between a network appliance and the premise wiring is an interface
between the building cabling infrastructure and equipment such as a switch or
computer. For the user, it is simply a port where a patch cord is plugged into the
network device. Unfortunately it is not simple. In fact, there are two networking
interfaces, which have rather complex electrical and mechanical characteristics
where the mechanical structures directly impact the transmission performance.
The networking equipment connectors are not covered by the premise wiring
Bel Stewart Connector 11118 Susquehanna Trail South Glen Rock, PA 17327
Tel: 717.235.7512
Page 2
The first interface is a connector; most often an RJ45 or ARJ45 (augmented
RJ45. It is usually a receptacle (a jack) at the network appliance side, and a plug
at the Premise Wiring cable side.
The first interface is the interruption and disturbance of the twisted pair media.
The second interface is a transformer that provides a safety barrier between
cables and a networking appliance. Often transformers, other magnetic elements
and other signal conditioning components are incorporated into the connectors:
such connectors are referred to as Integrated Connector Modules (ICM).
As a result, the connectors specified by TIA-568 series and ISO/IEC standards
for the premise wiring have the same name as network equipment connectors
but look and function differently. What they have in common are the interfaces.
The interface is subject to mechanical stresses and abuses. It is subject to
electrical discharges due to connect and disconnect under the electrical load, in
particular, in the Power-over-Ethernet (POE) applications.
And yet the interfaces have to be inexpensive and therefore not complex, must
be robust and be easy to use by millions of people.
FIGURE 1 shows the most common connector interfaces for and applicable
ISO/IEC standards. The jacks are shown. Table 1 lists the transmission classes
and typical applications.
Table 1. Transmission classes, categories and interfaces
class ISO/IEC
Typical Application
Class C
16 MHz
IEEE 802.5 TokenRing
RJ 45
Class D
100 MHz
10 to 1000baseT Ethernet
Class E
Class Ea
250 MHz
500 MHz
100-1000 baseT
1 to 10 GbE
Class F
600 MHz
Class Fa
1000 MHz
2000 MHz
1GbE over single pair
10 GbE
10 Gigabit over 2 pairs
10 GbE over 100 m
10 to 40 GbE
Bel Stewart Connector 11118 Susquehanna Trail South Glen Rock, PA 17327
GG45, ARJ45
Tel: 717.235.7512
Page 3
IEC standard
contains a switch
IEC standard
600 to 1000 to 2000 MHz
IEC standard
3 to 500 MHz
Figure 1. The connector interfaces and applicable international standards
The basic connector is a 12-contact category 7 connector described in the
ISO/IEC 60603-7-7. Its opening’s dimensions and contact positions were derived
from the traditional RJ45 jack. The 8-contact RJ45 and AR45 connectors are its
subsets. With an exception of the presence of bottom contacts all the dimensions
of ARJ45 are identical to RJ45.
The category 7 connector has a mechanical switch inside that would redirect the
signals from traditional split pairs 3/6 and 4/5 to new pairs located on the
opposite side of the cavity.
A category 7 jack can accept either category 6a or lower RJ45 plugs or category
7a and 7a plugs. The same plug is used for category 7 and 7a connectors. This
plug has a keying feature that prevents them from mating with Category 5e, 6
and 6a jacks. A category 7 and 7a plug has the front protrusion that activates a
mechanical switch within the category 7 connector, shown in figure 2.
The same patch cord can have a category 6a plug on one side and a category 7a
plug on another.
Bel Stewart Connector 11118 Susquehanna Trail South Glen Rock, PA 17327
Tel: 717.235.7512
Page 4
The category 7a connector ARJ45 (augmented RJ45) does not have any split
pairs. An alternative category 7a connector that was not derived from the RJ45
interface is described by ISO 61076-3-114.
Category 6a
Category 7a
Figure 2. Category 7a to 6a plugs combined in a single patch cord
Compensation vs. Isolation
The Major Difference between Category 7a and Lesser Categories.
The major difference between RJ45 and ARJ45 connectors and interfaces is how
the differential Near End Cross Talk is attenuated. That issue was one of the
prime factors why the International Standards Organization decided to change to
a new interface for applications above 500 MHz in the IEC/ISO standard
The RJ45 connector categories 5e to 6a use compensation to cancel the
differential NEXT. The compensation is a method of purposefully creating the
crosstalk in the near vicinity of the interface that is equal in amplitude but
opposite in phase to the NEXT “native” to the interface. In practice, it calls for
adding capacitive and some inductive elements and creating complex structures
within the connectors. The compensation in the RJ45 connectors is used in both:
shielded and unshielded designs.
The category 7a connectors rely on isolation to attenuate the NEXT. Category 7a
connectors are always shielded. A Faraday cage is built around each differential
pair that isolates them from each other, thus reducing NEXT. The ARJ45
Bel Stewart Connector 11118 Susquehanna Trail South Glen Rock, PA 17327
Tel: 717.235.7512
Page 5
interface design allows the isolation to be extended through a plug and a
An additional and valuable benefit of an isolation method design is a dramatic
improvement in the Transverse Conversion Loss in comparison to lesser
Transverse Conversion Loss.
The measure of the balance and Differential to Common Mode conversion
applicable to the interface is described in the connecting hardware standards as
the Transverse Conversion Loss (TCL), or Sdc11 as an S-parameter.
TCL (dB) = 20 LOG
Common Mode Voltage
(measured at the same end)
Differential Mode Voltage
It is useful to keep in mind that TCL is the measure of the quality of the interface
as a source of common mode.
Table 2. TCL in the standards
28-20Log (f/100)
68-20 Log(f)
Frequency Range, MHz
1 to 500
1 to 1000
Both standard expressions yield the same values shown in table 3 below.
Bel Stewart Connector 11118 Susquehanna Trail South Glen Rock, PA 17327
Tel: 717.235.7512
Page 6
Table 3. TCL values (dB) as specified in the IEC/ISO standards
Balanced Twisted Pair Environment
Outside the connector contacts proper, that form the mechanical interface, the
regions directly adjacent to the interface are twisted pairs.
On one side it is a patch cord, on another side it is a high performance LAN
transformer. The transformer windings made of twisted pairs are shown in
Figure 3
Figure 3. High Performance 10GbE LAN transformer utilizes twisted pairs
Bel Stewart Connector 11118 Susquehanna Trail South Glen Rock, PA 17327
Tel: 717.235.7512
Page 7
The differential pairs of the high performance 10GbE LAN transformer winding
are twisted. In addition to reducing the inter-winding capacitance which is always
parasitic, the twisting directly improves the DCMC. Also, the highest performing
LAN magnetics both chokes and transformers tend to use a controlled media,
such as ferrite, on all sides of the channels within the windings.
The impedance of a balanced twisted pair is derived from the impedances of two
conductors, as shown in figure 4. The differential impedance in this case does
not have a resistive component and consists of the inductance and capacitance.
Z1 = Z 2
Z d = Z 1 +Z 2
Figure 4. Differential Impedance of a balanced twisted pair
Bel Stewart Connector 11118 Susquehanna Trail South Glen Rock, PA 17327
Tel: 717.235.7512
Page 8
RJ45 contacts
Figure 5.
The prime source of TCL is change in the geometry defining the characteristic
impedance on the interface. Compare figures 4 and 5 . The Figure 5 is an
electrical structure of RJ45 - one the most popular connector in the world where
differential pairs defined as contacts 1-2, 3-6, 4-5, 7-8.
The positions of the contacts in the RJ45 interface are such as there is explicit
imbalance. The pair 45 is intrinsically better balanced as well as 36, though their
impedance is hard to match to each other. The intra-pair balance of differential
pairs 1-2 and 7-8 are affected by their positions where contacts 2 and 7
capacitively coupled to adjacent contacts, and
Z1 ≠ Z 2
The result is a skew and the degradation of the TCL.
There are other sources of imbalance. All category 6a connectors are supposed
to meet the minimum TCL requirements. Within the plug the patch cord is
changed from the twisted to the interface geometry. The balanced plug
incorporates the design where at least a portion of the transmission channel
within the plug is balanced or controlled. The technology, sometimes referred to
as a wire guide filter, in which pairs are guided through the plug to mimic the
balance of a twisted pair cable.
Bel Stewart Connector 11118 Susquehanna Trail South Glen Rock, PA 17327
Tel: 717.235.7512
Page 9
Thus, within the Category 6a connectivity there are subsets of balanced and
unbalanced connectors.
The category 7a connectors maintain the balance and low TCL to the spectra
well above 1000 MHz
Negative Effects of High TCL
The high TCL causes greater common mode noise. The common mode noise
reduces the signal energy. The common mode signal generally has a different
propagation velocity from the differential signal. It also has an arbitrary phase.
Two common noise signals can form an effectively parasitic differential signal
superimposed on the original useful transmission. The common noise causes
EMI and jitter. As a contributor to the Alien NEXT it may cause bit errors.
The shorter the channel length the greater are negative effects of the TCL
The reduction in the TCL and corresponding DCMC is translated in a lower noise
and potential increases in the useful channel length.
The direct contribution of the TCL to the channel length heavily depends on the
cable quality, specifically the cable’s insertion loss. Though the experimental
evaluation is still in progress, the estimated improvement in length due to 10 dB
reduction in the TCL can be as high as 10m for 500 MHz applications
Transverse Conversion Loss: Comparative Test Data
The computer modeling conducted by Bel’s R&D, using the coaxial structures,
established a theoretical limit for Sdc11 as 98 dB at 1000 MHz.
Table 4 provides a comparison of the TCL values for balanced and unbalanced
category 6a connectors and category 7a ARJ45 connectors. The TCL limit is
used instead of data of unbalanced Category 6a connectors.
Table 4. Test data: selected (worst) TCL values of Network Connectors, dB
Frequency, MHz
Category 6a
Bel Stewart Connector 11118 Susquehanna Trail South Glen Rock, PA 17327
Category 7a
Tel: 717.235.7512
Page 10
Figure 6. TCL of balanced category 6a connector with a Wire Guide Filter
Figure 7
TCL of category 7a ARJ45 connector
Bel Stewart Connector 11118 Susquehanna Trail South Glen Rock, PA 17327
Tel: 717.235.7512
Page 11
The Transverse Conversion Loss defines the interface as a source of Differential
to Common Mode Conversion. The common mode noise negatively affects the
network transmission. The Internet traffic from 1 GbE to 10 GbE and, in
particular, new 40 GbE systems should benefit from balanced category 6a
connectors. In the frequency spectra above 500 MHz the category 7a
connectors can provide improvement of over 15 dB in comparison to category 6a
and extend the channel bandwidth to 2000 MHz and above
Yakov Belopolsky, Manager, Research & Development, Member of the US TAG and
a member of the ISO/IEC committees on Connectors and Cabling.
Yakov published over 30 technical papers and awarded 78 US patents.
Rich Marowsky,
Principal Electrical Engineer, Member of the TIA committees.
Rich has over 35 years experience in Development, Simulation, and Testing High Speed
Connector Products for Computer Systems and the Premise Wire Industry
Bel Stewart Connector, Glen Rock, PA , USA
Bel Stewart Connector 11118 Susquehanna Trail South Glen Rock, PA 17327
Tel: 717.235.7512
Page 12