Bel Stewart
Connector
May 1
2012
Network Connectors Interfaces and
Differential to Common Mode Conversion
AN12012
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
Conversion
Abstract
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
standards.
Bel Stewart Connector 11118 Susquehanna Trail South Glen Rock, PA 17327
Tel: 717.235.7512
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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
Transmission
class ISO/IEC
standards
Class C
Class D
Class E
Class Ea
Class F
Class Fa
NA
Connector
category
Frequency
bandwidth
Typical Application
Connecting
Hardware
Interface
RJ 45
RJ45
RJ45
RJ45,
GG45, ARJ45
ARJ45,
ARJ45
3
5e
6
6a
7
7a
NA
16 MHz
100 MHz
250 MHz
500 MHz
600 MHz
1000 MHz
2000 MHz
IEEE 802.5 TokenRing
10 to 1000baseT Ethernet
100-1000 baseT
1 to 10 GbE
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
Tel: 717.235.7512
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STANDARD CONNECTOR INTERFACES
IEC standard
60603-7-7
contains a switch
IEC standard
61076-3-110
ARJ45
600 to 1000 to 2000 MHz
IEC standard
60603-7
RJ45
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
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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
60603-7-7.
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
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