Doug Kerr
Well-known member
I noted this morning that the new Canon EOS-1D X body provides a LAN interface, accessed by what was described as an "RJ-45" connector.
This designation has a sad and curious history, and I thought I would talk a little bit about it.
First, some background.
The "RJ-" series of designations emerged as extension of the Universal Service Order Code (USOC) system, a system of alphanumeric designations used in the US telephone industry to designate equipment arrangements and functionalities in telephone station installations. They were used, among other places, in the orders for the installation of telephone stations (thus the name of the system), and on the records of what each customer had.
For example, in fairly modern times, "W" meant a wall-mounted telephone set. The ordinary desk telephone set had no USOC code; it was the default "hardware" as part of many types of telephone service (which each had their own USOC codes).
Earlier, however, a desk telephone set of the "modern" type was coded "HC". The "H" meant that it had a handset (as contracted to the separate receiver and transmitter that were characteristic of the "desk stand" telephone sets that were, at that time, the "default"). The "C" meant "combined"; that is, the transmission circuitry was inside the telephone set itself, rather than being in an external box, which most people thought only contained the ringer ("bell"). (These were both "upgrades" at the time.)
The codes defined overall physical arrangement and functionality, not specific "models" of equipment. Thus, in a Bell Telephone company, "W" might have been implemented with a Western Electric (of course) 354-type telephone set, or the later 554 type. An implication of this is that the customer didn't usually get to choose - they had arranged for a wall telephone set, and they would get one.
When regulatory changes provided (finally) for telephone customers to be able to buy their own telephone sets, and connect them to the holy switched network, standard industry interfaces had to be defined.
Each interface had a complete (sort of) electrical/logical definition, and in addition was physically implemented on a certain type of connector (jack).
Now, a customer planning to buy his own telephone set for a new home would have the telephone company provide not a certain style of telephone set but instead a standard interface. Thus the interfaces had to be given USOC codes. These were assigned in the "RJ-" series, "RJ" standing for "registered jack". The rationale was that, to the customer, what they wanted was a jack to plug their telephone set into.
Of course, the designation meant more than a certain type of jack. It referred to the entire interface (only one of whose properties was the kind of jack on which it terminated).
For example, RJ-11 referred to an interface that carried one traditional telephone line, with its electrical and logical characteristics codified, to be physically/electrically terminated in a female 6-position modular telecommunications connector. (Only two contacts were used.)
Then, an RJ-14 interface carried two conventional telephone lines, terminated on the same kind of connector. (Now, four contacts were used.)
Sadly, no "industry" designation came into use for this family of connectors (comparable, for example, to "D subminiature" or "BNC", or for the old-timers, "Amphenol 83").
*******
We are now ready to meet our beast.
The RJ-45 interface was very specialized. It provided access to a telephone line for the connection of a fancy type of "dialup" modem, with a special wrinkle. A resistor in the interface (accessed through two contacts of the jack) gave the modem an analog clue of the loss of the line from the station to the central office. The resistor was put in place (on screw terminals on the actual jack "block") during installation; the installation order told the installer what value to use, based on knowledge of the line by the cognizant engineering people.
The modem then "read" the resistance, and based on its observed value, set the transmitting output power to a certain value. The object was to have the signal at a standard level at the serving central office. That allowed as high a level as was prudent at the entry to the network interior, for optimum performance.
This interface terminated in an 8-position modular telecommunications connector, with a special feature. It had an "ear" ("index key") that stuck out in a certain place. The jack used had a notch to receive that. The object was that such a modem could not be (inappropriately) plugged into other interfaces that used the "standard" 8-position modular connector.
Now, eventually, the (normal, "earless") 8-pin modular telecommunications connector became the standard physical manifestation of various Ethernet over twisted pair data interfaces. But what to call it? There was no industry designation system.
Many people involved were familiar with the RJ-45 interface, and probably thought that it used a (normal) 8-position modular telecommunications connector. So they began referring to the (normal) 8-position modular telecommunications connector as an "RJ-45".
So to summarize:
• "RJ-45" does not properly refer to a connector type, but rather to a telephone line interface, physically terminating at a certain connector type.
• That connector type is not the type used for Ethernet interfaces.
The whole thing is as disappointing as the use of "APS-H" and "APS-C" to refer to digital camera sensor size families, or the use of the quantity "Ev" to refer to scene luminance.
******
By the way, standard industry nomenclature for this family of connectors has (belatedly) come into existence. The "Ethernet" connector we are speaking of is today called "8P8C MTC". "8P" means "8-position", and describes the overall body dimensions and layout as the one that can accommodate 8 contacts. "8C" means that 8 contacts are in fact populated. "MTC" means "modular telecommunications connector". The two genders are called "plug" and "jack".
Sometimes suffix letters are used to describe keying "ears", of which there are now several flavors. There is no good standardization of that.
Best regards,
Doug
This designation has a sad and curious history, and I thought I would talk a little bit about it.
First, some background.
The "RJ-" series of designations emerged as extension of the Universal Service Order Code (USOC) system, a system of alphanumeric designations used in the US telephone industry to designate equipment arrangements and functionalities in telephone station installations. They were used, among other places, in the orders for the installation of telephone stations (thus the name of the system), and on the records of what each customer had.
For example, in fairly modern times, "W" meant a wall-mounted telephone set. The ordinary desk telephone set had no USOC code; it was the default "hardware" as part of many types of telephone service (which each had their own USOC codes).
Earlier, however, a desk telephone set of the "modern" type was coded "HC". The "H" meant that it had a handset (as contracted to the separate receiver and transmitter that were characteristic of the "desk stand" telephone sets that were, at that time, the "default"). The "C" meant "combined"; that is, the transmission circuitry was inside the telephone set itself, rather than being in an external box, which most people thought only contained the ringer ("bell"). (These were both "upgrades" at the time.)
The codes defined overall physical arrangement and functionality, not specific "models" of equipment. Thus, in a Bell Telephone company, "W" might have been implemented with a Western Electric (of course) 354-type telephone set, or the later 554 type. An implication of this is that the customer didn't usually get to choose - they had arranged for a wall telephone set, and they would get one.
When regulatory changes provided (finally) for telephone customers to be able to buy their own telephone sets, and connect them to the holy switched network, standard industry interfaces had to be defined.
Each interface had a complete (sort of) electrical/logical definition, and in addition was physically implemented on a certain type of connector (jack).
Now, a customer planning to buy his own telephone set for a new home would have the telephone company provide not a certain style of telephone set but instead a standard interface. Thus the interfaces had to be given USOC codes. These were assigned in the "RJ-" series, "RJ" standing for "registered jack". The rationale was that, to the customer, what they wanted was a jack to plug their telephone set into.
Of course, the designation meant more than a certain type of jack. It referred to the entire interface (only one of whose properties was the kind of jack on which it terminated).
For example, RJ-11 referred to an interface that carried one traditional telephone line, with its electrical and logical characteristics codified, to be physically/electrically terminated in a female 6-position modular telecommunications connector. (Only two contacts were used.)
The name "modular" came from the fact that this line of low-cost connectors was originally developed as part of the "modular telephone set" concept. That meant that the telephone set "body", the handset, and both cords, all equipped with connectors of the new style, were stocked separately, to be assembled to suit at the time of installation.
Then, an RJ-14 interface carried two conventional telephone lines, terminated on the same kind of connector. (Now, four contacts were used.)
Sadly, no "industry" designation came into use for this family of connectors (comparable, for example, to "D subminiature" or "BNC", or for the old-timers, "Amphenol 83").
*******
We are now ready to meet our beast.
The RJ-45 interface was very specialized. It provided access to a telephone line for the connection of a fancy type of "dialup" modem, with a special wrinkle. A resistor in the interface (accessed through two contacts of the jack) gave the modem an analog clue of the loss of the line from the station to the central office. The resistor was put in place (on screw terminals on the actual jack "block") during installation; the installation order told the installer what value to use, based on knowledge of the line by the cognizant engineering people.
The modem then "read" the resistance, and based on its observed value, set the transmitting output power to a certain value. The object was to have the signal at a standard level at the serving central office. That allowed as high a level as was prudent at the entry to the network interior, for optimum performance.
This interface terminated in an 8-position modular telecommunications connector, with a special feature. It had an "ear" ("index key") that stuck out in a certain place. The jack used had a notch to receive that. The object was that such a modem could not be (inappropriately) plugged into other interfaces that used the "standard" 8-position modular connector.
Now, eventually, the (normal, "earless") 8-pin modular telecommunications connector became the standard physical manifestation of various Ethernet over twisted pair data interfaces. But what to call it? There was no industry designation system.
Many people involved were familiar with the RJ-45 interface, and probably thought that it used a (normal) 8-position modular telecommunications connector. So they began referring to the (normal) 8-position modular telecommunications connector as an "RJ-45".
So to summarize:
• "RJ-45" does not properly refer to a connector type, but rather to a telephone line interface, physically terminating at a certain connector type.
• That connector type is not the type used for Ethernet interfaces.
The whole thing is as disappointing as the use of "APS-H" and "APS-C" to refer to digital camera sensor size families, or the use of the quantity "Ev" to refer to scene luminance.
******
By the way, standard industry nomenclature for this family of connectors has (belatedly) come into existence. The "Ethernet" connector we are speaking of is today called "8P8C MTC". "8P" means "8-position", and describes the overall body dimensions and layout as the one that can accommodate 8 contacts. "8C" means that 8 contacts are in fact populated. "MTC" means "modular telecommunications connector". The two genders are called "plug" and "jack".
Sometimes suffix letters are used to describe keying "ears", of which there are now several flavors. There is no good standardization of that.
Best regards,
Doug