Registered Jack

In the mid 1960’s the Bell System companies used the 505A plug, a round connector with four prongs.  We’ve moved to the de facto standard of Registered Jacks.

Common Registered Jacks

Code Connector Usage
RJ2MB 50-pin micro ribbon 2–12 telephone lines with make-busy arrangement
RJ11(C/W) 6P2C For one telephone line (6P4C if power on second pair)
RJ12(C/W) 6P6C For one telephone line ahead of the key system
RJ13(C/W) 6P4C For one telephone line behind the key system
RJ14(C/W) 6P4C For two telephone lines (6P6C if power on third pair)
RJ15C 3-pin weatherproof For one telephone line
RJ18(C/W) 6P6C For one telephone line with make-busy arrangement
RJ21X 50-pin micro ribbon For up to 25 lines
RJ25(C/W) 6P6C For three telephone lines
RJ26X 50-pin micro ribbon For multiple data lines, universal
RJ27X 50-pin micro ribbon For multiple data lines, programmed
RJ31X 8P4C Allows an alarm system to seize the telephone line to make an outgoing call during an alarm. Jack is placed ahead of all other equipment. (Only 4 conductors are used)
RJ38X 8P4C Similar to RJ31X, with continuity circuit. If the plug is disconnected from the jack shorting bars allows the phone circuit to continue to the site phones. (Only 4 conductors are used)
RJ41S 8P8C, keyed For one data line, universal (fixed loop loss and programmed)
RJ45S 8P8C, keyed For one data line, with programming resistor
RJ48C 8P4C For four-wire data line (DSX-1)
RJ48S 8P4C, keyed For four-wire data line (DDS)
RJ48X 8P4C with shorting bar For four-wire data line (DS1)
RJ49C 8P8C For ISDN BRI via NT1
RJ61X 8P8C For four telephone lines
RJ71C 50-pin micro ribbon 12 line series connection using 50-pin connector (with bridging adapter) ahead of customer equipment. Mostly used for call sequencer equipment.

Many of the basic names have suffixes that indicate subtypes:

  • C: flush-mount or surface mount
  • F: flex-mount
  • W: wall-mount
  • L: lamp-mount
  • S: single-line
  • M: multi-line
  • X: complex jack

T1 Termination

The RJ48C is used for T1 service.
The RJ48C is used for T1 service.

An RJ-48 plug is often mistaken for RJ-45. On the outside, the two look identical—both are housed in miniature 8-position jacks. The difference is in the wire pairing. RJ-48 connectorIn RJ-48, two of the wires are for transmit, two are for receive, and two are for the drain. The last two wires are reserved for future use


An RJ48X is wired like an RJ48C but shorting bars are added so when nothing is plugged into the jack, the shorting bars loop back the line toward the far end (pin 1 shorts to pin 4 and pin 2 shorts to pin 5.)
An RJ48X is wired like an RJ48C but shorting bars are added so when nothing is plugged into the jack, the shorting bars loop back the line toward the far end (pin 1 shorts to pin 4 and pin 2 shorts to pin 5.)


Three subsets

There are three subsets within RJ-48: RJ-48C, RJ-48X, and RJ-48S. RJ-48C and RJ-48X are very similar, though RJ-48C is more common. Both use lines 1, 2, 4, and 5, and connect T1 lines. RJ-48X connectors, however, have shorting bars. RJ-48S uses lines 1, 2, 7, and 8, and generally connects 56K DDS lines.

Here’s how RJ-48C pinning compares to RJ-48S pinning:

Pin RJ-48C RJ-48S
1 Receive ring Receive data +
2 Receive tip Receive data –
3 No connection No connection
4 Transmit ring No connection
5 Transmit tip No connection
6 No connection No connection
7 No connection Transmit data +
8 No connection Transmit data –

(*T568B is equivalent to AT&T 258A so for reasons of tradition, it’s likely the wire scheme the telco is going to drop off)


DS0 / DDS Termination

An RJ48S is used for subrate data services or a 56K DS0.
An RJ48S is used for subrate data services or a 56K DS0.

DMARC / Network Interface Devices

The demarcation point (DMARC) is the point at which the public switched telephone network (PSTN)ends and connects with the customer’s on-premises wiring (called Inside Wire or IW.)  Don’t confuse this with a NIU, it’s the same but different.

For residential locations you might have started off with an old Western Electric Company (WECO) lightning protector:

Lightning Protector
Used around 1915. The phone company line from the pole connected at the top binding posts marked “L” for “Line”, the house wiring connected to “I” for “Inside” and G for Ground. In later years the Bell System added copper straps that bypassed the fuses… unsure why.
The Western Electric Type 58A Protector, circa 1900, protects against lightning and other high voltages.
The Western Electric Type 58A Protector, circa 1900, protects against lightning and other high voltages.


After that they went to the carbon and porcelain blocks which were much smaller:

The carbon and porcelain block! In the presence of high voltage, the carbon blocks under the pressure of the spring would be moved to earth after the restraining glue melted. The whole thing had to be replaced.
The carbon and porcelain block! In the presence of high voltage, the carbon blocks under the pressure of the spring would be moved to earth after the restraining glue melted. The whole thing had to be replaced.
In the 1980s, Ma Bell started using a gas-filled protector. In the presence of high voltage, the gas ionizes and provides a path to earth (ground). When the voltage is removed, the protector returns to its normal state. Not a one shot deal like the older carbon ones.

 They got fancy with mounts  & attached them to the side of your house:

Use before the advent of Network Interfaces.
Use before the advent of Network Interfaces.

Then we come to the modern Network Interface Device, Telephone Network Interface, NID, NI, dmarc:

Modern NID


NID with Telco access opened.
NID with Telco access opened.

Check out


What about businesses?  Do they just have 66 of these on the wall, one for each line?

Don’t be crazy,  they use a 66 block / M-Block / B-Block as a type of punchdown block to terminate the line.

Introduced in 1962, the term "66 block" simply reflects its Western Electric model number.
Introduced in 1962, the term “66 block” simply reflects its Western Electric model number.

They could also have a 110 block:

110 style blocks allow a much higher density of terminations in a given space than older style termination blocks
110 style blocks allow a much higher density of terminations in a given space than older style termination blocks.

Networking Telecommunications


A Data Service Unit/Channel Service Unit (DSU/CSU) WAN Interface Card ( WIC) these days is usually a blade on a router.  In the past, these were separate.  The CSU originated at AT&T as an interface to their non-switched digital data system. The DSU provides an interface to the data terminal equipment (DTE) using a standard (EIA/CCITT) interface. It also provides testing capabilities.  They evolved from standalone hardware, to shelf type systems and are now just a blade or Wan Interfacde Card (WIC) in a router.

Internal CSU/DSU WIC
Internal CSU/DSU WIC


External CSU/DSU Universal Shelf
External CSU/DSU Universal Shelf


External CSU/DSU
External CSU/DSU


The functions of the LEDs

WIC-1DSU-56K4 Front Panel DSU/CSU WIC to a 56/64-kbps Services Wall Jack RJ48S

LED Description
TD Data is being transmitted to the DTE interface.
RD Data is being received from the DTE interface.
LP Internal DSU/CSU is in loopback mode.
AL One of these alarm conditions is present: no receive signal, loss of frame signal from the remote station, or out of service signal from the remote station. This LED is off during normal operation.
CD Internal DSU/CSU in the WIC is communicating with another DSU/CSU. This LED is on during normal operation
WIC-1DSU-T1 Front Panel
Push this button to place the WIC into loopback mode. The service provider can send a signal to test the connection from your site to the central office switch. Push this button again to turn off loopback mode.

DSU/CSU WIC to a 56/64-kbps Services Wall Jack RJ48S


There are many manufactures, each with their own ideas of abbreviations so TD, TX, or TXD all mean you’re transmitting data. You may not have every LED but in general…


Power, PWR Power
ERR, ER Error indicator
AL, ALARM Critical alarm indicator
Loop, LP Diagnostic loopback indicator
SYNC, RS DTE sync indicator (Receive signa from telco)
 TD, TX, or TXD Transmit data
 RD, RX, or RXD Receive data
 CTS Clear to send (per flow control)
 CLOS Carrier loss of signal
 RLOS Receiver loss of signal

What you could see

Scenario Power Err Alarm Loop Sync TD RD CTS CLOS RLOS Description
1 flash flash Normal – up and passing traffic
2 Loopback mode detected from telco or configured in CPE
3 flash flash flash Circuit is experiencing errors, but still passing traffic.
4 CSU detects a total disconnect. Circuit disconnected/no cable.
5 Carrier loss of signal. Possible timing, switch misconfiguration, or circuit degradation
6 Receiver loss of signal. Possible timing, switch misconfiguration
11 No Power

See Also:

DataSMART-558 DataSMART_558 DataSmart_500_Series


Telecom & Transport Speed Schemes

North American Digital Signal Hierarchy

In the 1960’s The Bell System / AT&T came up with a  transport system based off 64K (bits) “channels.”  To come up with 64K,  consider the  voice frequency (VF) or voice band frequencies used in  telephony, approximately 300 Hz to 3400 Hz.  You can refer to this as baseband or narrowband.  (Telecom likes to have several names for the same thing.)

A single voice transmission channel is about 4 kHz and is sampled at  8 kHz.  Why?  Because the  Nyquist theorem says so.

(4,000 Hz  is an adequate sweet-spot for human speech, it’s not exact but when reproduced at the other end, you can recognize it’s grandma when she calls.)

So what ya get is:

2 x 4K = 8K samples per second,  each one of those sample is/used 8-bit pulse-code modulation which ends up as 8K x 8 = 64K bits per second – a DS0. (It will be called D-S-O or D-S-Zero interchangeably.)

Robbed bits for signaling

Here’s the catch –  the low order bit is used for signaling purposes.  For voice this created noise that you really can’t hear so Ma Bell didn’t care.  For digital data you can’t fudge it like that, only 7 bits can be used. 8,000 7-bit samples gives you  56 kbps. Today you can get around this using different line codes and bit stuffing.


So the hierarchy using a DS0 of 64K or 64,000 bits per second:

Hierarchy Speed Digital
Carrier DS0’s Notes
1st 1.544 Mbit/s DS1 T-1 24 In ISDN PRI = 23B (user) + 1D (signaling) channels
IntermediateLevel 3.152 Mbit/s DS1C   48 DS1C uses two DS1 signals combined and sent on a 3.152 megabit per second carrier which allows 64 kilobits per second for synchronisation and framing using pulse stuffing.  Never common, you won’t see this in use.
2nd 6.312 Mbit/s DS2 T-2 96 4 x DS1. Never common, you won’t see this in use.
3rd 44.736 Mbit/s DS3 T-3 672 28 x DS1
Intermediate Level 139.264 Mbit/s DS4NA   2016 3 x DS3 Highest designed in ANSI T1.107
4th Level 274.176 Mbit/s DS4 T-4 4032 Replaced with Optical Carrier / OCx
5th Level 400.352 Mbit/s DS5 T-5 5760 Replaced with Optical Carrier / OCx
HOLD UP –  24 x  64,000 bits per second won’t get you 1.544 Mbit/s. What you have is 24 x 64,000 = 1.536 Mbit/s. Bits are lost between frames because a frame separator is needed for every 8 bit sample of the of the 24 channels .  So yes, 24 x 8 = 192 but adding the separator, 193 bits per frame  x 8K samples = 1.544 Mbit/s.
Speed DS0 Carrier
64 Kbps 1
1.544 Mbit/s 24 T-1
2.048 Mbit/s 32
6.312 Mbit/s 96 T-2
7.786 Mbit/s 120
8.448 Mbit/s 128
32.064 Mbit/s 480
34.368 Mbit/s 512
44.736 Mbit/s 672 T-3
97.728 Mbit/s 1440
139.264 Mbit/s 2016 DS4NA
139.264 Mbit/s 2048
274.176 Mbit/s 4032 T-4
400.352 Mbit/s 5760 T-5
565.148 Mbit/s 8192

Anything above a T3 is now optical/fiber.

Optical Carrier

SONET (Synchronous Optical Network) in North America or SDH (Synchronous Digital Hierarchy) elsewhere is the modern day optical transmission systems.  It’s nice because everything is a multiple of the OC-1 rate of 51.84 Mbps.

Hierarchy Data Rate SONET SDH OCx
Level Zero 155.52 STS-3 STM-1 OC-3
Level One 622.08 STS-12 STM-4 OC-12
Level Two 2488.32 Mbit/s STS-48 STM-16 OC-48
Level Three 9953.28 Mbit/s STS-192 STM-64 OC-192

Optical Carrier Rates

Optical Carrier Data Rate Payload-SONET (SPE) User Data Rate SONET SDH
OC-1 51.84 Mbit/s 50.112 Mbit/s 49.536 STS-1
OC-3 155.52 Mbit/s 150.336 Mbit/s 148.608 STS-3 STM-1
OC-9 466.56 Mbit/s 451.044 Mbit/s 445.824 STS-9 STM-3
OC-12 622.08 Mbit/s 601.344 Mbit/s 594.824 STS-12 STM-4
OC-18 933.12 Mbit/s 902.088 Mbit/s 891.648 STS-18 STM-6
OC-24 1244.16 Mbit/s 1202.784 Mbit/s 1188.864 STS-24 STM-8
OC-36 1866.24 Mbit/s 1804.176 Mbit/s 1783.296 STS-36 STM-12
OC-48 2488.32 Mbit/s 2.4 Gbps 2377.728 STS-48 STM-16
OC-192 9953.28 Mbit/s 9.6 Gbps 9510.912 STS-192 STM-64
OC-768 40Gbit/s STS-768 STM-256
OC-3072 160Gbit/s STS-3072 STM-1024

Virtual Tributary

To slice up the 51.84 Mbit/s, you can have a sub-STS-1 facilitie.  VT1.5 is  common because it can carry 1.728 Mbit/s (enough room for a DS1/T1 signal.)  There’s a lot of overhead in SONET.

Name Speed
VT-1.5 1.728Mbit/s
VT-2 2.304Mbit/s
VT-3 3.456Mbit/s
VT-6 6.912Mbit/s
STS-1 50.112Mbit/s
STS-3 150.336Mbit/s

Networking Telecommunications

Traffic Shaping

Traffic shaping (also known as “packet shaping”) is a computer network traffic management technique which delays some or all datagrams to bring them into compliance with a desired traffic profile. Traffic shaping is a form of rate limiting.

So… Let’s say you’re a business that processes credit cards.  You have 15 stores and 1 corporate headquarters that has your computer network.  They 15 stores only need a little bandwidth to send the transactions to HQ but the HQ needs more bandwidth to accept the data from all of the 15 stores.  What you have is a mismatch in bandwidth (or CIR, Committed Information Rate.)  The stores would only need a 56K DS0 (56,000 bits per second) circuit but the HQ would need a full T1 (DS1) running at 1.544 megabits per second to handle all the traffic coming from and going to the stores.

So the problem is, the HQ is a fire hose and the stores are a garden hose… You can’t spray a fire-hose into a garden-hose and not expect some water is going splash out.  In data, that water would be drops or lost packets.

Networks are often asymmetrical, that is, the access rate at one site may differ from the access rate at another. In such cases, it may be necessary to configure the faster rate to shape to the access rate of the slower rate.

To limit bandwidth, you can shape or you can police.

Traffic policing propagates bursts. When the traffic rate reaches the configured maximum rate, excess traffic is dropped (or remarked). The result is an output rate that appears as a saw-tooth with crests and troughs. In contrast to policing, traffic shaping retains excess packets in a queue and then schedules the excess for later transmission over increments of time. The result of traffic shaping is a smoothed packet output rate.


Policing Versus Shaping


Shaping implies the existence of a queue and of sufficient memory to buffer delayed packets, while policing does not. Queueing is an outbound concept; packets going out an interface get queued and can be shaped. Only policing can be applied to inbound traffic on an interface. Ensure that you have sufficient memory when enabling shaping. In addition, shaping requires a scheduling function for later transmission of any delayed packets. This scheduling function allows you to organize the shaping queue into different queues. Examples of scheduling functions are Class Based Weighted Fair Queuing (CBWFQ) and Low Latency Queuing (LLQ).

Simply stated, both shaping and policing use the token bucket metaphor. A token bucket itself has no discard or priority policy. Let’s look at how the token bucket metaphor works:

  • Tokens are put into the bucket at a certain rate.
  • Each token is permission for the source to send a certain number of bits into the network.
  • To send a packet, the traffic regulator must be able to remove from the bucket a number of tokens equal in representation to the packet size.
  • If not enough tokens are in the bucket to send a packet, the packet either waits until the bucket has enough tokens (in the case of a shaper) or the packet is discarded or marked down (in the case of a policer).
  • The bucket itself has a specified capacity. If the bucket fills to capacity, newly arriving tokens are discarded and are not available to future packets. Thus, at any time, the largest burst a source can send into the network is roughly proportional to the size of the bucket. A token bucket permits burstiness, but bounds it.

Use the police command to specify that a class of traffic should have a maximum rate imposed on it, and if that rate is exceeded, an immediate action must be taken. In other words, with the police command, it is not an option to buffer the packet and later send it out, as is the case for the shape command.

 This example shows the configuration of two traffic-shaped interfaces on a router. Ethernet interface 0 is configured to limit User Datagram Protocol (UDP) traffic to 1 Mbps. Ethernet interface 1 is configured to limit all output to 5 Mbps.

access-list 101 permit udp any any
interface Ethernet0
traffic-shape group 101 1000000 125000 125000
interface Ethernet1
traffic-shape rate 5000000 625000 625000