Chapter 2

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2. ELECTRICAL INTERFACE

2.1 Transmission Media The means through which data is transformed from one place to another is called transmission or communication media. There are two categories of transmission media used in computer communications.

Bounded / guided media

Unbounded / unguided media 2.1.1 Bounded media Bounded media are the physical links through which signals are confined to narrow path. These are also called guide media. Bounded media are made up o a external conductor (Usually Copper) bounded by jacket material. Bounded media are great for LABS because they offer high speed, good security and low cast. However, some time they cannot be used due distance communication. Three common types of bounded media are used of the data transmission. These are

Coaxial Cable

Twisted Pairs Cable

Fiber Optics Cable 2.1.1.1 Coaxial cable: Coaxial cable is very common & widely used commutation media. For example TV wire is usually coaxial. Coaxial cable gets its name because it contains two conductors that are parallel to each other. The center conductor in the cable is usually copper. The copper can be either a solid wire or stranded martial. Outside this central Conductor is a non-conductive material. It is usually white, plastic material used to separate the inner Conductor form the outer Conductor. The other Conductor is a fine mesh made from Copper. It is used to help shield the cable form EMI. Outside the copper mesh is the final protective cover. (as shown in Fig 2.1) The actual data travels through the center conductor in the cable. EMI interference is caught by outer copper mesh. There are different types of coaxial cable vary by gauge & impedance. Gauge is the measure of the cable thickness. It is measured by the Radio grade measurement, or RG number. The high the RG number, the thinner the central conductor core, the lower the number the thicker the core.

Fig 2.1 Coaxial cable Here the most common coaxial standards.

50-Ohm RG-7 or RG-11 : used with thick Ethernet. 50-Ohm RG-58 : used with thin Ethernet 75-Ohm RG-59 : used with cable television

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93-Ohm RG-62 : used with ARCNET

Characteristics of coaxial cable

Low cost Easy to install Up to 10Mbps capacity Medium immunity form EMI Medium of attenuation

ADVANTAGES COAXIAL CABLE

Inexpensive Easy to wire Easy to expand Moderate level of EMI immunity

DISADVANTAGE COAXIAL CABLE

Single cable failure can take down an entire network

2.1.1.2 Twisted pair cable The most popular network cabling is Twisted pair. It is light weight, easy to install, inexpensive and support many different types of network. It also supports the speed of 100 mps. Twisted pair cabling is made of pairs of solid or stranded copper twisted along each other. The twists are done to reduce vulnerably to EMI and cross talk. The number of pairs in the cable depends on the type. The copper core is usually 22-AWG or 24-AWG, as measured on the American wire gauge standard.

Fig 2.2 UTO & STP cable

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There are two types of twisted pairs cabling :

Unshielded twisted pair (UTP)

Shielded twisted pair (STP) a) Unshielded twisted pair (UTP)

UTP is more common. It can be either voice grade or data grade depending on the condition. UTP cable normally has an impedance of 100 ohm. UTP cost less than STP and easily available due to its many use. There are five levels of data cabling

o Category 1 o These are used in telephone lines and low speed data cable. o Category 2 o These cables can support up to 4 mps implementation. o Category 3 o These cable supports up to 16 mps and are mostly used in 10 mps. o Category 4 o These are used for large distance and high speed. It can support 20mps. o Category 5 o This is the highest rating for UTP cable and can support up to 100mps.

UTP cables consist of 2 or 4 pairs of twisted cable. Cable with 2 pair use RJ-11 connector and 4 pair cable use RJ-45 connector.

Characteristics of UTP

low cost

easy to install

High speed capacity

High attenuation

Effective to EMI

100 meter limit

Advantages of UTP

Easy installation

Capable of high speed for LAN

Low cost

Disadvantages of UTP

Short distance due to attenuation b) Shielded twisted pair (STP)

It is similar to UTP but has a mesh shielding that’s protects it from EMI which allows for higher transmission rate.

IBM has defined category for STP cable. o Type 1 o STP features two pairs of 22-AWG o Type 2 o This type include type 1 with 4 telephone pairs o Type 6 o This type feature two pairs of standard shielded 26-AWG o Type 7 o This type of STP consist of 1 pair of standard shielded 26-AWG o Type 9 o This type consist of shielded 26-AWG wire

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Characteristics of STP

Medium cost

Easy to install

Higher capacity than UTP

Higher attenuation, but same as UTP

Medium immunity from EMI

100 meter limit

Advantages of STP: Shielded

Faster than UTP and coaxial

Disadvantages of STP:

More expensive than UTP and coaxial

More difficult installation

High attenuation rate

2.1.1.2 Fiber Optics Fiber optic cable uses electrical signals to transmit data. It uses light. In fiber optic cable light only moves in one direction for two way communication to take place a second connection must be made between the two devices. It is actually two stands of cable. Each stand is responsible for one direction of communication. A laser at one device sends pulse of light through this cable to other device. These pulses translated into “1’s” and “0’s” at the other end. In the center of fiber cable is a glass stand or core. The light from the laser moves through this glass to the other device around the internal core is a reflective material known as CLADDING. No light escapes the glass core because of this reflective cladding. Fiber optic cable has bandwidth more than 2 Gbps (Gigabytes per Second)

Characteristics Of Fiber Optic Cable: Expensive

Very hard to install

Capable of extremely high speed

Extremely low attenuation

No EMI interference

Advantages Of Fiber Optic Cable: Fast

Low attenuation

No EMI interference

Disadvantages Fiber Optics: Very costly

Hard to install

2.1.2. Unbounded / unguided media Unguided Transmission Media is data signals that flow through the air. They are not guided or bound to a channel to follow. They are classified by the type of wave propagation.

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RF Propagation There are 3 types of RF (Radio Frequency) Propagation:

Ground Wave, Ionospheric and Line of Sight (LOS) Propagation.

Ground Wave Propagation follows the curvature of the Earth. Ground Waves have carrier frequencies up to 2 MHz. AM radio is an example of Ground Wave Propagation.

Fig 2.3 Unguided media: Type Ground Wave Propagation or Ground Terrestrial Propagation

Ionospheric Propagation bounces off of the Earths Ionospheric Layer in the upper atmosphere. It is sometimes called Double Hop Propagation. It operates in the frequency range of 30 - 85 MHz. Because it depends on the Earth's ionosphere, it changes with weather and time of day. The signal bounces off of the ionosphere and back to earth. Ham radios operate in this range. (See Fig 2.4)

Fig 2.4 Unguided media: Type Ionospheric Propagation or Terrestrial Propagation

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Line of Sight Propagation transmits exactly in the line of sight. The receive station must be in the view of the transmit station. It is sometimes called Space Waves or Tropospheric Propagation. It is limited by the curvature of the Earth for ground based stations (100 km: horizon to horizon). Reflected waves can cause problems. Examples of Line of Sight Propagation are: FM Radio, Microwave and Satellite

Fig 2.5 Unguided media: Type Line of Sight (LOS) Propagation Radio Frequencies Radio Frequencies are in the range of 300 kHz to 10 GHz. We are seeing an emerging technology called wireless LANs. Some use radio frequencies to connect the workstations together, some use infrared technology. Microwave Microwave transmission is line of sight transmission. The Transmit station must be in visible contact with the receive station. This sets a limit on the distance between stations depending on the local geography. Typically the line of sight due to the Earth's curvature is only 50 km to the horizon! Repeater stations must be placed so the data signal can hop, skip and jump across the country

Fig 2.6 Unguided media : Microwave Propagation

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Microwaves operate at high operating frequencies of 3 to 10 GHz. This allows them to carry large quantities of data due to the large bandwidth.

Advantages:

They require no right of way acquisition between towers.

They can carry high quantities of information due to their high operating frequencies.

Low cost land purchase: each tower occupies small area.

High frequency/short wavelength signals require small antenna.

Disadvantages:

Attenuation by solid objects: birds, rain, snow and fog.

Reflected from flat surfaces like water and metal.

Diffracted (split) around solid objects

Refracted by atmosphere, thus causing beam to be projected away from receiver. Satellite Satellites are transponders that are set in a geostationary orbit directly over the equator. A transponder is a unit that receives on one frequency and retransmits on another. The geostationary orbit is 36,000 km from the Earth's surface. At this point, the gravitational pull of the Earth and the centrifugal force of Earths rotation are balanced and cancel each other out. Centrifugal force is the rotational force placed on the satellite that wants to fling it out to space.

The uplink is the transmitter of data to the satellite. The downlink is the receiver of data. Uplinks and downlinks are also called Earth stations due to be located on the Earth. The footprint is the "shadow" that the satellite can transmit to. The shadow being the area that can receive the satellite's transmitted signal.

Fig 2.7 Satellite Transmission

Fig 2.7 Satellite Uplink & Downlink Communication

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2.2 Serial Interface Standard (RS232C/V.24) 2.2.1 Development of the RS 232 standard The RS 232 standard for data communications was devised in 1962 when the need to be able to transmit data along a variety of types of line started to grow. The idea for a standard had grown out of the realisation in the USA that a common approach was required to allow interoperability. As a result the Electrical Industries Association in the USA created a standard for serial data transfer or communication known as RS232. It defined the electrical characteristics for transmission of data between a Data Terminal Equipment (DTE) and the Data Communications Equipment (DCE). Normally the data communications equipment is the modem (modulator/demodulator) which that encodes the data into a form that can be transferred along the telephone line. A Data Terminal Equipment could be a computer. The RS 232 standard underwent several revisions, the C issue known as RS232C was issued in 1969 to accommodate the electrical characteristics of the terminals and devices that were being used at the time. The RS 232 standard underwent further revisions and in 1986 Revision D was released (often referred to as RS232D). This revision of the RS 232 standard was required to incorporate various timing elements and to ensure that the RS 232 standard harmonised with the CCITT standard V.24, while still ensuring interoperability with older versions of RS 232 standard. Further updates and revisions have occurred since then and the current version is TIA-232-F issued in 1997 under the title: "Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange." The name of the RS 232 standard has changed during its history, several times as a result of the sponsoring organisation. As a result it has variously been known as EIA RS-232, EIA 232, and most recently as TIA 232. 2.2.2 Variations of the RS 232 standard There are number of different specifications and standards that relate to RS 232. A description of some of the RS 232 standards and the various names and references used is given below:

EIA/TIA-232 This reference to the RS 232 standard includes the names of the first and current sponsoring organizations, the Electronic Industries Alliance (EIA) the Telecommunications Industry Alliance (TIA).

RS-232C This was the designation given to the release of RS 232 standard updated in 1969 to incorporate many of the device characteristics.

RS-232D This was the release of the RS 232 standard that occurred in 1986. It was revised to incorporate various timing elements and to ensure that the RS 232 standard harmonised with the CCITT standard V.24.

RS-232F This version of the RS 232 standard was released in 1997 to accommodate further revisions to the standard. It is also known as TIA-232-F.

V24 The International Telecommunications Union (ITU) / CCITT (International Telegraph and Telephone Consultative Committee) of the ITU developed a standard known as ITU v.24, often just written as V24. This standard is compatible with RS232, and its aim was to enable manufacturers to conform to global standards and thereby allow products that would work in

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all countries around the world. It is entitled "List of definitions for interchange circuits between data terminal equipment (DTE) and data circuit-terminating equipment (DCE)."

V28 V.28 is an ITU standard defining the electrical characteristics for unbalanced double current interchange circuits, i.e. a list of definitions for interchange circuits between data terminal equipment (DTE) and data circuit-terminating equipment (DCE).

V10 V.10 is an ITU standard or recommendation for unbalanced data communications circuits for data rates up to 100 kbps that was first released in 1976. It can inter-work with V.28 provided that the signals do not exceed 12 volts. Using a 37 pin ISO 4902 connector it is actually compatible with RS423.

2.2.3 RS-232 Applications The RS-232 standard has come a long way since its initial release in 1962. Since then the standard has seen a number of revisions, but more importantly, RS232 has been used in an ever increasing number of applications. Originally it was devised as a method of connecting telephone modems to teleprinters or teletypes. This enabled messages to be sent along telephone lines - the use of computers was still some way off. As computers started to be used, links to printers were required. The RS-232 standard provided an ideal method of connection and therefore it started to be used in a rather different way. However its use really started to take off when personal computers were first introduced. Here the RS-232 standard provided an ideal method of linking the PC to the printer. The RS-232 standard provided an ideal method of linking many other remote items to computers and data recorders. As a result, RS-232 became an industry standard, used in a host of applications that were never conceived when it was first launched in 1962. The RS-232 Serial Interface Standard added the mechanical characteristics to the RS-232C Standard. The RS-232 standard defines:

The Mechanical Characteristics of the Interface

The Electrical Characteristics of the Interface

The Function of Each Signal

Subsets of the Signals for Certain Applications The European version of RS-232 is defined in:

V.24 - Mechanical Standard

V.28 - Electrical Standard 2.2.1 Mechanical Characteristics of the RS-232 Mechanical Characteristics of the RS-232 Interface defines:

The connector is a DB25 connector. DB9 is not universally accepted.

The connector gender is Male at the DTE and Female at the DCE.

The assignments of signals to pins

The maximum cable length is 50 ft.

The maximum cable capacitance = 2500 pF. Typical cable has 50 pF/foot capacitance. 2.2.4 Electrical Characteristics of the RS-232 Electrical Characteristics of the RS-232 Interface defines:

The transmitter side generates a voltage between +5 and +25 Volts for a Space (digital 0 or Low) and generates a voltage between -5 and -25 Volts for a Mark (digital 1 or High).

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Fig 2.8 Electrical Characteristic of RS232 for transmitting

The receiving side recognizes a Space (digital 0 or Low) as any voltage between +3 and +25V and a Mark (digital 1 or High) as any voltage between -3 and -25V. The standard allows for a voltage loss through the cable and noise immunity by reducing the receive requirements to +/-3 Volts!

Fig 2.9 Electrical Characteristic of RS232 for receiving

Function of Each Signal

Pin Name Description EIA Circuit

1 GND Chassis ground AA

2 TXD Transmit Data (TXD) BA

3 RXD Receive Data (RXD) BB

4 RTS Ready to Send CA

5 CTS Clear to Send CB

6 DSR Data Set Ready (DCE Ready) CC

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7 SGND Signal ground AB

8 DCD Carrier Detect (CD or RLSD) (RLSD - Received Line Signal Detector)

CF

9 n/u

10 n/u

11 n/u

12 DCD2 Secondary Carrier Detect (SRLSD) SCF

13 CTS2 Secondary Clear to Send SCB

14 TXD2 Secondary Transmit Data SBA

15 TxSigC Transmitter Signal Element Timing - DCE DB

16 RXD2 Secondary Receive Data SBB

17 RxSig Receive Signal Element Timing - DCE DD

18 LL Local Loopback

19 RTS2 Secondary Ready to Send SCA

20 DTR Data Terminal Ready (DTE Ready) CD

21 SQ/RL Signal Quality/Remote Loopback CG

22 RI Ring Indicator CE

23 DSRS Data Signal Rate Selector CH/CI

24 TxSigT Transmitter Signal Element Timing - DTE DA

25 TM Test Mode

The signals in Bold/Italic are required for a basic asynchronous modem connection.

Fig 2.10 RS232 Interface

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Subsets of Signals for Certain Applications

Data Signals

2 TXD Transmit Data Data generated by DTE BA

3 RXD Receive Data Data generated by DCE BB

Control Signals

4 RTS Ready to Send DTE wishes to transmit

5 CTS Clear to Send DCE ready to receive

6 DSR Data Set Ready (DCE Ready) DCE powered on & ready to go

20 DTR Data Terminal Ready (DTE Ready) DTE powereded on & ready to go

22 RI Ring Indicator Phones ringing

Test Modes

18 LL Local Loopback Initiate Local Loopback Self-Test

21 SQ/RL Signal Quality/Remote Loopback Initiate Remote Loopback Self-Test

25 TM Test Mode Initiate Test Mode

Synchronous Control Signals

21 SQ/RL Signal Quality/Remote Loopback Error in received data!

23 DSRS Data Signal Rate Selector DTE can dynamically select 1 of 2 data rates

Synchronous Timing Signals

15 TxSigC Transmitter Signal Element Timing - DCE DCE generated

17 RxSig Receive Signal Element Timing - DCE DCE generated

24 TxSigT Transmitter Signal Element Timing - DTE DTE generated

Ground/Shield

1 GND Chassis ground Shield DTE side only for noise protection.

Do NOT connect to signal ground!

7 SGND Signal ground Signal return path

2.3 RS422/V.11 Transmissions using RS-232 are limited in their speed and the length over which data can be transferred. Normally the maximum is 19.2 k baud and the distance 15 metres, although for slow transmission speeds longer lengths can sometimes be used. However care must be taken because stray

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pick up becomes a problem as the length is increased. This can result in the data becoming corrupted even when slow transmission speeds are used. To enable high speed data to be transmitted another specification was devised. Known as RS-422 ( RS422 ), it allows for speeds of 10 M bits per second to be achieved. However the distance over which data can be transmitted at this rate is limited to 50 feet. The overall maximum distance over which data can be transmitted is 4000 feet and for this length the data rate is limited to 100 kbps To achieve much greater data speed, balanced transmission techniques are used within RS-422 and as a result differential drivers and receivers are required. Lower voltage line levels are used for RS422 than those used for the older RS232 standard. A space is represented by a line voltage level in the band between +2 and +6 volts while a mark is represented by a voltage in the range -2 to -6 volts. The range between +2 and -2 volts provides a good noise margin for the system (refer Fig 2.11). Additionally the RS422 standard allows for line impedances down to 50 ohms while supporting the high data rates. To enable the differential driver to be used, the RS-422 standard uses a four conductor cable. Additionally up to ten receivers can be placed on a single cable, providing a multi-point network or bus. Although RS422 is significantly different to RS232, it can often be used as a direct interface in many instances. RS422 specification overview

Attribute Specification

Cabling Single ended

Multi-drop

Number of devices 5 transmitters 10 receivers

Communications modes half duplex full duplex

Maximum distance 4000 feet @ 100 kbps

Maximum data rate 10 Mbps @ 50 feet

Signalling Balanced

Mark (data = 1) condition

2V to 6V (B greater than A)

Space (data = 0) condition

2V to 6 V (A greater than B)

Driver output current capability 150 mA

Interface As the features of the V.24/RS-232-C interface are limited, the V.11/RS-422 interface was developed. This interface is also standardized, but operates symmetrically. The RS-422/V.11 serial interface is suitable for data transfer rates up to 10 Mbps. At a baud rate of 38,400 bauds, data can be transferred over 1 km cable.

Hardware The V.11/RS-422 standard operates with differential voltages. This offers the advantage that interferences act on both signal lines equally and simultaneously over the transmission distance.

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As the receiver only evaluates the differential voltages of the two signal lines, interferences are not relevant. By this means, considerably longer lines can be installed, and the transfer rate is much higher, as interferences are limited.

Signal levels With the V.11/RS-422 interface the signals are both transmitted and received as differential voltage. A positive differential voltage means a logical zero (OFF), a negative differential voltage means a logical one (ON). Differential voltages between, Fig 2.11 :

Udmin = 2 V and Udmax = 5 V are output; the control detects the differential voltages between Udmin = 0.2 V and Udmax = 6 V as logically defined levels.

Fig 2.11 RS422 Signal Level

(b) RSS 422, 15pin (a) RSS 422 Pin assignment

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Pin no. Assignment Designation

1 Shield Chassis Ground

2 RxD Receive Data

3 CTS Clear to Send

4 TxD Transmit Data

5 RTS Request to Send

6 DSR Data Set Ready

7 DTR Data Terminal Ready

8 GND Signal Ground

9 RxD Receive Data

10 CTS Clear to Send

11 TxD Transmit Data

12 RTS Request to Send

13 DSR Data Set Ready

14 DTR Data Terminal Ready

15 Do not assign

(c) Pin number and designation

Fig 2.12 RS-422/V.11 Data Interface, 15-pin, D-sub (Flange socket with female insert) 2.4. Registered Jack 45 (RJ 45)

The RJ system was originally invented by AT&T and Bell Labs in the 60’s . The idea was to design a standard interface that could be used throughout the United States and the world when dealing with different instruments. The RJ45 jack was originally designed for the AT&T telephone system often called the Merlin System in the 70’s and 80’s. All other phone systems used the RJ11 jack which is considerably smaller and only uses 3 pairs of wires and is called a six position jack. The RJ45 is an eight position jack and in telecommunications terminology it is often called a 4-pair jack. The RJ45 is now primarily used for Ethernet over copper twisted pair wire computer network systems. When used in the Ethernet system, it will support all the categories from category 3 up through and beyond category 6. Each jack has to be rated for the category of it’s intended use in the network design. In other words, if you have cat6 cable and patch panels, you must use cat6 RJ45’s. The RJ45 also can be used with shielded cable. In that case, the jack itself is surrounded in a metal case that must be grounded. Using shielded cable and shielded jacks together insure the highest protection from radio waves. This type of cabling system is unusual and rarely used. An application would be for use near a strong radio station or equipment that sometimes transmits in the radio frequencies. Some government organizations also use shielded twisted pair and shielded jacks for use in a secure network.

Fig 2.13 RJ-45 connector female type (left) and male type (right)

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RJ-45 Connector Pin Out

Pin # Signal Name Signal Description DTE DCE

1 RI Ring Indicator Used by the Data Set to indicate to the Data Terminal that a ringing condition has been detected

In Out

2 DCD Carrier Detect Used by the Data Set to indicate to the Data Terminal that the Data set has detected a carrier (of another device).

In Out

3 DTR Data Terminal Ready

Used by the Data Terminal to signal to the Data Set that it is ready for operation, active high.

Out In

4 GND Signal Ground / Common

The common return for all signals on the interface

5 RXD Receive Data The data sent from the Data Set and received by the Data Terminal

In Out

6 TXD Transmit Data The data sent from the Data Terminal and received by the Data Set

Out In

7 CTS Clear To Send Used by the Data Set to signal the Data Terminal that it may begin sending data. The Data Terminal will not send out data with out this signal, active high.

In Out

8 RTS Request To Send Used by the Data Terminal to signal the Data Set that it may begin sending data. The Data Set will not send out data with out this signal, active high.

Out In

2.4.1 Wiring T568A and T568B termination Perhaps the widest known and most discussed feature of TIA/EIA-568-B.1-2001 is the definition of pin/pair assignments for eight-conductor 100-ohm balanced twisted-pair cabling, such as Category 3 (CAT-3), Category 5 (CAT-5) and Category 6 (CAT-6) unshielded twisted-pair (UTP) cables. These assignments are named T568A and T568B and they define the pinout, or order of connections, for wires in 8P8C (often incorrectly referred to as RJ45) eight-pin modular connector plugs and sockets. Although these definitions consume only one of the 468 pages in the standards documents, a disproportionate amount of attention is paid to them. This is because cables that are terminated with differing standards on each end will not function normally.

TIA/EIA-568-B specifies that horizontal cables should be terminated using the T568A pin/pair assignments, "or, optionally, per [T568B] if necessary to accommodate certain 8-pin cabling systems.", refer Fig 2.14. Despite this instruction, many organizations continue to implement T568B for various reasons, chiefly associated with tradition (T568B is equivalent to AT&T 258A). The United States National Communication Systems Federal Telecommunications Recommendations do not recognize T568B.

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The primary color of pair one is blue, pair two is orange, pair three is green and pair four is brown. Each pair consists of one conductor of solid color, and a second conductor which is white with a stripe of the same color. The specific assignments of pairs to connector pins varies between the T568A and T568B standards.

Note that the only difference between T568A and T568B is that pairs 2 and 3 (orange and green) are swapped. Both configurations wire the pins "straight through", i.e., pins 1 through 8 on one end are connected to pins 1 through 8 on the other end. Also, the same sets of pins are paired in both configurations: pins 1 and 2 form a pair, as do 3 and 6, 4 and 5, and 7 and 8. One can use cables wired according to either configuration in the same installation without significant problem; problems involving crosstalk can occur (which is normally minimized by correctly twisting a pair together), but are usually insignificant in all but the most stringent specifications such as Category 6 cable. The primary thing one has to be careful of is not to accidentally wire the ends of the same cable according to different configurations (unless one intends to create an Ethernet crossover cable).

Pin T568A

Pair T568B

Pair Wire T568A Color T568B Color Pins on plug face (socket is reversed)

1 3 2 tip

white/green stripe

white/orange stripe

2 3 2 ring green solid

orange solid

3 2 3 tip

white/orange stripe

white/green stripe

4 1 1 ring blue solid

blue solid

5 1 1 tip white/blue stripe

white/blue stripe

6 2 3 ring orange solid

green solid

7 4 4 tip

white/brown stripe

white/brown stripe

8 4 4 ring brown solid

brown solid

Fig 2.14 RJ45 pin pairing for T568A and T568B

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2.5 RS449 Interface RS449, which is identical to V.11, relies on balanced differential signaling (RS422) to achieve longer range, higher speeds, and obtain some immunity against common-mode noise. The standard uses a 37-pin D-connector and is intended for synchronous wide area networking applications. Each pair of differential signals are labeled as "A" and "B". The "A" wire always connects to "A" on the other interface, and "B" connects to "B".

Fig 2.15 RS449 Connector (DB-37)

Fig 2.16 RS449 Speed vs. Distance

Terminated Unterminated

Speed Distance Speed Distance

10 MHz 10 m 1 MHz 10 m

2 MHz 40 m 100 kHz 100 m

1 MHz 100 m 56 kHz 110 m

100 kHz 1 km 10 kHz 1 km

Pin Signal Description Mnemonic

1 Shield Frame ground

2 Signal rate Indicator SI (unbalanced)

3 Not used -

4 Send Data A SDA

5 Send Timing A STA

6 Receive Data A RDA

7 Request To Send A RTSA

8 Receive Timing A RTA

9 Clear To Send A CTSA

10 Local Loopback LL (unbalanced)

11 Data Mode A DMA (like DSR)

12 Terminal Ready A TRA (like DTR)

13 Receiver Ready A RRA (like DCD)

14 Remote Loopback RLB (unbalanced)

15 Incoming Call IC (unbalanced) (like RI)

16 Signal Rate Selector SI (unbalanced)

17 Terminal Timing A TTA

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Fig 2.17 RS449 Pinout

2.6 V.35 Interface

V.35 has been around for quite some time. It was originally designed for a 48kbps synchronous modem - that's right, officially its top rated speed is 48kbps. However, V.35 has been used for many years in applications running from 20kbps up to and past 2Mbps. Most of the V.35 signals are for control and handshake purposes (like RTS, CTS, DSR, DTR) and these are implemented in unbalanced fashion, similar to RS232 / V.24. This approach is simple, inexpensive, and is usually adequate for these relatively invariant signals.

V.35 gets its superior speed and noise immunity by using differential signaling on the data and clock lines. Unlike RS232 / V.24 which uses signals with reference to ground, V.35 receivers look for the difference in potential between a pair of wires. The wires can be at any potential, the signal is carried by voltage differences between the two wires. Now the secret; by twisting these two wires, it becomes likely that noise picked up on one wire will also be picked up on the other. When both wires pick up the same noise it has the affect of cancelling itself - as the same noise impulse on both wires is invisible to the receiver. Remember the receivers are only looking at the difference in voltage level of each wire to the other, not to ground. Many high speed interfaces use this same technique, examples are: RS530, RS449, 10/100/1000baseT.

The differential signals for V.35 are commonly labeled as either "A" and "B". Wire A always connects to A, and B connects to B. Crossing the wires just inverts the data or clock. I have never seen any piece of equipment damaged from this, but they don't work this way, either

18 Test Mode TM (unbalanced)

19 Signal Ground SG

20 Receive Common RC

21 Not used

22 Send Data B SDB

23 Send Timing B STB

24 Receive Data B RDB

25 Request to Send B RTSB

26 Receive Timing B RTB

27 Clear To Send B CTSB

28 Terminal In Service IS (unbalanced)

29 Data Mode B DMB

30 Terminal Ready B TRB

31 Receiver Ready B RRB

32 Select Standby SS (unbalanced)

33 Signal Quality SQ (unbalanced)

34 New Signal NS

35 Terminal Timing B TTB

36 Standby Indicator SB (unbalanced)

37 Send Common SC

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Fig 2.18 Pin layout for V.35

V.35 Cable Design The design of your cable depends on what you are connecting together and the interfaces involved. There are two standard interface types "Data Terminal Equipment" (DTE) and " Data Communication Equipment" (DCE). Usually, but not always, the interface "facing away" from the network is the DCE, and the interface "facing toward" the network is the DTE. The DCE normally supplies the clock. All differential pairs must be twisted

DTE DCE

P ------------------- P

S ------------------- S

R ------------------- R

T ------------------- R

C ------------------- C

D ------------------- D

E ------------------- E

H ------------------- H

Y ------------------- Y

AA ------------------- AA

V ------------------- Y

X ------------------- X

Fig 2.19 DTE to DCE

Chapter 2 E5124

38 Sargunan Ainal (JKE,PUO)

This cable design assumes that both devices provide their own transmit clock. Not all equipment does, in which case a modem eliminator with clock will be needed. If only one device has a clock you might be able to get away with using the one clock to drive transmit and receive in both devices. However, if it is the old type interface it probably won't work (the impedance will be too low)

DTE DTE

P ------------------- R

S ------------------- T

R ------------------- P

T ------------------- S

C ------------------- D

D ------------------- C

E ------------------- H

H ------------------- E

Y & U ------------------- V

W & AA ------------------- X

V ------------------- Y & U

X ------------------- W & AA

Fig 2.10 DTE to DTE