Twisted-Pair Cabling; Crossover Cables, Exams of Computer Networks

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Twisted-Pair Cabling;

Crossover Cables

Wiring standards for Ethernet cables using

twisted-pair wires;

Making and testing cables

UTP and STP

 UTP – U nshielded T wisted P air; four pairs are bundled in a plastic “jacket” as a cable  Inexpensive; most commonly used  STP – S hielded T wisted P air; each pair is wrapped in a conductive shield for additional electrical isolation, then bundled in the jacket  Pricier, used where required

Which standard is

used in this connector?

✦ Hint:

the clip is on the back side

in this photo.

✦ Answer:

T-568B

RJ-45 Connectors and Signals

This table shows transmit (TX) and receive (RX) pins for a DTE (computer) device –

✦ 10Base-T and 100Base-T “Fast

Ethernet” only use two pairs

‣ DTE (active) nodes transmit on pins 1&2, receive on 3& ‣ DCE (hubs, switches) receive on pins 1&2, transmit on 3&

✦ 1000Base-T “GigE”

(and older 100Base-T4) require

all four pairs

‣ GigE actually transmits and receives on each pair Pin 10Base-T, 100Base-T 1000Base-T, 10GBase-T 1 Tx^ data +^ Tx A + 2 Tx data - Tx A - 3 Rx data +^ Rx B + 4 unused^ Tx C + 5 unused^ Tx C - 6 Rx data^ -^ Rx B - 7 unused Rx D + 8 unused^ Rx D -

What are Rolled Cables?

“In the Cisco world, a rolled (or rollover ) cable is a cable used to manage a router or switch from the console port of that device. This cable is usually used when configuring one of these devices for the first time. A rolled cable connects the PC’s serial port (using a DB9 to RJ45 adaptor) to the router or switch’s console port. A rolled cable looks just like an Ethernet cable. The funny thing is that a rolled cable isn’t an Ethernet cable at all. In fact, it is a serial cable. In a rolled cable, Pin 1 connects to Pin 8 on the other side.”

What kind of cable is this?

✦ Hint: check the wiring pattern carefully!

✦ Answer:

✦ Non-standard cable ‣ Left side is T-568B ‣ Right side is neither standard ‣ Not a "rollover" cable either

Making a “Patch” Cable

 For a straight-through cable, use the same standard on both ends - either T- 568A or T-568B  For a Gigabit Ethernet crossover cable, exchange all four pairs  Either T-568A or T-568B can be used  Pin pairs 1&2, and 3&6, are crossed with each other  Pin pairs 4&5, and 7&8, are crossed with each other  For a 10Mbps or 100Mbps crossover cable , you can use T-568A wiring on one end and T- 568B on the other  Pin pairs 1&2, and 3&6, are crossed with each other  Pairs 4&5 and 7&8 don’t matter

1000BaseT Crossover-Cable

– Two Possible Connection Patterns

T568B

T568A

Use the pattern above, or the pattern below - not both!

Gigabit Ethernet crossover wiring – T568B - from

en.wikipedia.org/wiki/Ethernet_crossover_cable

UTP cable, showing about 1 inch of sheathing

stripped off

Do NOT strip the individual wires, just untwist them

Making a Cable

…Look closely…

 On one side, the wires

reach all the way to the

end.

 But on the other, they

don't get as far as the

electrical contacts.

 This cable has not been

crimped yet. It can still be

corrected.

 The wires should also be shortened, so that the sheath reaches into the back of the connector.

Crimping Tool

Which Connector Has Been Crimped?

Uncrimped Crimped