Twisted-Pair Cabling; Crossover Cables

Transcription

1 Twisted-Pair Cabling; Crossover Cables Wiring standards for Ethernet cables using twisted-pair wires; Making and testing cables UTP and STP UTP Unshielded Twisted Pair; four pairs are bundled in a plastic jacket as a cable Inexpensive; most commonly used STP Shielded Twisted Pair; each pair is wrapped in a conductive shield for additional electrical isolation, then bundled in the jacket Pricier, used where required 1

2 Twisted-Pairs and RJ-45 Connectors Ethernet uses RJ-45 connectors for UTP and STP Standard EIA/TIA 568 specifies the wiring patterns for 10Base-T, 100Base-T EIA, TIA organizations collaborated on the standard T-568A and T-568B differ slightly in which pairs go where A cable with the same pattern on each end (both T-568A or T-568B) is a straight-through cable T-568B is more common Which standard is used in this connector? Hint: the clip is on the back side in this photo. Answer: T-568B 2

3 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&6 DCE (hubs, switches) receive on pins 1&2, transmit on 3&6 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 - Straight-Through and Crossover Cables Straight-through cables connect a DTE (e.g., computer) to a DCE (e.g., switch) The same wiring pattern on each end Crossover cables are used to connect one DTE directly to another Transmit-pins 1&2 must be connected to Receivepins 3&6, and vice-versa For Gigabit crossover, Transmit-pins 4&5 must also be connected to Receive-pins 7&8, and viceversa 3

4 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. You cannot use a rolled cable for an Ethernet cable, nor can you use an Ethernet cable for a rolled cable. What you can do is take a straight-through Ethernet cable, cut off one end, flip over the wires, and recrimp it to make your own rolled cable. 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 4

5 Using Twisted-Pair Cables Original, non-smart situation: DTE nodes connect to DCE hub/switch via straightthrough cables hub/switch performs cross-over effect internally DTE nodes must use crossover cables to connect directly to each other Likely situation today: switch is smart enough to do auto-sensing some NICs can auto-sense Straight-through / crossover cables less important Crossover cables are used with backbone switches Oh, yeah Gig-E crossover cable will work for 100Mbps Ethernet 100Mbps-only crossover cable will not work for Gig-E 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 5

6 1000BaseT Crossover-Cable Two Possible Connection Patterns T568A Use the pattern above, or the pattern below - not both! T568B Gigabit Ethernet crossover wiring T568A - from en.wikipedia.org/wiki/ethernet_crossover_cable 6

7 Gigabit Ethernet crossover wiring T568B - from en.wikipedia.org/wiki/ethernet_crossover_cable Inspect the connector before crimping it Much too much of the sheath has been removed here. It might work, but it would be susceptible to electrical noise. At least the wires look okay from the top 7

8 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 8

9 Testing Crossover Cables Test a crossover cable by connecting two computers with it. 1. If the NICs have LEDs, and they don't light up, the cable isn't electrically sound 2. Set an IP address on each machine Linux: ifconfig eth x MS-Windows: netsh interface ip set address name="local Area Connection" static y Choose a different x and y for the two machines one of them should be Replace "eth0" and "Local Area Connection" with correct NIC names (shown by ifconfig or ipconfig) 3. Send a command between them Ping one from the other (if a ping server is running) Establish a "netcat" session (if netcat or nc.exe is installed) Establish a "telnet" session (if telnet is enabled; this is deprecated) 9

10 Netcat "the Swiss Army knife of networking" Netcat program can run as either server or client Netcat clients and servers can "talk" to each other Server mode: netcat "listens" on your choice of port number Example: Linux: nc -l 8080 Windows: nc -l -p 8080 Port 8080 is a common choice for mimicking a web server Client mode: netcat connects to IP-and-port-number combo Example: nc Server and client can both run on the same machine Use or "localhost" as the IP address 10