02.Ethernet Basics And Cable Types.pdf

8/14/2019 02.Ethernet Basics And Cable Types.pdf

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The Ultimate CCNA Study Package - ICND 1 Chris Bryant, CCIE #12933 http://www.thebryantadvantage.com

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Ethernet Basics And Cable TypesOverview

In this section, we're going to take a look at a few different Ethernetstandards as well as the different cable types we use in our network. Wecan have everything else in place, but if we have the wrong cable - well,nothing's going to happen!

As you'll see in the Switching section, a typical Local Area Network (LAN)will have the hosts connected to their own individual switch ports.

With each host connected to its own switch port, we no longer have toworry about collisions when hosts send data simultaneously. In the olddays of networking, though, that wasn't the case. While you may not seethe following physical topology very often in your career, if at all, I'm

presenting it here so you know how hosts on a shared Ethernet segmentpractice CSMA/CD - Carrier Sense Multiple Access with CollisionDetection .

CSMA / CD

Ethernet Types and Standards

Pins And Transmissions

Crosstalk

Crossover, Straightthrough, And Rollover Cables

Ethernet Addressing


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These four hosts are all connected to the same cable. This topology onlyallows one host at a time to transmit; otherwise, a collision will result.When there's a data collision, both signals are rendered unusable.

We needed a solution for this problem, and the solution is CSMA/CD. I'llnumber the hosts and we'll walk through the CSMA/CD process. When ahost wants to send data, it listens to the wire. It's listening to see if any

other host is currently transmitting.

If another host is transmitting, Host 1 will not send its data; if there is noother host currently transmitting, Host 1 will send its data.

That sounds great, but there is the danger that two hosts will decide tosend data, both listen to the wire, and both hear nothing - so they sendtheir data at the same time!


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And we know what happens next - a collision. The collision renders bothhosts' transmissions useless. The collision actually results in a change inthe voltage on the wire, so the hosts that just sent data will hear thatchange - "noise" - and will realize they need to try sending their dataagain.

If both Host 1 and Host 4 send data at the exact same time again, acollision would occur again! To lessen the chances of that happening,both hosts invoke a random backoff timer set to "x" number of milliseconds. Since "x" is random, the chances of the hosts beginning theentire CSMA/CD process at the exact same time again is low. When therandom timer expires, the host will not just send its data again, but insteadwill begin the entire process again by listening to the wire.

Ethernet, Fast Ethernet, and Gigabit Ethernet

I do advise you to pay special attention to the speeds, IEEE standards,and the different names that these three versions of Ethernet have. Iwouldn't be surprised to see this come up on your CCENT and CCNA

exams more than once.

The Ethernet standard you're most likely to be familiar with is 10Base-T,


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specified by IEEE 802.3. The "T" stands for twisted-pair cable, and themaximum length of a 10Base-T copper cable is 100 meters. The "10"refers to the 10 MegaBits Per Second (MBPS) capacity.

You may be asking "Why twist the cable pairs?" Twisting pairs of wires

inside the cable cuts down on the possibility of electromagneticinterference, whether that interference comes from another cable or anoutside source - elevators are notorious for generating such interference.

In the previous illustrations, we looked at a network with a single coaxialcable and multiple hosts connected to that coax cable. That topology wasused by the first Ethernet standards, 10Base5 and 10Base2 . The solephysical components were the Ethernet cards in the computers andcoaxial cable, which is the topology we looked at in the previous example.

The cable made up a bus that all the connected devices would use. Thistype of bus is referred to as a shared bus, and Ethernet is considered tobe a logical bus topology .

The ending numbers in the terms 10Base5 and 10Base2 allegedlyrefer to the limit on the length of the cable, expressed in units of 100meters.This is true for 10Base5; the limit on the cable length is 500meters. Its not quite accurate for 10Base2, though; the limit on that cableis 185 meters , NOT 200 meters.

Fast Ethernet is defined by IEEE 802.3u, and has a maximum capacity of 100 MBPS. Fast Ethernet copper cables also have a maximum cablelength of 100 meters.

Defined by IEEE 802.3z and IEEE 802.3ab, Gigabit Ethernet has amaximum capacity of 1000 MBPS, also expressed as 1 GBPS (GigaBitsPer Second). The maximum cable length is 100 meters here as well.

To review:

Ethernet runs at 10 MBPS, defined by IEEE 802.3, and its copper cable has a maximum length of 100 meters. Variations include10Base-T, 10Base-2, and 10Base-5, with the last two involving ashared cable bus.

Fast Ethernet runs at 100 MBPS, is defined by IEEE 802.3u, and itscopper cable has a maximum length of 100 meters.

Gigabit Ethernet runs at 1000 MBPS (1 GBPS), is defined by IEEE802.3z, and also has a 100 meter cable length maximum. GigEthernet has two specifications to keep in mind - IEEE 802.3z and802.3ab.

How Bits Are Sent Over Ethernet Cabling

A standard Ethernet cabling type is Category 5 Unshielded Twisted-Pair,commonly known as CAT 5 UTP. The connector on the end of a typical


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Cat 5 UTP cable is an RJ-45 connector. This type of connector has a tabon the bottom that snaps into place when the connector is correctlyplaced into the device. (You can usually hear the snap sound, unlessyoure in a very loud wiring closet!)

The cable will contain separate wires inside, and the endpoints of thesewires are referred to as pins . While you now know that bits are sent over these wires, its important to know that the same set of pins is alwaysused to transmit, and a separate set of pins is always used to receive.

Note that the cabling type is referred to as twisted-pair. Why bother twisting the wires inside the cable? Twisting the cables reduces theamount of electromagnetic interference that the wires themselves cause.

(If EMI is a real problem, fiber-optic cable is your best bet, and also your most expensive.)

What Is Crosstalk?

Crosstalk is caused by the electromagnetic interference mentioned amoment ago. Basically, a signal crosses over from one pair of cables toanother, causing the signals to become unusable.

NEXT (near-end crosstalk) is a condition generally caused by crossed or crushed pairs of wires. The conductors inside the wires dont even haveto be exposed but if the conductors are too close, the signal traveling onone wire can actually interfere with the signal on another wire. The near-end is a relative term, referring to the end of the cable being tested (asopposed to far-end crosstalk, or FEXT).

In a typical RJ-45 connection, the crosstalk is actually at its highest levelas data enters the cable.

You may occasionally see the term PSNEXT. This is short for Power Sum Near End Cross Talk, and refers to the calculation carried out whena NEXT test is run. When the NEXT results for each pair of wires isadded, the result is the PSNEXT value.

Crossover, Straightthrough, And Rollover Cables

In the following exhibit, we've got three separate physical connections:

A laptop connected to a switch (Cable 3)Two switches connected to each other (Cable 2)

A PC connected to a switch (Cable 1)

This is a very basic and common topology, but we've got three differentcable types in there! Working from top to bottom, let's take a look at thedifferent cables we'll need to make this work.

Pins 1 and 2 TransmitPins 3 and 6 Receive


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Two cables down, one to go! To connect that laptop directly to a switch,we've got to be careful of two things:

Using the right cableConnecting the right cable to the right port on the switch

The cable we need is a rollover cable. All eight wires in the cable will "rollover" to another pin at the remote end, with the wire on Pin 1 at one endrolling over to Pin 8 at the other end, the wire on Pin 2 at one end rollingover to Pin 7 at the remote end, and so forth.

You may also need an adapter for your rollover cable, since one end of the cable is a DB-9 connector, and few if any of today's laptops have sucha port. You probably do have USB ports on your laptop, and you can getan adapter that allows you to connect a rollover cable to your laptop'sUSB port from just about any cable dealer.

As for the connection to the switch, you need to make sure you connectthe RJ-45 connector on the other end of the rollover cable to the Consoleport of the switch. I'll drive this point home at least one more timeelsewhere in the course!

Ethernet / NIC / Physical / LAN / BIA Addressing

You may be thinking "oh, man, that's a lot of addressing!" Actually, it'snot, because these are all different names for the same thing. The fifthname is the term you hear most often -- MAC address, short for Media


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Access Control.

The MAC address is used by switches to send frames to the proper destination, as you'll see in the LAN Switching section. Before you get tothat section, let's take a look at the format of the MAC address.

The entire MAC address is a 48-bit address that looks a little somethinglike this:

aa-bb-cc-11-22-33

A MAC address consists of hexadecimal values, and if you're notcomfortable or familiar with hex numbering, there is a supplement to theCCENT Study Package that will help you out. It's nowhere as mysteriousas it looks, believe me.

That MAC address actually has two parts, the first beingthe Organizationally Unique Identifier (OUI). The OUI is assigned tohardware vendors by the Institute of Electrical and Electronics Engineers(IEEE). A given OUI is assigned to one and only one vendor.

The second half of the MAC address is a value not yet used by thatparticular vendor. Looking at the MAC address example given earlier, wenow know that:

The OUI is aa-bb-ccThe vendor has not yet used 11-22-33 with that particular OUI, so thevendor is doing so now

Here's the list of different names used for the MAC address:

ethernet addressNIC (Network Interface Card) addressphysical addressLAN address (don't hear this term often anymore)BIA

The MAC is sometimes called the physical address because it physicallyexists on the network card. The address is burned into the card, giving ityet another name - the Burned-In Address (BIA).

As with IP addresses, we have broadcast and multicast MAC addresses.It's a good idea to be able to identify these addresses, and here's how todo it!

The broadcast MAC address is the "all-Fs" address: ff-ff-ff-ff-ff-ff (or FF-FF-FF-FF-FF-FF, as case does not matter in hexadecimal)

There is a range of multicast MAC addresses, and the first half of amulticast MAC address is always 0100.5e. The second half of amulticast MAC address will fall in the range 00-00-00 through 7F-FF-FF. Watch that 7! :)

WAN And Home Lab Cabling


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In this section we concentrated on LANs and the cables needed toconnect hosts, switches, and routers to each other. If we had expandedour discussion to include WANs, we'd need to add these cable types:

Routers typically use Serial cables for connections to a Frame Relaynetwork. Easy to remember, since they're using their serial interfaces todo so!

In a home lab, you may find it helpful to connect two Cisco routers directlyvia their Serial interfaces. In that case, you'll need a DTE/DCE cable.

Runts And Giants

There are some curious terms in the output of show interface ethernet0:

R2#show int e0Ethernet0 is up, line protocol is up Hardware is Lance, address is 0010.7b39.c5e9 (bia 0010.7b39.c5e9)

Internet address is 172.23.23.2/27 MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, rely 255/255, load 1/255 Encapsulation ARPA, loopback not set, keepalive set (10 sec) ARP type: ARPA, ARP Timeout 04:00:00 Last input 00:00:05, output 00:00:07, output hang never Last clearing of "show interface" counters never Queueing strategy: fifo Output queue 0/40, 0 drops; input queue 0/75, 0 drops 5 minute input rate 1000 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 65971 packets input, 15358846 bytes, 0 no buffer Received 65971 broadcasts, 0 runts, 0 giants , 0 throttles

Both of these terms represent packets that have been dropped, but for very different reasons. Runts are packets that have been droppedbecause they're smaller than the legal limit - that's 64 bytes.

On the other hands, giants are discarded because they're too big, but youprobably guessed that from the name! Giants are packets larger than1518 bytes.

Having suffered through a network that had quite a few runts and giantspolluting the network, I can tell you the #1 reason you have these are badand/or cheap Network Interface Cards (NICs). Cables can be an issue aswell, but I'd check the NICs first. Trust me - I've been there.

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02.Ethernet Basics And Cable Types.pdf

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