Senior Acquisitions Editor: Kenyon Brown Development Editor: Kim Wimpsett

Ethernet at the Physical Layer

Ethernet was first implemented by a group called DIX, which stands for

Digital, Intel, and Xerox. They created and implemented the first

Ethernet LAN specification, which the IEEE used to create the IEEE

802.3 committee. This was a 10 Mbps network that ran on coax and then

eventually twisted-pair and fiber physical media.

The IEEE extended the 802.3 committee to three new committees known

as 802.3u (Fast Ethernet), 802.3ab (Gigabit Ethernet on category 5), and

then finally one more, 802.3ae (10 Gbps over fiber and coax). There are

more standards evolving almost daily, such as the new 100 Gbps Ethernet

(802.3ba)!

When designing your LAN, it’s really important to understand the

different types of Ethernet media available to you. Sure, it would be great

to run Gigabit Ethernet to each desktop and 10 Gbps between switches,

but you would need to figure out how to justify the cost of that network

today! However, if you mix and match the different types of Ethernet

media methods currently available, you can come up with a cost-effective

network solution that works really great.

The EIA/TIA (Electronic Industries Alliance and the newer

Telecommunications Industry Association) is the standards body that

creates the Physical layer specifications for Ethernet. The EIA/TIA

specifies that Ethernet use a registered jack (RJ) connector on

unshielded twisted-pair (UTP) cabling (RJ45). But the industry is

moving toward simply calling this an 8-pin modular connector.

Every Ethernet cable type that’s specified by the EIA/TIA has inherent

attenuation, which is defined as the loss of signal strength as it travels the

length of a cable and is measured in decibels (dB). The cabling used in

corporate and home markets is measured in categories. A higher-quality

cable will have a higher-rated category and lower attenuation. For

example, category 5 is better than category 3 because category 5 cables

have more wire twists per foot and therefore less crosstalk. Crosstalk is

the unwanted signal interference from adjacent pairs in the cable.

Here is a list of some of the most common IEEE Ethernet standards,

starting with 10 Mbps Ethernet:

10Base-T (IEEE 802.3) 10 Mbps using category 3 unshielded twisted

pair (UTP) wiring for runs up to 100 meters. Unlike with the 10Base-2

and 10Base-5 networks, each device must connect into a hub or switch,

and you can have only one host per segment or wire. It uses an RJ45

connector (8-pin modular connector) with a physical star topology and a

logical bus.

100Base-TX (IEEE 802.3u) 100Base-TX, most commonly known as

Fast Ethernet, uses EIA/TIA category 5, 5E, or 6 UTP two-pair wiring.

One user per segment; up to 100 meters long. It uses an RJ45 connector

with a physical star topology and a logical bus.

100Base-FX (IEEE 802.3u) Uses fiber cabling 62.5/125-micron

multimode fiber. Point-to-point topology; up to 412 meters long. It uses

ST and SC connectors, which are media-interface connectors.

1000Base-CX (IEEE 802.3z) Copper twisted-pair, called twinax, is a

balanced coaxial pair that can run only up to 25 meters and uses a special

9-pin connector known as the High Speed Serial Data Connector

(HSSDC). This is used in Cisco’s new Data Center technologies.

1000Base-T (IEEE 802.3ab) Category 5, four-pair UTP wiring up to

100 meters long and up to 1 Gbps.

1000Base-SX (IEEE 802.3z) The implementation of 1 Gigabit

Ethernet running over multimode fiber-optic cable instead of copper

twisted-pair cable, using short wavelength laser. Multimode fiber (MMF)

using 62.5- and 50-micron core; uses an 850 nanometer (nm) laser and

can go up to 220 meters with 62.5-micron, 550 meters with 50-micron.

1000Base-LX (IEEE 802.3z) Single-mode fiber that uses a 9-micron

core and 1300 nm laser and can go from 3 kilometers up to 10 kilometers.

1000Base-ZX (Cisco standard) 1000BaseZX, or 1000Base-ZX, is a

Cisco specified standard for Gigabit Ethernet communication.

1000BaseZX operates on ordinary single-mode fiber-optic links with

spans up to 43.5 miles (70 km).

10GBase-T (802.3.an) 10GBase-T is a standard proposed by the IEEE

802.3an committee to provide 10 Gbps connections over conventional

UTP cables, (category 5e, 6, or 7 cables). 10GBase-T allows the

conventional RJ45 used for Ethernet LANs and can support signal

transmission at the full 100-meter distance specified for LAN wiring.

If you want to implement a network medium that is not

susceptible to electromagnetic interference (EMI), fiber-optic cable

provides a more secure, long-distance cable that is not susceptible to

EMI at high speeds.

Armed with the basics covered so far in this chapter, you’re equipped to

go to the next level and put Ethernet to work using various Ethernet

cabling.

Interference or Host Distance Issue?

Quite a few years ago, I was consulting at a very large aerospace

company in the Los Angeles area. In the very busy warehouse, they

had hundreds of hosts providing many different services to the

various departments working in that area.

However, a small group of hosts had been experiencing intermittent

outages that no one could explain since most hosts in the same area

had no problems whatsoever. So I decided to take a crack at this

problem and see what I could find.

First, I traced the backbone connection from the main switch to

multiple switches in the warehouse area. Assuming that the hosts

with the issues were connected to the same switch, I traced each

cable, and much to my surprise they were connected to various

switches! Now my interest really peaked because the simplest issue

had been eliminated right off the bat. It wasn’t a simple switch

problem!

I continued to trace each cable one by one, and this is what I found:

As I drew this network out, I noticed that they had many repeaters in

place, which isn’t a cause for immediate suspicion since bandwidth

was not their biggest requirement here. So I looked deeper still. At

this point, I decided to measure the distance of one of the intermittent

hosts connecting to their hub/repeater.

This is what I measured. Can you see the problem?

Having a hub or repeater in your network isn’t a problem, unless you

need better bandwidth (which they didn’t in this case), but the

distance was! It’s not always easy to tell how far away a host is from

its connection in an extremely large area, so these hosts ended up

having a connection past the 100-meter Ethernet specification, which

created a problem for the hosts not cabled correctly. Understand that

this didn’t stop the hosts from completely working, but the workers

felt the hosts stopped working when they were at their most stressful

point of the day. Sure, that makes sense, because whenever my host

stops working, that becomes my most stressful part of the day!

Ethernet Cabling

A discussion about Ethernet cabling is an important one, especially if you

are planning on taking the Cisco exams. You need to really understand

the following three types of cables:

Straight-through cable

Crossover cable

Rolled cable

We will look at each in the following sections, but first, let’s take a look at

the most common Ethernet cable used today, the category 5 Enhanced

Unshielded Twisted Pair (UTP), shown in

Figure 2.9

.

FIGURE 2.9

Category 5 Enhanced UTP cable

The category 5 Enhanced UTP cable can handle speeds up to a gigabit

with a distance of up to 100 meters. Typically we’d use this cable for 100

Mbps and category 6 for a gigabit, but the category 5 Enhanced is rated

for gigabit speeds and category 6 is rated for 10 Gbps!

Straight-Through Cable

The straight-through cable is used to connect the following devices:

Host to switch or hub

Router to switch or hub

Four wires are used in straight-through cable to connect Ethernet

devices. It’s relatively simple to create this type, and

Figure 2.10

shows

the four wires used in a straight-through Ethernet cable.

FIGURE 2.10

Straight-through Ethernet cable

Notice that only pins 1, 2, 3, and 6 are used. Just connect 1 to 1, 2 to 2, 3

to 3, and 6 to 6 and you’ll be up and networking in no time. However,

remember that this would be a 10/100 Mbps Ethernet-only cable and

wouldn’t work with gigabit, voice, or other LAN or WAN technology.

Crossover Cable

The crossover cable can be used to connect the following devices:

Switch to switch

Hub to hub

Host to host

Hub to switch

Router direct to host

Router to router

The same four wires used in the straight-through cable are used in this

cable—we just connect different pins together.

Figure 2.11

shows how the

four wires are used in a crossover Ethernet cable.

FIGURE 2.11

Crossover Ethernet cable

Notice that instead of connecting 1 to 1, 2 to 2, and so on, here we connect

pins 1 to 3 and 2 to 6 on each side of the cable.

Figure 2.12

shows some

typical uses of straight-through and crossover cables.

FIGURE 2.12

Typical uses for straight-through and cross-over Ethernet

cables

The crossover examples in

Figure 2.12

are switch port to switch port,

router Ethernet port to router Ethernet port, and router Ethernet port to

PC Ethernet port. For the straight-through examples I used PC Ethernet

to switch port and router Ethernet port to switch port.

It’s very possible to connect a straight-through cable between

two switches, and it will start working because of autodetect

mechanisms called auto-mdix. But be advised that the CCNA

objectives do not typically consider autodetect mechanisms valid

between devices!

UTP Gigabit Wiring (1000Base-T)

In the previous examples of 10Base-T and 100Base-T UTP wiring, only

two wire pairs were used, but that is not good enough for Gigabit UTP

transmission.

1000Base-T UTP wiring (

Figure 2.13

) requires four wire pairs and uses

more advanced electronics so that each and every pair in the cable can

transmit simultaneously. Even so, gigabit wiring is almost identical to my

earlier 10/100 example, except that we’ll use the other two pairs in the

cable.

FIGURE 2.13

UTP Gigabit crossover Ethernet cable

For a straight-through cable it’s still 1 to 1, 2 to 2, and so on up to pin 8.

And in creating the gigabit crossover cable, you’d still cross 1 to 3 and 2 to

6, but you would add 4 to 7 and 5 to 8—pretty straightforward!

Rolled Cable

Although rolled cable isn’t used to connect any Ethernet connections

together, you can use a rolled Ethernet cable to connect a host EIA-TIA

232 interface to a router console serial communication (COM) port.

If you have a Cisco router or switch, you would use this cable to connect

your PC, Mac, or a device like an iPad to the Cisco hardware. Eight wires

are used in this cable to connect serial devices, although not all eight are

used to send information, just as in Ethernet networking.

Figure 2.14

shows the eight wires used in a rolled cable.

FIGURE 2.14

Rolled Ethernet cable

These are probably the easiest cables to make because you just cut the

end off on one side of a straight-through cable, turn it over, and put it

back on—with a new connector, of course!

Okay, once you have the correct cable connected from your PC to the

Cisco router or switch console port, you can start your emulation

program such as PuTTY or SecureCRT to create a console connection and

configure the device. Set the configuration as shown in

Figure 2.15

.

FIGURE 2.15

Configuring your console emulation program

Notice that Baud Rate is set to 9600, Data Bits to 8, Parity to None, and

no Flow Control options are set. At this point, you can click Connect and

press the Enter key and you should be connected to your Cisco device

console port.

Figure 2.16

shows a nice new 2960 switch with two console ports.

FIGURE 2.16

A Cisco 2960 console connections

Notice there are two console connections on this new switch—a typical

original RJ45 connection and the newer mini type-B USB console.

Remember that the new USB port supersedes the RJ45 port if you just

happen to plug into both at the same time, and the USB port can have

speeds up to 115,200 Kbps, which is awesome if you have to use Xmodem

to update an IOS. I’ve even seen some cables that work on iPhones and

iPads and allow them to connect to these mini USB ports!

Now that you’ve seen the various RJ45 unshielded twisted-pair (UTP)

cables, what type of cable is used between the switches in

Figure 2.17

?

FIGURE 2.17

RJ45 UTP cable question #1

In order for host A to ping host B, you need a crossover cable to connect

the two switches together. But what types of cables are used in the

network shown in

Figure 2.18

?

FIGURE 2.18

RJ45 UTP cable question #2

In

Figure 2.18

, there’s a whole menu of cables in use. For the connection

between the switches, we’d obviously use a crossover cable like we saw in

Figure 2.13

. The trouble is that you must understand that we have a

console connection that uses a rolled cable. Plus, the connection from the

router to the switch is a straight-through cable, as is true for the hosts to

the switches. Keep in mind that if we had a serial connection, which we

don’t, we would use a V.35 to connect us to a WAN.

Fiber Optic

Fiber-optic cabling has been around for a long time and has some solid

standards. The cable allows for very fast transmission of data, is made of

glass (or even plastic!), is very thin, and works as a waveguide to transmit

light between two ends of the fiber. Fiber optics has been used to go very

long distances, as in intercontinental connections, but it is becoming

more and more popular in Ethernet LAN networks due to the fast speeds

available and because, unlike UTP, it’s immune to interference like cross-

talk.

Some main components of this cable are the core and the cladding. The

core will hold the light and the cladding confines the light in the core. The

tighter the cladding, the smaller the core, and when the core is small, less

light will be sent, but it can go faster and farther!

In

Figure 2.19

you can see that there is a 9-micron core, which is very

small and can be measured against a human hair, which is 50 microns.

FIGURE 2.19

Typical fiber cable.

Dimensions are in um (10

–6

meters). Not to scale.

The cladding is 125 microns, which is actually a fiber standard that allows

manufacturers to make connectors for all fiber cables. The last piece of

this cable is the buffer, which is there to protect the delicate glass.

There are two major types of fiber optics: single-mode and multimode.

Figure 2.20

shows the differences between multimode and single-mode

fibers.

FIGURE 2.20

Multimode and single-mode fibers

Single-mode is more expensive, has a tighter cladding, and can go much

farther distances than multimode. The difference comes in the tightness

of the cladding, which makes a smaller core, meaning that only one mode

of light will propagate down the fiber. Multimode is looser and has a

larger core so it allows multiple light particles to travel down the glass.

These particles have to be put back together at the receiving end, so

distance is less than that with single-mode fiber, which allows only very

few light particles to travel down the fiber.

There are about 70 different connectors for fiber, and Cisco uses a few

different types. Looking back at

Figure 2.16

, the two bottom ports are

referred to as Small Form-Factor Pluggables, or SFPs.

Data Encapsulation

When a host transmits data across a network to another device, the data

goes through a process called encapsulation and is wrapped with protocol

information at each layer of the OSI model. Each layer communicates

only with its peer layer on the receiving device.

To communicate and exchange information, each layer uses protocol

data units (PDUs). These hold the control information attached to the

data at each layer of the model. They are usually attached to the header in

front of the data field but can also be at the trailer, or end, of it.

Each PDU attaches to the data by encapsulating it at each layer of the OSI

model, and each has a specific name depending on the information

provided in each header. This PDU information is read only by the peer

layer on the receiving device. After its read, it’s stripped off and the data

is then handed to the next layer up.

Figure 2.21

shows the PDUs and how they attach control information to

each layer. This figure demonstrates how the upper-layer user data is

converted for transmission on the network. The data stream is then

handed down to the Transport layer, which sets up a virtual circuit to the

receiving device by sending over a synch packet. Next, the data stream is

broken up into smaller pieces, and a Transport layer header is created

and attached to the header of the data field; now the piece of data is

called a segment (a PDU). Each segment can be sequenced so the data

stream can be put back together on the receiving side exactly as it was

transmitted.

FIGURE 2.21

Data encapsulation

Each segment is then handed to the Network layer for network

addressing and routing through the internetwork. Logical addressing (for

example, IP and IPv6) is used to get each segment to the correct network.

The Network layer protocol adds a control header to the segment handed

down from the Transport layer, and what we have now is called a packet

or datagram. Remember that the Transport and Network layers work

together to rebuild a data stream on a receiving host, but it’s not part of

their work to place their PDUs on a local network segment—which is the

only way to get the information to a router or host.

It’s the Data Link layer that’s responsible for taking packets from the

Network layer and placing them on the network medium (cable or

wireless). The Data Link layer encapsulates each packet in a frame, and

the frame’s header carries the hardware addresses of the source and

destination hosts. If the destination device is on a remote network, then

the frame is sent to a router to be routed through an internetwork. Once

it gets to the destination network, a new frame is used to get the packet to

the destination host.

To put this frame on the network, it must first be put into a digital signal.

Since a frame is really a logical group of 1s and 0s, the physical layer is

responsible for encoding these digits into a digital signal, which is read by

devices on the same local network. The receiving devices will synchronize

on the digital signal and extract (decode) the 1s and 0s from the digital

signal. At this point, the devices reconstruct the frames, run a CRC, and

then check their answer against the answer in the frame’s FCS field. If it

matches, the packet is pulled from the frame and what’s left of the frame

is discarded. This process is called de-encapsulation. The packet is

handed to the Network layer, where the address is checked. If the address

matches, the segment is pulled from the packet and what’s left of the

packet is discarded. The segment is processed at the Transport layer,

which rebuilds the data stream and acknowledges to the transmitting

station that it received each piece. It then happily hands the data stream

to the upper-layer application.

At a transmitting device, the data encapsulation method works like this:

1.  User information is converted to data for transmission on the

network.

2.  Data is converted to segments, and a reliable connection is set up

between the transmitting and receiving hosts.

3.  Segments are converted to packets or datagrams, and a logical address

is placed in the header so each packet can be routed through an

internetwork.

4.  Packets or datagrams are converted to frames for transmission on the

local network. Hardware (Ethernet) addresses are used to uniquely

identify hosts on a local network segment.

5.  Frames are converted to bits, and a digital encoding and clocking

scheme is used.

To explain this in more detail using the layer addressing, I’ll use

Figure

2.22

.

Remember that a data stream is handed down from the upper layer to the

Transport layer. As technicians, we really don’t care who the data stream

comes from because that’s really a programmer’s problem. Our job is to

rebuild the data stream reliably and hand it to the upper layers on the

receiving device.