CCNA Exploration Chapter 8 physical layer. Study questions.

Answers in white font. Select and change colour to read them.

8.1

How does the OSI physical layer correspond to the TCP/IP model layers?

 

It corresponds to the lower part of the network access layer.

 

What is the purpose of the OSI physical layer?

 

To take the frame from the data link layer, create the electrical, optical, or microwave signal that represents the bits in each frame, and place the signals  on the media.  It also retrieves signals from the media, restores them to their bit representations, and passes the bits up to the Data Link layer.

 

What are the three basic types of physical media used in networking?

 

Copper cable, Fibre optic cable, Wireless

 

On copper cable the bits are represented as patterns of:

 

Electrical pulses.

 

On copper cable the bits are represented as patterns of:

 

Light

 

On copper cable the bits are represented as patterns of:

 

Radio transmissions

 

The protocols of the TCP/IP suite, used in the upper layers, are defined by the Internet Engineering Task Force (IETF) in RFCs. Who defines protocols for the physical layer?

 

Electrical and communications engineering organizations such as ISO,  IEEE,  ANSI,  ITU,  EIA/TIA.

 

8.2

What is the difference between encoding and signalling?

 

Encoding is converting the bits into codes and signalling is putting signals to represent the coded bits on the medium.

 

How does Non-Return to Zero (NRZ) represent a 0 and a 1?

 

0 is represented by one voltage level on the media during the bit time and  1 is represented by a different voltage on the media during the bit time.

 

How does Manchester Encoding represent a 0 and a 1?

 

0 is represented by a transition in voltage from high to low in the middle of the bit time and 1 is represented by a transition from low to high.

 

What are the problems with using NRZ signalling alone?

 

There can be long runs of 1s or 0s, giving the receiving host no chance to resynchronise with the transmitting host, so that the boundaries between bits may be lost. It uses bandwidth inefficiently and is susceptible to electromagnetic interference.

 

How does Manchester Encoding let hosts keep synchronised?

 

There is a voltage transmission in the middle of each bit time.

 

Which versions of Ethernet use Manchester Encoding?

 

10Mbps Ethernet. (On UTP and older versions on coaxial cable.)

 

What are the advantages of having a separate coding step before signalling?

 

Control codes can be added without risk of confusing them with data.
Errors can be detected at the physical layer.
Long runs of 0 or 1 can be prevented.
The number of 0s and 1s can be balanced to prevent too much energy entering the system.

 

What is the difference between bandwidth and throughput?

 

Bandwidth is the theoretical maximum amount of data that can flow on a network segment in a certain length of time, and is determined by the physical properties of the medium and the technologies used for signalling and detecting signals. Throughput is the actual number of bits flowing per second when measured at a given time. It includes all the overhead, e.g. headers.

 

What units are used to measure bandwidth and throughput?

 

Bits per second and its multiples, kilobits per second (kbps), megabits per second (Mbps), gigabits per second (Gbps).

 

8.3

What is the maximum segment length for Cat 5 UTP cable?

 

100 metres.

 

What are the most common types of copper cable?

 

Coaxial cable, unshielded twisted pair (UTP) cable.

 

What are the sources of interference (noise) originating outside copper cabling?

 

Wireless signals and electromagnetic waves from devices such as motors and fluorescent lights.

 

How many wires are there in UTP cable, and how are they arranged?

 

8 wires in individual colour-coded plastic sheaths, twisted together in pairs.

 

Why is the twisting important in UTP cable?

 

Twisting of pairs of wires in the same circuit (one carrying the signal and one for the return current) keeps them close together and causes a cancellation effect that helps to protect against errors caused by noise.

 

What is the source of the interference called crosstalk?

 

It comes from other wires in the same or nearby cables.

 

What is the minimum acceptable UTP cable type to meet the standards for new Ethernet networks?

 

Category 5e

 

Which is the recommended UTP cable type for new Ethernet networks?

 

Category 6

 

Where should a straight through UTP cable be used?

 

Between a computer and a switch or hub. Between a router and a switch or hub.

 

Where should a crossover UTP cable be used?

 

Between similar devices, e.g. computer to computer, switch or hub to switch or hub, router to router, also computer direct to router.

 

Where is a rollover cable used?

 

Between the serial port of a computer and the console port of a router or switch. This is not a network connection. It is used to configure and manage the router or switch.

 

How is a coaxial cable constructed?

 

Copper conductor in the middle to carry the signals. Insulation layer. Copper braid sheath to act as a return path and shield against interference. Outer plastic jacket.

 

What is coaxial used for now, and what was it used for?

 

Coaxial cable is used for transporting high radio frequency signals, so it is used for antennae and aerials. It is used for cable TV and Internet connections. It was formerly used for Ethernet networks.

 

Name three types of coaxial cable connector.

 

BNC, F type, N type.

 

How is STP cable constructed?

 

It has 8 wires arranged in twisted pairs. Each pair is surrounded by a metal foil shield. There is another braided metal shield round all the wires, just inside the jacket.

 

STP cable provides protection from noise, so why is it not commonly used in Ethernet networks?

 

It is much more expensive than UTP.

 

What are the components of a fibre optic cable, and what is their purpose?

 

The core carries the light signals. The cladding stops the signals from escaping from the core. The buffer provides a protective layer. The strengthening material also provides protection and allows the cable to be pulled without damage. The outer plastic jacket is also for protection and to keep the contents together. There are several fibres in one cable.

 

Why are fibre optic cables always in pairs?

 

To provide full duplex operation: signals can travel in both directions at the same time.

 

What are the advantages of fibre optic cable over UTP?

 

No noise, operates over much greater distances.

 

What are the disadvantages of fibre optic cable over UTP?

 

More expensive. Needs different skills for installation. Needs careful handling to avoid damage.

 

In an Ethernet network, where is fibre optic cable most likely to be used?

 

On point to point links where there is a lot of traffic. Between buildings.

 

How does single mode fibre optic cable differ from multimode?

 

Single mode has a narrower core, 8 – 10 micrometres rather than 50-60 for multimode. Single mode used lasers for signalling, and these but one focussed beam of light on the core. Multimode uses LEDs which put out a less focussed beam so that light enters the core at different angles and travels at slightly different speeds so that eventually the pulses become less distinct (modal dispersion). Single mode cables could operate up to 100km, but multimode up to 2km. Single mode is more expensive than multimode.

 

What are the main problems with wireless networks?

 

Signals being blocked by materials in buildings, interference, security.

 

What is the IEEE standard for WiFi?

 

IEEE 802.11

 

Are all RJ45 connectors wired in the same way?

 

No. Ethernet has standards for straight through and crossover cables. Other technologies (e.g. ISDN) have different wiring. It is important not to use the wrong wiring because it may cause damage.

 

How much untwisting of the wire pairs should you do when attaching an RJ45 connector to an UTP cable?

 

As little as possible – just enough to get the wires into the connector.