Chapter 3

• CISCO Semester I

• Unit 3 - Media

Karl Wick SUNY Ulster

MEDIA

• Media refers to the physical layer

• Media includes;– NICs– Cabling– Connectors– Layer 1, 2, and 3 devices– Media is Hardware

MEDIA

• For Cisco’s purposes – Media is the hardware that connects the devices in a network.

• Copper – inexpensive, easy to work with

• Fiber – longer distance, high bandwidth

• Wireless – portability of nodes

Cat 5 Cables

• Category 5 is the standard type of wire for Ethernet cables.

• Category 5 cable contains four pairs of wire. Each pair is twisted around itself and the rate of twist varies from pair to pair.

• 10 base T and 100 base T ethernet use two pairs. Gigabit ethernet uses all 4 pairs.

REVIEW

Atomic Structure

• Free Electrons are the key to electricity

Conductor

• Any material that allows easy passage of electrical current.

• Most metals are good conductors

• Any material with free electrons or Ions

Conductor

• Any material that allows easy passage of electrical current.

• Most metals are good conductors

• Any material with free electrons or Ions

– Gold

– Silver

– Copper

– Nickel

– Mercury

– Tin

– Lead

– Iron - A fair conductor

– Most Water (Has ions)

– Humans

Insulator

• Any material that does not allow easy passage of electrical current

Insulator

• Any material that does not allow easy passage of electrical current– Wood– Plastic– Teflon– Cotton– Dry air– PURE Water (Distilled and de-ionized)

Current

• Def: The flow of electrons through a material.

• Measured in Amperes.– 6.02 x 10 23 coulombs per second

• (This is a somewhat simplified definition)

Voltage

• A force created by the separation of positive and negative electrical charges.

• Also called EMF or Electromotive Force.

• Voltage causes current to flow.

Resistance

• A material’s reluctance to pass current

• A conductor has very little resistance

• An insulator has very high resistance

• A semi-conductor is in between and resistance may be variable

Ohm’s Law

• Ohm said that current flow is proportional to the amount of voltage applied and inversely proportional to the amount of resistance in the circuit

• I = V/R, or as the rule reads: V = I*R,

• It is sometimes written as E = I*R

Complete Circuit

Static Electricity

• Loose Electrons at rest

• No conducting path to let them move

• Very high voltage, no current until discharge, then low current.

• Very damaging to electronic equipment.

Modes of Transmission

• Simplex

• Half-Duplex

• Full Duplex

Copper Media

UTP, STP, Coaxial, etc.

• 10 base T One pair simplex

• 100 base T Two pairs duplex

• 1000 base T Four pairs duplex

• Fiber Two fibers duplex

RJ-45

• The RJ-45 connector is the standard connector for UTP cables (the most commonly used cables for ethernet)

• The RJ-45 standard specifies 8 pins and a certain minimum quality of connector

• There are also RJ-45 jacks. These are used in patch panels and devices (examples?)

EIA568A and EIA568B

Bad and Good Connections

Cable Types - Straight

• All 8 pins connect to the same pins on each end of the cable

• Used for most Ethernet connections

• EIA 568A or 568B, same on both ends

Crossover

• A special cable cable used to connect two devices of the same type.– Workstation to workstation– Sometimes between other devices

• EIA 568A on one end, EIA 568B on other

• Transmit and receive pairs are swapped

Rollover

• Used to connect a workstation serial port to the console port on a router or programmable switch

• Also called a console cable

• All pins swap end to end (1-8, 2-7, etc)

Types of Electrical Signals

DC, AC, Digital

DC

• Electricity where electrons flow in only one direction.

• A battery generates DC electricity

AC

• Electricity where electrons flow in both directions

• Electricity where polarity reverses periodically

Digital Signal

Impedance

• A Property similar to resistance but more complex.

• Impedance opposes the flow of AC current

• Impedance opposes the flow of Digital Signals

Data Transmission Concerns

Collisions

• A collision occurs when two devices try to send data over shared media at the same time.

• Collisions destroy data one bit at a time.

Collisions

• A collision occurs when two devices try to send data over shared media at the same time.

• Collisions destroy data one bit at a time.

Collision Domain

• Collision Domain means “the place where collisions may take place AND all of the devices that can send data to that place”.

Hubs and Repeaters extend Collision Domains

Bridges, Switches and Routers segment Collision Domains

Other Concerns

• Noise Pickup– Twisted pair minimizes pickup of external

noise due to canceling effect.

• Attenuation

• Crosstalk

Optical Media

Single Mode or Multi Mode

Optical Terms

• Reflection• Angle of incidence = angle of reflection• Refraction

– Bending of ray as light crosses a boundary.

• Index of Refraction– Speed in material / Speed in a vacuum

• Micron– a metric unit of length equal to one millionth of a

meter

Single Mode Fiber Optic Transmission

• Small Core = Low Dispersion.

• Long Distance – Up to 10,000 feet.

• Laser light source.

• Single Duplex Conversation at a time (all fiber uses TWO fibers per connection).

• Fastest data flow

Multi Mode Fiber Transmission

• Large Core = more dispersion

• Medium long distance – 2km 0r 6,500 feet.

• Can use LED’s as a light source. – Cheaper.

• Slower data flow than single mode fiber.

Bending of fiber should be minimized

A straight single mode fiber conducts light very well.

Any light at less than the critical angle will remain in the fiber.

Bends change the critical angle. More chance of loss.

With severe bends all of the light is lost.

Signal Loss

• Signal loss is usually greatest at terminations.– Terminations must be perfectly square.– Terminations must be perfectly smooth.– Terminations must be perfectly clean.– Terminations must touch perfectly (no gaps)

• Otherwise signal loss will occur.

Wireless Networks

802.11, 802.11b, 802.11a, 802.11g

Standards, stamdards

• 802.11 - The oldest. Up to2Mbps• 802.11b - Up to 11Mbps, typically 5+Mbps• 802.11a - Up to 54 Mbps (or proprietary X2),

but in practice closer to 25Mbps.• 802.11g – Compatible with both 802.11b and

802.11a• All have low throughputs due to overhead.

Non Standard – up to 108 Mbps?

Access Point

• Used to cover a specific area and provide connectivity to a wired LAN or to the WAN.

• Generally cover a 300 to 500 foot diameter but this is variable by building materials.

• “Cells” with coverage areas overlapping by 20-30% are often used.

Wireless Frames

• Differ from Ethernet 802.3– Control (RTS, CTS, ACK)– Access

• Probe Request / Response by Node• Association Request / Response by Node• Beacons by the Access Point• Authentication

– Data – only these are similar to 802.3 May however be longer (2346 bytes vs 1518).

Adaptive Rate Selection (ARS)

• As signal strength becomes weaker, or interference increases, the Access Point may invoke ARS to give better data integrity.

• Basically speed cuts in half. Longer bit lengths = reliability.

Connecting to the Network

• Unauthenticated and unassociated – The node is disconnected from the network and

not associated to an access point.

• Authenticated and unassociated – The node has been authenticated on the network

but has not yet associated with the access point.

• Authenticated and associated – The node is connected to the network and able to

transmit and receive data through the access point.

Security? What Security?

• Cisco – “Where wireless networks exist there is little security.”

• WLANs often extend outside the perimeter of the home or office in which they are installed and without security intruders may infiltrate the network with little effort.

• A number of new security solutions and protocols, such as Virtual Private Networking (VPN) and Extensible Authentication Protocol (EAP) are emerging.

Security? What Security?

• With EAP, the access point does not provide authentication to the client, but passes the duties to a more sophisticated device, possibly a dedicated server, designed for that purpose.

• Using an integrated server, VPN technology creates a tunnel on top of an existing protocol such as IP. This is a Layer 3 connection as opposed to the Layer 2 connection between the AP and the sending node.

Modulation Adding data to an AC signal.

• Module 3.3.5 does an OK job but shirks some issues: See also a good flash activity.

• Signal strength is inversely proportional to distance squared.

• Phase modulation dynamically alters the phase or starting point of each cycle of the carrier wave.

Common Modulation Types

End