Lesson 3: Topology and Cables

I. Topology

Topology — ways of connecting computers together

I. Bus + Ring

First generation networks use buses and rings.Buses need terminators to prevent a signal reflections from one end to of computer to another:

Rings have traffic moves in a circle in the same direction:

II. Star Topology

* uses a central connection  
* offers fault tolerance ( if once cable breaks, the computer can still communicate)

III. Hybrids

* ex: Token ring, the star-ring topology
* looks like a star topology, but the ring is shrink down to fit inside a small box
* ** Physical vs. signaling/logical topology ** - how cabels look vs. how signals travel electronically

IV. Mesh and Point-to-Multipoint

* Common among wireless networks  
* ** Fully meshed ** - every computer connects to every other computer via two or more routes.  
* ** Partially Meshed ** - at least two machines hace redudant connections

FORMULA TO CALC NUM CONNECTIONS:

y = num of computers

Num Connections = y(y-1) / 2

Ex: 6 computers equal 6(5)/2 = 15 connections

** Point to Multipoint **

+ needs an intelligent device in the center whereas the star topology only need a device to provide a path/signal down to all connections

V.Point to Point

For connections between network with no central device.

II. Cabling

There are two categories of cables networks use:

• coaxial
• twisted-pair
• fiber optic

I. Coaxial Cabels

Central conductire wire surrounded by insulating material, then surrounded by 3rd layer of braided metal shield.

Coaxial bc the center wire and braided metal shield share common centerline.

Bus topology earlier days used BNC connectors

Modern coaxial cables no connect ISP with F-type connectors

• Cable modems connect with either RG-6 or RG-59. Both are 75 Ohms.
• RG-6 is common, wheras RG-59 is rarer and usually used for television.
• Ohm rating — relative measure of the resistance (the characteristic impedance) on the cable.
• Impedance — set of charc. that define how much a cable resists the flow of electricity.
• Capacitance — how long it takes a wire to get a full charge.

II. Twisted Pair Cables

• most common cables in network
• composed of multiple pairs of wires, twisted around each other to reduce crosstalk (interference)

Shielded Twisted Pair — protect against EMI, rare

• common in IBM Type1, in Token Ring network

Unshielded Twisted — common, no protection against EMI

Categories for UTP

• CAT ratings are in megahetz (Mhz) for the highest frequencey the cable can handle
• focus on CAT 3, 5, 5a, 6 for the exam
• CAT 5e is currently the most common, although CAT 6 is gaining popularity
• ethernet standar have 100-meter limitation
• most common cable is the Registered Jacket (RJ) connectors:
• RJ-11 is mostly for telephones
• RJ-45 common for networks

III. Fiber Optic Cable

* transmit light, not electricity
* good for high EMI areas + long distance
* most common size is 62.5/125 μm (micrometer)

Multimode Fiber (MMF) — uses LED lght, has problem with model distortion

• all MMF transmit 850mm wavelengths

Single mode fiber (SMF) — cables that uses lasers, prevents problem with model distortion MMF

• SMF transmits 1310–1550 nanamter

Three Comon Connector Types for Fiber Optic Cable

• ST — “straight tip”
• SC — “standard connector”
• LC — “Lucent connector” ?

Older Cables

A. Clasic Serial Cables

• Use only for point to point connections w/ only 56,000 bps data rates
• Port of 9pin D-submarine (DB-9) male connector

B. Parallel Connectors

• only 2 Mbps, pt-to-pt topology
• 25-pin female connector
• set by IEEE 1284 committee

C. Firewire

• based on IEEE 1394
• fast, 800 Mbps
• pt-to-pt

III.Networking Industry Standards: IEEE

Institute of Electrical and Electronic Engineers defines the industry-wide standards

• IEEE 802 working group defines the frames, speeds, distances, tupes of cabling use in network environment
• 802.3 — Ethernet
• 802.11 — Wireless LAN (WLAN), like wifi

NOTES: