Network Essentials Study Guide

Note: This exam has retired.

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Network Types
Topologies
Cabling
Network Devices
OSI Model
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--TYPES OF NETWORKS--
1) PEER TO PEER
A peer to peer network is one in which lacks a dedicated server and every computer acts as both a client and a server. This is a good networking solution when there are 10 or less users that are in close proximity to each other. A peer to peer network can be a security nightmare, because the people setting permissions for shared resources will be computer idiots and the right people will never have access to the right resources. Thus is only recommended in situations where security is not an issue.

2) CLIENT/SERVER
This type of network is designed to support a large Number of users and uses dedicated server/s to accomplish this. Clients log on to the server/s in order to run applications or obtain files. Security and permissions can be managed by 1 or more administrators which cuts down on the aforementioned computer illiterates from medling with things that they shouldn't be. This type of network also allows for convenient backup services, reduces network traffic and provides a host of other services that come with the network operating system(NOS).

3) CENTRALIZED
This is also a client/server based model that is most often seen in UNIX environments, but the clients are "dumb terminals". This means that the client may not have a floppy drive, hard disk or CDROM and all applications and processing occur on the server/s. As you can imagine, this requires fast and damn expensive server/s. Security is very high on this type of network, although a similar level of security can be achieved using an NT server and setting appropriate permissions.

--NETWORK TOPOLOGIES--
1) BUS
This topology is old school and essentially has each of the computers on the network daisy-chained to each other. This type of network is usually peer to peer and uses Thinnet(10base2) cabling. It is configured by connecting a "T-connector" to the network adapter and then connecting cables to the T-connectors on the computers on the right and left. At both ends of the chain the network must be terminated with a 50 ohm impedance terminator.
ADVANTAGES: Cheap, simple to set up.
DISADVANTAGES: Excess network traffic, a failure may affect many users, Problems are difficult to troubleshoot.

As you can see if computer #1 sends a packet to computer #4, it must pass through computers #2 and #3, creating excess traffic.
2) STAR
The star is probably the most commonly used topology today. It uses twisted pair(10baseT or 100baseT) cabling and requires that all devices are connected to a hub.
ADVANTAGES: centralized monitoring, failures do not affect others unless it is the hub, easy to modify.
DISADVANTAGES: If the hub fails then everything connected to it is down. This is like if you were to burn down the phone company's central office, then anyone connected to it wouldn't be able to make any phone calls.

3) RING
The ring topology looks the same as the star, except that it uses special hubs and ethernet adapters. The Ring topology is used with Token Ring networks(will be discussed later).
ADVANTAGES: Equal access.
DISADVANTAGES: Difficult to troubleshoot, network changes affect many users, failure affects many users.
4) MESH
Mesh topologies are combinations of the above and are common on very large networks. For example, a star bus network has hubs connected in a row(like a bus network) and has computers connected to each hub.

--CABLING--
TYPES

Cable Type	Also Known As	Connector	Maximum Length
10Base5	RG-8 or RG-11, Thicknet coax	AUI/DIX	500 meters(1640 ft)
10Base2	RG-58, thinnet coax	BNC connector	185 meters(607 ft)
10BaseT	Cat 3, 4, 5 twisted pair	RJ-45	100 meters(328 ft)
100BaseT	Cat 5 twisted pair	RJ-45	100 meters(328 ft)
10baseFL	Fiber Optic	Fiber Optic connector	2 Kilometers(6562 feet)

SPEED

Cable Type	Transmission Speed
Thicknet	10mbps
Thinnet	10 mbps
cat 2 twisted pair	4 mbps
cat 3 twisted pair	10 mbps
cat 4 twisted pair	16 mbps
cat 5 twisted pair	100 mbps
Fiber Optic	100 mbps - 1 gbps

MISC CABLE STUFF
--Shielded twisted pair(STP) differs from UTP in that it has a foil jacket that helps prevent crosstalk. Crosstalk is overflow from an adjacent wire.

--The 5-4-3 rule: this rule states that on a 10base2 network can have 5 cable segment connected with 4 repeaters, but only 3 of these segments can be occupied by computers. There is also a maximum of 30 computers per segment.

--Thicknet cables are 0.5 inches thick and have a 50 ohm impedance.

--Thinnet cables are 0.25 inches thick and have a 50 ohm impedance.

--Plenum grade cabling is required if the cabling will be run between the ceiling and the next floor(this is called the plenum). Plenum grade is resistant to fire and does not emit poisonous gasses when burned.

--Thicknet is often used as a backbone. A transceiver with a vampire tap penetrates the core of the cable. From the transceiver a DB-15 connector plugs into the AUI port on a given device.

--Fiber Optic cabling has an built in security as you can't intercept data as you can with other cable mediums.

--Baseband= Digital, single frequency, bidirectional communications and uses a repeater to regenerate signals. Broadband= Analog, multiple frequencies, unidirectional communications, uses amplifiers to boost signals.

--NETWORK HARDWARE--
INTERRUPT REQUESTS(IRQ)

IRQ	DEVICE
IRQ 0	System Timer
IRQ 1	Keyboard
IRQ 2/9	Video Card
IRQ 3	Open unless needed for Com 2 or 4
IRQ 4	Com 1, Com 3
IRQ 5	Open unless needed for LPT2 or sound card
IRQ 6	Floppy Disk Controller
IRQ 7	LPT1(parallel port)
IRQ 8	Real time clock
IRQ 9/2	linked to IRQ 2
IRQ 10	Open
IRQ 11	Open
IRQ 12	PS/2 Mouse
IRQ 13	Math Co-processor
IRQ 14	Hard Disk Controller
IRQ 15	Open

-Often, if an network card will not function it is due to an IRQ or memory conflict.
-The NDIS specification allows for multiple protocols to be bound to a single network adapter.
-ODI is a specification defined by Novell and Apple that also allows multiple protocols to be bound to a single network adapter. Similar to NDIS.

LAN CONNECTIVITY DEVICES
1) REPEATERS
Boost signal in order to allow a signal to travel farther and prevent attenuation. Attentuation is the degradation of a signal as it travels farther from its origination. Repeaters do not filter packets and will forward broadcasts. Both segments must use the same access method, meaning that you can't connect a token ring segment to an Ethernet segment. Repeaters will connect different cable types.
2) BRIDGES
Functions the same as a repeater, but can also divide a network in order to reduce traffic problems. A bridge can also connect unlike network segments(ie. token ring and ethernet). Bridges create routing tables based on the source address. If the bridge can't find the source address it will forward the packets to all segments.
3) ROUTERS
A router will do everthing that a bridge will do and more. Routers are used in complex networks because they do not pass broadcast traffic. A router will determine the most efficient path for a packet to take and send packets around failed segments. Unroutable protocols can't be fowarded.
4) BROUTERS
A brouter has the best features of both routers and bridges in that it can be configured to pass the unroutable protocols by imitating a bridge, while not passing broadcast storms by acting as a router for other protocols.
5) GATEWAYS
Often used as a connection to a mainframe or the internet. Gateways enable communications between different protocols, data types and environments. This is achieved via protocol conversion, whereby the gateway strips the protocol stack off of the packet and adds the appropriate stack for the other side.

--OSI 7 LAYER MODEL--
Here is an easy way to memorize the order of the 7 layer model and it is as follows:
All People Seem To Need Data Processing. The first letter of each word corresponds to the first letter of one of the layers. It is dumb as hell, but it works.

Layer	Description	Device	Protocol
Application	Provides network access for applications, flow control and error recovery	Gateway	NCP, SMB, SMTP, FTP, SNMP, Telnet, Appletalk
Presentation	Performs protocol conversion, encryption and data compression	Gateway and redirectors	NCP, AFP, TDI
Session	Allows 2 applications to communicate over a network by opening a session and synchronizing the involved computers	Gateway	NetBios
Transport	Repackages messages into smaller formats, provides error free delivery and error handling functions	Gateway	NetBEUI, TCP, SPX, and NWLink
Network	Handles addressing, translates logical addresses and names to physical addresses, routing and traffic management.	Router and brouter	IP, IPX, NWLink, NetBEUI
**Data Link	Packages raw bits into frames and includes a cyclical redundancy check(CRC)	Switch, bridge and brouter	None
Physical	Transmits data over physical medium	Multiplexer and repeater	None

**The Data Link layer is divided into 2 sublayers called Media Access Control(MAC) and Logical Link Control(LLC).
-MAC Sublayer= Communicates with network card and delivers error-free delivery between 2 computers.
-LLC Sublayer= Defines service access points(SAPs) which are used to transfer information to the upper layers of the OSI model.
If you want to look like a real geek you can get a free poster of the 7-layer model here All you have to do is sign up and they will send it too you.

ACCESS METHODS
1) CSMA/CD
This stands for "carrier-sense multiple access with collision detection" and is the method used on ethernet networks whereby all computers on the network check the cable for traffic before attempting to transmit a packet. If more than 1 transmits at the same time then there will be a collision and both computers will wait a random amount of time and retransmit.
2) CSMA/CA
Stands for "carrier-sense multiple access with collision avoidance". This access method prevents collisions by having computers broadcast an intent to send a packet. This is the access method used by Localtalk and is sometimes described as "chatty". This broadcasting of intent to send can cause excess network traffic and slow things down.
3) TOKEN PASSING
Token passing is the access method used by token ring networks. With this method, a packet called a token is passed around the network. A computer that wishes to transmit must wait until it can take control of the token, allowing only one computer to transmit at a time. This is sort of like the "conch" in Lord of the Flies. Piggy had all of this crap that he wanted to whine about all of the time, but could only do so if he possessed the conch.
4) DEMAND PRIORITY
This access method is used with 100VG-AnyLAN networks. The repeaters, bridges, routers or hubs search the network for requests that are waiting to be sent. If 2 or more requests are received by the network hardware at once, the data with the highest priority is sent. Priority for different data types can be controlled by the administrator. A real advantage is that computers can receive and transmit at the same time with this access method.

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