computer networking notes

Network Orientation
Peer to Peer Networks:
No dedicated server or hierarchy, also called a workgroup.
Usually 10 or fewer workstations.
Users act as their own administrator and security.
Computers are in same general area.
Limited growth.

Server Based Networks:
10 or more users.
Employs specialized servers.
1.File and Print
2.Application
3.Mail
4.Fax
5.Communications (gateways)
Central administration.
Greater security.
Centralized backup.
Data Redundancy.
Supports many users
Combination Networks:
Combines the features of both Peer to Peer and Server based networks
Users can share resources among themselves as well as access server-based resources.

Network Topologies:
There are 4 basic topologies with variations
Bus Topology:
Bus consists of a single linear cable called a trunk.
Data is sent to all computers on the trunk. Each computer examines EVERY packet on the wire to determine who the packet is for and accepts only messages addressed to them.
Bus is a passive topology.
Performance degrades as more computers are added to the bus.
Signal bounce is eliminated by a terminator at each end of the bus.
Barrel connectors can be used to lengthen cable.
Repeaters can be used to regenerate signals.
Usually uses Thinnet or Thicknet
both of these require 50 ohm terminator
good for a temporary, small (fewer than 10 people) network
But its difficult to isolate malfunctions and if the backbone goes down, the entire network goes down.
Star Topology:
Computers are connected by cable segments to a centralized hub.
Signal travels through the hub to all other computers.
Requires more cable.
If hub goes down, entire network goes down.
If a computer goes down, the network functions normally.
most scalable and reconfigurable of all topologies
Ring Topology:
Computers are connected on a single circle of cable.
usually seen in a Token Ring or FDDI (fiber optic) network
Each computer acts as a repeater and keeps the signal strong => no need for repeaters on a ring topology
No termination required => because its a ring
Token passing is used in Token Ring networks. The token is passed from one computer to the next, only the computer with the token can transmit. The receiving computer strips the data from the token and sends the token back to the sending computer with an acknowledgment. After verification, the token is regenerated.
relatively easy to install, requiring ;minimal hardware
Mesh
The mesh topology connects each computer on the network to the others
Meshes use a significantly larger amount of network cabling than do the other network topologies, which makes it more expensive.
The mesh topology is highly fault tolerant.
Every computer has multiple possible connection paths to the other com-puters on the network, so a single cable break will not stop network communications between any two computers.
Star Bus Topology:
Several star topologies linked with a linear bus.
No single computer can take the whole network down. If a single hub fails, only the computers and hubs connected to that hub are affected.
Star Ring Topology:
Also known as star wired ring because the hub itself is wired as a ring. This means it's a physical star, but a logical ring.
This topology is popular for Token Ring networks because it is easier to implement than a physical ring, but it still provides the token passing capabilities of a physical ring inside the hub.
Just like in the ring topology, computers are given equal access to the network media through
the passing of the token.
A single computer failure cannot stop the entire network, but if the hub fails, the ring that the hub controls also fails.
Hybrid Mesh
most important aspect is that a mesh is fault tolerant
a true mesh is expensive because of all the wire needed
another option is to mesh only the servers that contain information that everyone has to get to. This way the servers (not all the workstations) have fault tolerance at the cabling level.


Connecting Network Components:

Primary Cable Types:
Coaxial Cable
Twisted-pair
UTP - Unshielded Twisted Pair
STP - Shielded Twisted Pair
Fiber-optic

Wireless Local Area Networks
Used where cable isn't possible - remote sites; also when mobility is important.
Use transceivers or access points to send and receive signals between the wired and wireless network.
There are 4 techniques for transmitting data
Infrared transmission consists of four types;
Line of sight
Scatter: good within 100 ft.
Reflective
Broadband optical telepoint: used for multimedia requirements; as good as cable.
Laser requires direct line-of-sight.
Narrow-band (single frequency) radio
Cannot go through steel or load-bearing walls.
Requires a service handler.
Limited to 4.8 Mbps
Spread-Spectrum Radio
Signals over a range of frequencies.
Uses hop timing for a predetermined length of time.
Coded for data protection.
Quite slow; Limited to 250 Kbps.
Point to Point Transmission
Transfers data directly from PC to PC (NOT through cable or other peripherals)
Uses a point to point link for fast error-free transmission.
Penetrates objects.
Supports data rates from 1.2 to 38.4 Kbps up to
200 feet indoors or
1/3 of a mile with line of site transmission.
Also communicates with printers, bar code readers, etc.
Multipoint Wireless Bridge
Provides a data path between two buildings.
Uses spread-spectrum radio to create a wireless backbone up to three miles.
Long-Range Wireless Bridge
Uses spread-spectrum technology to provide Ethernet and Token-Ring bridging for up to 25 miles.
This costs less than T1, but T1 will transmit at 1.544 Mbps
Mobile Computing
Uses wireless public carriers to transmit and receive using;
Packet-radio communication.
Uplinked to satellite, broadcast only to device which has correct address.
Cellular networks.
CDPD same as phone, subsecond delays only, real time transmission, can tie into cabled network.
Satellite stations.
Microwave, most common in USA, 2 X directional antennas, building to building, building to satellite
Slow transmission rate: 8 Kbps - 19.2 Kbps

Network Adapter Cards
The role of the network Adapter card it to:
Prepare data from the computer for the network cable
Send the data to another computer
Control the flow of data between the computer and the cabling system
NIC's contain hardware and firmware (software routines in ROM) programming that implements the
Logical Link Control and
Media Access Control
functions of the Data Link layer of the OSI
Preparing Data
data moves along paths in the computer called a BUS - can be 8, 16, 32 bits wide.
on network cable, data must travel in a single bit stream in what's called a serial transmission (b/c on bit follows the next).
The transceiver is the component responsible for translating parallel (8, 16, 32-bit wide) into a 1 bit wide serial path.
A unique network address or MAC address is coded into chips in the card
card uses DMA (Direct Memory Access) where the computer assigns memory space to the NIC
if the the card can't move data fast enough, the card's buffer RAM holds it temporarily during transmission or reception of data
Sending and Controlling Data
The NICs of the two computers exchanging data agree on the following:
Maximum size of the groups of data being sent
The amount of data to be sent before confirmation
The time intervals between send data chunks
The amount of time to wait before confirmation is sent
How much data each card can hold before it overflows
The speed of the data transmission
Base I/O port: Channel between CPU and hardware
specifies a channel through which information flows between the computer's adapter card and the CPU. Ex. 300 to 30F.
Each hardware device must have a different base I/O port
Base Memory address: Memory in RAM used for buffer area
identifies a location in the computer's RAM to act as a buffer area to store incoming and outgoing data frames. Ex. D8000 is the base memory address for the NIC.
each device needs its own unique address.
some cards allow you to specify the size of the buffer ( 16 or 32 k, for example)
Transceiver:
sometimes selected as on-board or external. External usually will use the AUI/DIX connector: Thicknet, for example
Use jumpers on the card to select which to use
Data Bus Architecture
The NIC must
match the computer's internal bus architecture and
have the right cable connector for the cable being used
ISA (Industry Standard Architecture): original 8-bit and later 16-bit bus of the IBM-PC.
EISA (Extended Industry Standard Architecture): Introduced by consortium of manufacturers and offers a 32-bit data path.
Micro-Channel Architecture (MCA): Introduced by IBM in its PS/2 line. Functions as either 16 or 32 bit.
PCI (Peripheral Component Interconnect): 32-bit bus used by Pentium and Apple Power-PC's. Employs plug and play.
Improving Network Card Performance
Direct Memory Access (DMA):
data is moved directly from the network adapter card's buffer to computer memory.
Shared Adapter Memory:
network adapter card contains memory which is shared with the computer.
The computer identifies RAM on the card as if it were actually installed on the computer
Shared System Memory:
the network adapter selects a portion of the computer's memory for its use.
MOST common
Bus Mastering:
the adapter card takes temporary control of the computer's bus, freeing the CPU for other tasks.
moves data directly to the computer's system memory
Available on EISA and MCA
can improve network performance by 20% to 70%
RAM buffering:
Ram on the adapter card acts as a buffer that holds data until the CPU can process it.
this keeps the card from being a bottleneck
On-board microprocessor:
enables the adapter card to process its own data without the need of the CPU
Wireless Adapter Cards
Used to create an all-wireless LAN
Add wireless stations to a cabled LAN
uses a wireless concentrator, which acts as a transceiver to send and receive signals
Remote-Boot PROMS (Programmable Read Only Memory)
Enables diskless workstations to boot and connect to a network.
Used where security is important.


How a Network Functions
The OSI Model:
International Standards Organization (ISO) specifications for network architecture.
Called the Open Systems Interconnect or OSI model.
Seven layered model, higher layers have more complex tasks.
Each layer provides services for the next higher layer.
Each layer communicates logically with its associated layer on the other computer.
Packets are sent from one layer to another in the order of the layers, from top to bottom on the sending computer and then in reverse order on the receiving computer.
OSI Layers:
Application
Presentation
Session
Transport
Network
Data Link
Physical

Application Layer:
Serves as a window for applications to access network services.
Handles general network access, flow control and error recovery.
Presentation Layer
Determines the format used to exchange data among the networked computers.
Translates data from a format from the Application layer into an intermediate format.
Responsible for protocol conversion, data translation, data encryption, data compression, character conversion, and graphics expansion.
Redirector operates at this level.
Session Layer
Allows two applications running on different computers to establish use and end a connection called a Session.
Performs name recognition and security.
Provides synchronization by placing checkpoints in the data stream.
Implements dialog control between communicating processes.
Transport Layer
Responsible for packet creation.
Provides an additional connection level beneath the Session layer.
Ensures that packets are delivered error free, in sequence with no losses or duplications.
Unpacks, reassembles and sends receipt of messages at the receiving end.
Provides flow control, error handling, and solves transmission problems.
Network Layer
Responsible for addressing messages and translating logical addresses and names into physical addresses.
Determines the route from the source to the destination computer.
Manages traffic such as packet switching, routing and controlling the congestion of data.
Data Link Layer
Sends data frames from the Network layer to the Physical layer.
Packages raw bits into frames for the Network layer at the receiving end.
Responsible for providing error free transmission of frames through the Physical layer.
Physical Layer
Transmits the unstructured raw bit stream over a physical medium.
Relates the electrical, optical mechanical and functional interfaces to the cable.
Defines how the cable is attached to the network adapter card.
Defines data encoding and bit synchronization.

The 802 Project Model
Defines Standards for the Data Link and Physical Layers.
Network Adapter Cards
WAN components
Components used to create twisted-pair and coaxial cable networks.

OSI Model Enhancements
The bottom two layers - Data Link and Physical - define how multiple computers can simultaneously use the network without interfering with each other.
Divides the Data-link layer in to the Logical Link Control and Media Access Control sublayers.
Logical Link Control
manages error and flow control and
defines logical interface points called Service Access Points (SAP's). These SAP's are used to transfer information to upper layers
Media Access Control
communicates directly with the network adapter card and
is responsible for delivering error-free data between two computers.

Drivers:
a device driver is software that tells the computer how to drive or work with the device so that the device performs the job it's supposed to.
Drivers are called
Network Drivers,
MAC drivers,
NIC drivers.
Provide communication between a network adapter card and the redirector in the computer.
Resides in the Media Access Control sublayer of the Data Link layer. Therefore, the NIC driver ensures direct communication between the computer and the NIC
the Media Access Control driver is another name for the network card device driver
When installing a driver, you need to know these things
IRQ
I/O Port Address
Memory Mapped (Base Memory Address)
Transceiver Type

Packets
Data is broken down into smaller more manageable pieces called packets.
Special control information is added in order to:
disassemble packets
reassemble packets
check for errors
Types of data sent includes
Can contain information such as messages or files.
Computer control data and commands and requests.
Session control codes such as error correction and retransmission requests.
Original block of data is converted to a packet at the Transport layer.
Packet Components :
Header
Alert signal to indicate packet is being transmitted
Source address.
Destination address.
Clock synchronization information.
Data
Contains actual data being sent.
Varies from 512 to 4096 bytes (4K), depending on the network
Trailer
Content varies by protocol.
Usually contains a CRC.
Packet Creation:
Look at the example on pp. 201 - 204
Begins at the Application layer where data is generated.
Each layer subsequently adds information to the packet; the corresponding layer on the receiving machine reads the information.
Transport layer breaks the data into packets and adds sequencing information needed to reassemble data at the other end => the structure of the packets is defined by the common protocol being used between the two computers.
Data is passed through the Physical layer to the cable.
Packet Addressing :
every NIC sees all packets sent on its cable segment but only interrupts the computer if the packet address matches the computer's address
a broadcast type address gets attention of all computers on the network

Protocols
Protocols are rules and procedures for communication.

How Protocols Work
The Sending Computer :
Breaks data into packets.
Adds addressing information to the packet
Prepares the data for transmission.

The Receiving Computer (same steps in reverse) :
Takes the packet off the cable.
Strips the data from the packet.
Copies the data to a buffer for reassembly.
Passes the reassembled data to the application.

Protocol Stacks (or Suites):
A combination of protocols, each layer performing a function of the communication process.
Ensure that data is prepared, transferred, received and acted upon.

The Binding Process
Allows more than one protocol to function on a single network adapter card. (e.g. both TCP/IP and IPX/SPX can be bound to the came card
Binding order dictates which protocol the operating systems uses first.
binding also happens with the Operating System architecture: for example, TCP/IP may be bound to the NetBIOS session layer above and network card driver below it. The NIC device driver is in turn bound to the NIC.
Standard Stacks
ISO/OSI
IBM SNA (Systems Network Architecture)
Digital DECnet
Novell NetWare
Apple AppleTalk
TCP/IP
Protocol types map roughly to the OSI Model into three layers:
Application Level Service Users
Application Layer
Presentation Layer
Session Layer
Transport Services
Transport Layer
Network Services
Network Layer
Data Link Layer
Physical Layer
Application Protocols
Work at the upper layer of the OSI model and provide application to application interaction and data exchange.
Examples:
APPC-IBM's peer to peer SNA protocol used on AS400's
FTAM: an OSI file access protocol.
X.400: international e-mail transmissions.
X.500: file and directory services across systems.
SMTP: Internet e-mail.
FTP: Internet file transfer
SNMP: Internet network management protocol.
Telnet: Internet protocol for logging on to remote hosts.
Microsoft SMB: client shells and redirectors.
NCP: Novell client shells or redirectors.
AppleTalk and AppleShare: Apple's protocol suite.
AFP: Apple's protocol for remote file access.
DAP (data access protocol): DECnet file access protocol.
Transport Protocols
These protocols provide communication sessions between computers and ensure data is moved reliably between computers.
Examples:
TCP (transmission control protocol): internet protocol for guaranteed delivery of sequenced data.
SPX (sequenced packet exchange): Novell protocol suite.
NWLink: Microsoft implementation of IPX/SPX.
NetBEUI: establishes communications sessions between computers and provides the underlying data transport services.
ATP, NBP: Apple's communication session and transport protocols.
Network Protocols
These provide link services
They also
handle
addressing and routing,
error checking and
retransmission requests.
Define rules for Ethernet or Token Ring.
Examples:
IP (Internet Protocol): packet forwarding and routing.
IPX: (Internetwork Packet Exchange): Novell's protocol for packet forwarding and routing.
NWLink: Microsoft implementation of IPX/SPX.
NetBEUI: Transport for NetBIOS sessions and applications.
DDP (datagram delivery protocol): An AppleTalk data transport protocol.