Copyright: Created by: Jonathan Cretsinger


THE OSI MODEL - Application Layer
The Application Layer

The user communication interface is specified at this layer along with management of communications between applications.


THE OSI MODEL - Presentation Layer
The Presentation Layer

Here, data is formatted or translated into a format that is mutually understandable between applications / systems. Encryption and compression can also occur here.


THE OSI MODEL - Session Layer
The Session Layer

The session layer is responsible for dialog control. This includes the establishment, maintenance, and synchronization of dialog between applications


The OSI MODEL - Transport Layer
The Transport Layer

The primary purpose of the transport layer is to provide reliable delivery. This is performed by connection establishment, acknowledgments, sequence numbers, and flow control. Multiplexing is also a function of many transport layer implementations.


The OSI MODEL - Network Layer
The Network Layer

As the name implies, network addressing occurs at this layer when there are multiple interconnected networks.


OSI MODEL - Data Link
The Data Link Layer

Electrical impulses representing 1’s and 0’s are organized into frames. A frame is a logical grouping of information. Error detection, flow control, and identification also occur here.


THE OSI MODEL - Physical Layer
The Physical Layer

The mechanical and electrical specifications for network medium and interface hardware are defined here. Also defined is how data is placed on and retrieved from the medium.


Repeaters and Amlifiers
REPEATERS / AMPLIFIERS

Please select one ....


THE OSI MODEL - Introduction
The OSI MODEL

In 1977, the International Organization for Standardization created a subcommittee to develop data communications standards that promote multivendor interoperabilty. The committee developed the Open Systems Interconnection (OSI) Model.

The Model serves as a guideline for communications between systems and does not specify any standard for these communications. The analogy of building a house best describes the function of the OSI model.

Typically, when a home builder builds a house, many sub-contractors are employed on the project. The builder, via a blueprint, specifies what is needed, but it is up to each sub-contractor to rely on his own expertise and experience to get the job done. One sub-contractor may have a completely different solution than the next, however, the end result to the home buyer is the same. For example, one plumbing contractor may decide to run copper pipes in the basement or foundation while another might use flexible tubing in the attic. In either case, when the home buyer moves in and turns on the shower, water flows from the shower head.

Builders find this method of construction most efficient because all of their eggs aren’t in one basket. If, for some reason, a sub-contractor needs to be replaced, the entire process does not need to be revamped.

The OSI model is defined in layers. Each layer has its own responsibilities and as long as a solution in a layer can interface with the adjacent layers and perform its required function, how it does it is virtually irrelevant. Just as when a plumber runs the pipes, the cabinet installer doesn’t really care how they got there when she installs the vanity.


ROUTERS

THIS AREA IS UNDER CONSTRUCTION!


BRIDGES

THIS AREA UNDER CONSTRUCTION!


Gatways
GATEWAYS

THIS AREA IS UNDER CONSTRUCTION!


ETHERNET

THIS AREA IS UNDER CONSTRUCTION!


THE OSI MODEL  - Token Ring
TOKEN RING

THIS AREA IS UNDER CONSTRUCTION!


LOCAL TALK

THIS AREA IS UNDER CONSTRUCTION!


The OSI MODEL - Graphic #2


NCP

THIS AREA IS UNDER CONSTRUCTION!


NFS

THIS AREA IS UNDER CONSTRUCTION!


THE OSI MODEL - SNA
SNA

THIS AREA IS UNDER CONSTRUCTION!


SNA

Systems Network Architecture


NCP

Netware Core Protocol


SPX

Sequenced Packet Exchange

is a low speed, high reliability transport protocol. Like TCP it is connection oriented and provides reliable delivery. Some Netware services require SPX such as printing and Rconsole.

<< MORE >>


IPX

Internetwork Packet Exchange

provides high speed, low reliability messaging similar to that of UDP/IP. It is primarily responsible for addressing and internetwork routing functions. Most Netware transactions use only IPX.

<< MORE >>


NFS

Network File System

was developed by Sun Microsystems and allows users to access files on other systems. Unlike TELNET and FTP, NFS is transparent. Through remote procedure calls, NFS enables users of dissimilar computers and operating systems to access remote file systems without the need to learn new network-specific commands. Remote files appear to be part of the local system.


FTP

File Transfer Protocol

Enables a user to move file copies between two hosts. It provides login, directory inspection, file manipulation, command execution, format specification, and help. Although designed for use by programs, most implementations provide an interactive interface for humans.


TFTP

Trivial File Transfer Protocol


TELNET

Terminal Emulation

TELNET is a program which allows users to access other hosts over a network. Once attached and logged in, you can give the remote  computer commands.


SMTP

Simple Mail Transfer Protocol

is an electronic mail routing application which uses UDP and IP. It does not provide a user interface, thus a mail front end must be used.


TCP

Transmission Control Protocol

Roughly corresponds to the Transport layer of the OSI model. It accepts messages of any length from an upper layer protocol and provides full duplex, acknowledged, connection oriented, flow controlled transport to a TCP peer in a remote network station.

<< BACKGROUND >>


UDP

User Datagram Protocol

works at the Transport layer but unlike TCP it is not connection oriented and does not provide acknowledgments or flow control. Because of this, UDP is much faster than TCP and is preferred by many upper layer protocols which handle these needs themselves.


IP

Internet Protocol

Roughly corresponds to the Network layer of the OSI model and provides connectionless, non-guaranteed delivery of Transport layer packets across an internetwork. It is responsible for addressing and may also fragment Transport layer packets into smaller parts then reassemble them at a router or the destination host.

<< BACKGROUND >>


ICMP

Internet Control Message Protocol

handles error and control messages. Specifically, gateways and hosts use ICMP to send reports of problems about datagrams back to the original source that sent the datagram. ICMP also includes and echo request/reply used to test whether a destination is reachable and responding. The PING (Packet InterNet Groper) uses ICMP.


SNMP

Simple Network Management Protocol


ARP

Address Resolution Protocol


RARP

Reverse Address Resolution Protocol


LLC

Logical Link Control


MAC

Media Access Control


OSI

Open Systems Interconnection


THE OSI MODEL -Graphic #1
REVISION#                                                                                             << CREDITS >>


ARP / RARP

This Area is Under Construction!


ICMP

THIS AREA IS UNDER CONSTRUCTION!


IP

THIS AREA IS UNDER CONSTRUCTION!


Repeater
REPEATERS

Repeaters pick up the transmission, reconstruct it, and place a fresh signal onto the medium. This regeneration allows a signal to travel distances beyond the point where it would become corrupted.


Amplifiers
AMPLIFIER

Amplifiers are devices which simply receive a signal and increase it. These are not commonly used for data transmission since they do nothing to correct for attenuated (deteriorated) signals.


UPD
UDP

THIS AREA IS UNDER CONSTRUCTION!


THE OSI MODEL - TCP
TCP

THIS AREA IS UNDER CONSTRUCTION!


IPX

THIS AREA IS UNDER CONSTRUCTION!


THE OSI MODEL - SPX
SPX

THIS AREA IS UNDER CONSTRUCTION!


ROUTERS

Routers operate up through the Network Layer of the OSI model. Network addresses are logical addresses usually assigned by the administrator. Routers are capable of determining the best path on which to send data and they can be used to connect dissimilar networks such as an Ethernet network with a token ring network.


GATEWAYS

Gateways are used to connect networks which use different upper layer protocols. A common example of this is seen when a Netware network is integrated with an SNA network. The gateway is used to translate the upper 4 layer protocols.


ETHERNET

Ethernet was originally invented by the Xerox Corporation and released jointly by Digital, Intel, and Xerox (DIX). In fact Ethernet is proprietary. However, the IEEE adopted the 802.3 standard which was built on Ethernet. Although most implementations today are 802.3, we still refer to them as Ethernet. It employs a 10 Mbps signaling rate and CSMA/CD channel access over a variety of media, such as NIC's.


NICS

Generally there are three types of connectors used today.

The original Ethernet specification requires and AUI (or DIX) connector. This is used with 10BASE5 cabling. The installation involves the use of an external transceiver - the device which “listens” to the channel. This implementation is becoming increasingly rare.

One is more likely to find one of the following two types of cards being used. An RJ-45 connector is used with 10BASE-T cabling (twisted pair). And a BNC connector is used for 10BASE2 (Thinnet). A transceiver is mounted on the card, therefore, an external one is unnecessary.

Most cards will have a combination of two of the above connectors with the ability to choose between them. Some have only one and others even have all three. Although not widely used yet, there are available, fiber optic Ethernet cards. Currently, the expense of these cards has made them cost prohibitive for most organizations.

Ethernet card settings are usually set by jumpers or software and rarely with DIP switches.

All NIC’s also have a socket on them which accommodates the installation of a Remote Boot Prom which is used for booting diskless workstations on the network. Diskless workstations are primarily found in high security environments.

Token-Ring NICs typically are set with DIP switches, however, it is usually possible to alter these settings via software. In particular, the physical address can be substituted. This is especially convenient when attaching a node to an SNA network where it is common practice that the systems programmer determines addressing. Some settings found on token-ring NICs are ring speed, primary or secondary, DMA, cable type, memory address, I/O port, etc.

Two connector types found on token-ring cards are the 9 pin (which actually only uses 4) and the RJ-45, depending on the cable type.

As with other network interface cards, a boot prom socket is available for setting up diskless workstations.


10BASE2

This is a coaxial cable commonly known as Thinnet. It is RG-58/U cabling. The designation stands for 10 Mbps, Baseband signaling, 185m maximum segment length.


BRIDGES

Amplifiers and repeaters operate at the Physical layer. Bridges operate at the Datalink layer. Because of this, bridges have the ability to connect LAN’s together. They are very good for segmenting a network with heavy traffic since they can discern physical station addresses.


The OSI MODEL - Graphic #3


The OSI MODEL - Graphic #3a


The OSI MODEL - Graphic #3b


The OSI MODEL - Graphic # 3c


TCP Background Info.
TCP/IP BACKGROUND

Possibly the most popular set of communication protocols for connecting systems in  diverse physical layer environments, is The Internet Protocol Suite. Of this suite of protocols, TCP and IP are the most widely known. So much so, that often the term TCP/IP refers to the entire suite.

Research was originally funded by the Department of Defense and the suite developed by Stanford University and Bolt, Beranek, and Newman for the ARPANET (Advanced Research Projects Agency Network) which was used to connect government agencies, research institutes and universities. Today it has evolved into what is now The Internet, our Information Superhighway, with millions of users.


Token Ring
Token-Ring

IBMs Token-Ring is actually a physical star but a logical ring which uses a token passing channel access method. Nodes are attached to the network via MAUs or CAUs (token-ring hubs). Ring speed can be set at either 4 Mbps or 16 Mbps. The IEEE standard 802.5 reflects Token-Ring technology.

<< NIC's >>


LOCAL TALK

LocalTalk is Apples proprietary implementation of the first two OSI layers. All Macs are shipped with LocalTalk capability built in.

The physical interface is EIA RS-422 and shielded twisted pair cabling is used. LocalTalks signaling rate is 230.4 Kbps. While this may seem to be a low rate, especially when compared to Ethernet at 10 Mbps, the channel access method used makes up some of the difference in performance. LocalTalk uses CSMA/CA.

A unique feature of LocalTalk is the addressing system. Basically, when a node attaches to the network it announces its desired address. If there is a node on the network which is using that address already, it objects and the attaching node tries another address. This process continues until an unused address is determined.


RPC

Remote Procedure Calls

involve using a redirector (or shell) which determines whether a request can be satisfied locally or if it should be sent out on the network. Netware, Vines, and LAN Manager are popular systems which use RPC.


IPX/SPX Background Info.
IPX and SPX are Novells implementation of the Network and Transport layers of the OSI model in the Netware operating system.


NETWARE

is a network operating system modeled after Xeroxs XNS and developed by  Novell, Inc.


FILER SERVER
FILE SERVER

At the heart of the Netware network is the file server. It is a PC which runs the Netware operating system software. In many cases it is installed on a PC specifically design to be a server. Its purpose is to provide file storage and other sharing services to clients.


CLIENT

A personal computer attached to the network is known as a client. However, any device on the network which requests services from the server is also a client. This includes printers and other servers.


Distributed Processing

The Netware file server is not responsible for performing user application processing functions. Unlike in a mainframe environment a PC requests an application from the server and runs the application locally.


ODI

is the Open Datalink Interface. Developed by Novell and Apple, ODI provides a means in which on NIC can support multiple protocols simultaneously.


LAN Driver

 also known as an MLID (Multiple Link Interface Driver) in the ODI environment, provides the means for the software to communicate with the NIC .


Credits for HELPCNTR.HLP
OSI_MD.HLP

Graphics (Corel Draw),
 Stucture and layout created
 using VB AHA!!! and Microsoft Hulk
 by:

Jonathan Cretsinger

Initial data imported from
Microsoft Word
input by:

Victor Clement


How to contact the author of OSI_MD.HLP
Jonathan Cretsinger

Contact Via E-Mail:

From Within Hughes Aircraft: (cc:Mail, Office Vision)

SMPTGATE.CRETSIN2 at EMIS

Internet:

CRETSINGER@AOL.COM

OSI_MD.HLP created with VB AHA!!! version 1.36


OSI Model Revision Date
REV. #01.B 10/29/1994

Contact Jonathan Cretsinger in regards to updating or adding to this file.


How to contact the author of VB AHA!
VB AHA! SUPPORT

Via E-Mail

From Within Hughes Aircraft:

 SMTPGATE.VILLALON at EMIS

Internet:

villalon@crl.com

Regular U.S Mail:

Craig Villalon
2074 Pleasant Hill Rd.
Sebastopol, CA 95472-4948

<< REQUIREMENTS / ORDERING >>


REQUIREMENT TO USE VB AHA!
Introduction to VB AHA!!!

VB AHA!!! is an inexpensive and easy to use utility for writing Windows help files. You won't need any expensive wordprocessors or have to use any cryptic commands. With simple mouse clicks and Drag n Drop ease you will be able to construct your help files quickly.
If you use Visual Basic then VB AHA!!! will handle all of the HelpContextID values for you and even recompile your programs with the new values. No more hassles of having to insert the values by hand! If you program in C/C++ or Pascal then VB AHA!!! will create a header file containing all of the definitions that you will need for context sensitive help in your programs.

The Shareware version is VBAHA123.ZIP (VB AHA! Ver. 1.23) Copy llocated on AOL (America On-Line)

REQUIREMENTS

You must have the following files: (Included with Microsoft HULK 2.1)

HC.EXE - the Microsoft Help Compiler. Only needed to actually compile the
Help file, you can still design it without this program.
VBRUN300.DLL - the Visual Basic runtime library.
SHED.EXE -  Microsofts .SHG file editor. To make graphics that have
hotspots (buttons) on them.
Not really necessary to use VB AHA!!!.

<<< Registering / Ordering VB AHA!!! >>>

File MSHULK.ZIP is located on AOL (America On-Line)


HULK


Registering VB AHA!!!
Registering / Ordering

  You may register by credit card, Compuserve or check. See the
appropriate section below for more information.
CREDIT CARD ORDERS ONLY
 You can order with MC, Visa, Amex, or Discover from Public
(software) Library by calling:
800-2424-PsL or 713-524-6394
or by FAX to:
713-524-6398
or by CIS Email to:
71355,470
You can also mail credit card orders to:
PsL
P.O.Box 35705
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You will need the following information to register:
product: VB AHA!!!
product #11196
$50 US and $54 Canada/overseas will be billed to your credit card.

 THE ABOVE NUMBERS ARE FOR ORDERS ONLY.

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  Any questions about the status of the shipment of the order,
refunds, registration options, product details, technical support,
volume discounts, dealer pricing, site licenses, etc, must be
directed to me at one of the following:

Internet:     villalon@crl.com
CompuServe:    73261,2665 (Compuserve members only)
or regular mail to:
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599 Teresa Ct.
Sebastopol, CA 95472

  To insure that you get the latest version, PsL will notify me the
day of your order and I will ship the product directly to you.

Compuserve Members
  VB AHA!!! may now be registered in Compuserves Shareware
Registration forum, GO SWREG and use one of the following in
your search:

 Registration ID:    2021
 Program Title:      VB AHA!!! V1.31
 Author:                     Craig Villalon
 CompuServe ID:  [73261,2665]


Information on MS HULK
Microsoft HULK, the Help UniversaL Kit.  Microsoft HULK covers all the common tasks involved in editing, modifying, testing and building Help files.  The kit contains special Help tools and provides a standard interface that makes it easier to work in all of these areas.
Microsoft HULK is designed to work with all international help source files.  In particular, we have tested it on Western, Eastern European, Middle East and Far East versions of Windows with single byte, double byte and BiDi source files.

The Microsoft Help UniversaL Kit was developed to make it easier to use the many different tools used in the process of creating, modifying, translating, testing and building Windows based Help files.  By installing Microsoft HULK on your machine, you will automatically also install many of the frequently used Help tools.  In addition, Microsoft HULK ships with a number of unique tools that are not available anywhere else.


CMSA
CSMA

Carrier Sense Multiple Access

" contention schemes "


CSMA/CA

Carrier Sense Multiple Access
With Collision Avoidance

" contention schemes "


CSMA/CD

Carrier Sense Multiple Access
With Collision Detection.

" contention schemes "


Contention Schemes
Contention

Contention schemes are known by various names: CSMA - CSMA/CA - CSMA/CD .
There is little effective difference among these variations and it is practical to view them all simply as the contention access method.

In a contention scheme, access is on a first-com, first-served basis.  This is a probabilistic access scheme.  The interval between messages from a particular workstation can only be approximated.  Probabilistic schemes are satisfactory for office automation applications.

When a workstation needs to transmit, it first "listens" to the cable to  determine if another workstation is transmitting.  If the cable is clear, the workstation transmits.  If two or more workstations transmit simultaneously, their messages will collide.

As soon as the workstation detects a collision, it ceases transmission, monitors the network until it hears no other traffic, and retransmits.  Most contention implementations assign a unique retry algorithm to each workstation to vary the wait-and-retry period.  The retry algorithm reduces the likelihood that after a collision two workstations will transmit retries simultaneously.