Networking Model

The International Standards Organization (ISO) has defined a standard called the Open Systems Interconnection (OSI) reference model. This is a seven layer architecture listed below. Each layer is considered to be responsible for a different part of the communications. This concept was developed to accommodate changes in technology. The layers are arranged here from the lower levels starting with the physical (hardware) to the higher levels.

1. Physical Layer - The actual hardware. Concerned with the connection between the computer and the network.
2. Data Link Layer - Data transfer method (802x ethernet). Puts data in frames and ensures error free transmission. Also controls the timing of the network transmission. IEEE divided this layer into the two following sublayers.
1. Media Access Control (MAC) - Used to coordinate the sending of data between computers. The 802.3, 4, 5, and 12 standards apply to this layer. If you hear someone talking about the MAC address of a network card, they are referring to the hardware address of the card.
2. Logical Link control (LLC) - Maintains the Link between two computers by establishing Service Access Points (SAPs) which are a series of interface points. IEEE802.2.
3. Network Layer - IP network protocol. Routes messages using the best path available. Concerned with message priority, status, and data congestion.
4. Transport Layer - TCP, UDP. Provides properly sequenced and error free transmission. Recombines fragmented packets.
5. Session Layer - Determines when the session is begun or opened, how long it is used, and when it is closed. concerned with security and name recognition.
6. Presentation Layer - ASCII or EBCDEC data syntax. Makes the type of data transparent to the layers around it. Used to translate date to computer specific format such as byte ordering. It may include compression. It prepares the data, either for the network or the application depending on the direction it is going.
7. Application Layer - Provides the ability for user applications to interact with the network.

Many protocol stacks overlap the borders of the seven layer model. Transmission Control Protocol (TCP) provides the function of session and some of the transport layer. The Internet Protocol (IP) provides the function of the rest of the transport and most of the network layer. Netware Core Protocol (NCP) provides the function of the application, presentation, and the session layer.

When we talk about Local Area Network (LAN) technology the IEEE 802 standard may be heard. This standard defines networking connections for the interface card and the physical connections, describing how they are done. The 802 standards were published by the Institute of Electrical and Electronics Engineers (IEEE). The 802.3 standard is called ethernet, but the IEEE standards do not define the exact original true ethernet standard that is common today. There is a great deal of confusion caused by this.

The ethernet standard data encapsulation method is defined by RFC 894. RFC 1042 defines the IP to link layer data encapsulation for networks using the IEEE 802 standards. The 802 standards define the two lowest levels of the seven layer network model and primarily deal with the control of access to the network media. The network media is the physical means of carrying the data such as network cable. The control of access to the media is called media access control (MAC). The 802 standards are listed below:

• 02.1 - Internetworking
• 802.2 - Logical Link Control *
• 802.3 - Ethernet or CSMA/CD, Carrier-Sense Multiple Access with Collision detection LAN *
• 802.4 - Token-Bus LAN *
• 802.5 - Token Ring LAN *
• 802.6 - Metropolitan Area Network (MAN)
• 802.7 - Broadband Technical Advisory Group
• 802.8 - Fiber-Optic Technical Advisory Group
• 802.9 - Integrated Voice/Data Networks
• 802.10 - Network Security
• 802.11 - Wireless Networks
• 802.12 - Demand Priority Access LAN, 100 Base VG-AnyLAN

*The ones with stars should be remembered in order for network certification testing.

Network Access Methods

• Contention
• Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) - Used by Ethernet
• Carrier-Sense Multiple Access with Collision Avoidance
• Token Passing
• Demand Priority - Describes a method where intelligent hubs control data transmission. A computer will send a demand signal to the hub indicating that it wants to transmit. The hub will respond with an acknowledgement that will allow the computer to transmit. The hub will allow computers to transmit in turn. An example of a demand priority network is 100VG-AnyLAN (IEEE 802.12). It uses a star-bus topology.
• Polling - A central controller, also called the primary device will poll computers, called secondary devices, to find out if they have data to transmit. Of so the central controller will allow them to transmit for a limited time, then the next device is polled.