- The application layer essentially acts as the end-user interface. This is the layer where inter-action between the mail application (cc:Mail, MS Outlook, and so on) or communications package (SecureCRT for Telnet or FTP Voyager for FTP) and the user occurs.
- The presentation layer is responsible for the agreement and translation of the communication format (syntax) between applications.
- The session layer responsibilities range from managing the application layer’s transfer of information to the data transport portion of the OSI reference model. An example is Sun’s or Novell’s Remote Procedure Call (RPC), which uses Layer 5.
- The transport layer is responsible for the logical transport mechanism, which includes functions conforming to the mechanisms characteristics.
Provides a level of error checking and reliability (through sequence numbers) to the transmission of user data to the lower layers of the OSI reference model. This is the only layer that provides true source-to-destination, end-to-end connectivity through the use of routing protocols such as open shortest path first (OSPF) or the file transfer protocol (FTP) application as examples of TCP.
The most common usage of UDP is streaming media solutions, such as Real Audio.
- The data link layer provides framing, error, and flow control across the network media being used. An important characteristic of this layer is that the information that is applied to it is used by devices to determine if the packet needs to be acted upon by this layer (that is, proceed to Layer 3 or discard).
Serial interfaces do not normally require unique Layer 2 station addresses, such as MAC addresses, unless it is necessary to identify the receiving end in a multipoint network.
24 bits are dedicated for Organization Unique Identification (OUI) and 24 bits are for unique identification.
First 3 bytes of an Ethernet address are the company ID, and the last 3 bytes are assigned by the manufacturer.
- Physical layer is responsible for defining information regarding the physical media, such as electrical, mechanical, and functional specifications to connect two systems.
The physical layer is composed of three main areas: wires, connectors, and encoding.
The data portion of an information unit at a given OSI layer can potentially contain headers, trailers, and data from all the higher layers. This is known as encapsulation.
A protocol is a set of rules and conventions that govern how devices on a network exchange information.
This growth and popularity is primarily due to TCP/IP’s capability to connect different networks regardless of their physical environments.
The term segment describes a unit of data at the TCP layer. At the IP layer, it is called a packet, and at the lower layers, it is called a frame.
If a message is too large for the underlying network topology, it is up to the IP layer to fragment the datagram into smaller parts.
Different paths might be available through the Internet, between a source and a destination station. Fragments of a datagram might take different paths through a network. So, when messages arrive at the destination station, the IP protocol stack must sequence them and reassemble them into their original datagram. Each datagram or fragment is given an IP header and is transmitted as a frame by the lower layers.
TCP provides five important functions within the TCP/IP protocol suite:
- Provides format of the data and acknowledgments that two computers exchange to achieve a reliable transfer
- Ensures that data arrive correctly
- Distinguishes between multiple destinations on a given machine
- Explains how to recover from errors
- Explains how a data stream transfer is initiated and when it is complete
IP offers unreliable, connectionless service because it depends on TCP to detect and recover from lost packets when TCP is being used. Alternatively, when UDP is used, there is no recovery of lost packets because UDP does not have that capability. IP provides three important functions within the TCP/IP protocol suite:
- Defines the basic format and specifications of all data transfer used throughout the protocol suite
- Performs the routing function by choosing a path to the required destination over which data is to be sent
- Includes the previously mentioned functions as well as those covering unreliable packet delivery
Token Ring has multiple wires that connect stations by forming a ring and operates at speeds of 4 Mbps and 16 Mbps.
Fiber distributed data internetworking (FDDI) —A dual fiber optic ring that provides increased redundancy and reliability. FDDI operates at speeds of 100 Mbps. FDDI is still in use, but Gigabit Ethernet and Synchronous Optical Network (SONET), mentioned in the next section, might make FDDI obsolete.
Frame Relay—A good, connection-oriented, frame-switched protocol
Asynchronous transfer mode (ATM): small, fixed-size cells. ATM is a high-speed, low-delay multiplexing and switching technology that can support any type of user traffic, including voice, data, and video applications.
ATM is ideally suited to applications that cannot tolerate time delay, as well as for transporting IP traffic.
Integrated Systems Digital Network (ISDN) —Consists of digital telephony and data transport services using digitization over a specialized telephone network.
SONET: optical fiber-based network. The international equivalent of SONET is synchronous digital hierarchy (SDH). SONET defines interface standards at the physical layer of the OSI seven-layer model. The SONET ANSI standard defines a hierarchy of interface rates that allow data streams of different rates to be multiplexed from optical carrier (OC) levels, from 51.8 Mbps (about the same as a T-3 line) to 2.48 Gbps. The international equivalent of SONET, standardized by the ITU, is called SDH.
Dense wave division multiplexing (DWDM)—An optical multiplexing technique that is used to increase the carrying capacity of a fiber network beyond what can currently be accomplished by time-division multiplexing (TDM) techniques. Different wavelengths of light are used to transmit multiple streams of information along a single fiber with minimal interference. Using DWDM, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a light stream that is transmitted on a single optical fiber. Because each wavelength or channel is demultiplexed at the end of the transmission back into the original source, different data formats being transmitted at different data rates can be transmitted together. DWDM will allow SONET data and ATM data to be transmitted at the same time within the optical fiber.
RFC 1918:
- 10.0.0.0–10.255.255.255—Single Class A network numbers
- 172.16.0.0–172.31.255.255—Contiguous Class B network numbers
- 192.168.0.0–192.168.255.255—Contiguous Class C network numbers
- Class A—255.0.0.0 default mask
- Class B—255.255.0.0 default mask
- Class C—255.255.255.0 default mask
With CIDR, several IP networks appear to networks outside the group as a single, larger entity.
