How Information Travels on the Internet
Data travels across the internet in packets. Each packet can carry a maximum of 1,500 bytes. Around these packets is a wrapper with a header and a footer. The information contained in the wrapper tells computers what kind of data is in the packet, how it fits together with other data, where the data came from and the data's final destination.
When you send an e-mail to someone, the message breaks up into packets that travel across the network. Different packets from the same message don't have to follow the same path. That's part of what makes the Internet so robust and fast. Packets will travel from one machine to another until they reach their destination. As the packets arrive, the computer receiving the data assembles the packets like a puzzle, recreating the message.
All data transfers across the Internet work on this principle. It helps networks manage traffic -- if one pathway becomes clogged with traffic, packets can go through a different route. This is different from the traditional phone system, which creates a dedicated circuit through a series of switches. All information through the old analog phone system would pass back and forth between a dedicated connection. If something happened to that connection, the call would end.
That's not the case with traffic across IP networks. If one connection should fail, data can travel across an alternate route. This works for individual networks and the Internet as a whole. For instance, even if a packet doesn't make it to the destination, the machine receiving the data can determine which packet is missing by referencing the other packets. It can send a message to the machine sending the data to send it again, creating redundancy. This all happens in the span of just a few milliseconds.
If you're interested in switching to an IP convergence system, do some research first. Many companies provide installation services. While an IP system is relatively easy to maintain, it does require an up-front investment to put the system in place. You'll need to outfit your organization with the proper servers, handsets, video conferencing equipment and wiring to take full advantage of the system's capabilities.
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The OSI Reference Model and the TCP/IP Reference Model
Before two computers or network devices can exchange information, they must establish communication, and this is where protocols come in. A network protocol enables two devices to communicate by using one set of rules. The OSI model and protocol standards help to ensure that networking devices are capable of working together over a network.
Protocols are the hardware or software components that carry out the OSI model guidelines for transferring information on a network. A protocol may be one component or a collection of components that carry out a task. A protocol stack , or protocol suite , is made up of multiple protocols used to exchange information between computers. One protocol in the stack might specify how network interface cards (NICs) communicate, and another might specify how a computer reads information from the NIC( network interface card).
The OSI Seven-Layer Reference Model
In the early 1970s, a problem was brewing. There were many different computer manufacturers, and there were many incompatibilities among them. Furthermore, each manufacturer created different product lines, and even within one company, there were often incompatibilities between product lines. So the International Organization for Standardization (ISO; www.iso.org) got involved and created the Open Systems Interconnection (OSI) reference model, which is a blueprint for device manufacturers and software developers to use when creating products.
The OSI model, shown in Figure 5.8, has seven layers that describe the tasks that must be performed to transfer information on a network. When data is being transferred over a network, it must pass through each layer of the OSI model. As the data passes through each layer, information is added to that data. At the destination, the additional information is removed. Layers 4 through 7 occur at the end node, and Layers 1 through 3 are the most important to telecommunications networks.
Figure 5.8. The OSI reference model
It is important to understand that the OSI model is exactly that model. It is a conceptual framework useful for describing the necessary functions required of a network device or member. No actual networking product implements the model precisely as described.
Layer 7, the application layer, is responsible for exchanging information between the programs running on a computer and other services on a network. This layer supports application and end-user processes. It acts as a window for applications to access network services. It handles general network access, flow control, error recovery, and file transfer. Examples of application layer protocols include File Transfer Protocol (FTP), Telnet, Simple Mail Transfer Protocol (SMTP), and Hypertext Transfer Protocol (HTTP).
Layer 6, the presentation layer, formats information so that a software application can read it. It performs transformations on the data to provide a standardized application interface and common communication services. It offers services such as encryption, compression, and reformatting. The presentation layer adds a field in each packet that tells how the information within the packet is encoded. It indicates whether any compression has been performed and, if so, indicates what type of compression so that the receiver can decompress it properly. It also indicates whether there has been any encryption, and if there has, it indicates what type so that the receiver can properly decrypt it. The presentation layer ensures that the transmitter and receiver see the information in the same format. Typically, Layer 6 processing is handled by an application rather than by a separate process running on a computer. In some cases, Layer 6 processing is handled by a process running at Layer 5.
Layer 5, the session layer, supports connections between sessions and handles administrative tasks and security. It establishes and monitors connections between computers, and it provides the control structure for communication between applications. Examples of session layer protocols include Network Basic Input/Output System (NetBIOS) and Lightweight Directory Access Protocol (LDAP).
Layer 4, the transport layer, corrects transmission errors and ensures that the information is delivered reliably. It provides an end-to-end error recovery and flow control capability. It deals with packet handling, repackaging of messages, division of messages into smaller packets, and error handling. Examples of transport layer protocols include Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Sequenced Packet Exchange (SPX).
Layer 3, the network layer, identifies computers on a network and determines how to direct information transfer over that network. In other words, it is a routing and relaying layer. It defines how to move information between networks, providing the functional and procedural means of transferring variable-length data sequences from a source to a destination via one or more networks while maintaining the QoS requested by the transport layer. The key responsibility of this layer is to perform network routing, flow control, segmentation/desegmentation, and error control functions. Examples of network layer protocols are X.25, Internet Protocol (IP), Internetwork Packet Exchange (IPX), and Message Transfer Part (MTP; part of the PSTN).
Layer 2, the data link layer, groups data into containers to prepare that data for transfer over a network. It puts the ones and zeros into a container that allows the movement of information between two devices on this same network. The protocols at this layer specify the rules that must be followed in transmitting a single frame between one device and another over a single data link. Bits are packaged into frames of data, and they include the necessary synchronization, error control, and flow control information. Examples of data link layer protocols in a LAN environment include Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI). Examples of data link layer protocols in a WAN environment include Frame Relay and Asynchronous Transfer Mode (ATM). Examples of data link layer protocols within the PSTN are Signaling System 7 (SS7) and MTP2.
Layer 1, the physical layer, defines how a transmission medium connects to a computer as well as how electrical or optical information is transferred on the transmission medium. The physical layer defines the types of cables or wireless interfaces that are allowed, the voltage levels used to represent the bits or the optical levels, the types of connectors that are allowed, and the types of transmission rates that can be supported. Every network service and every network device has definitions at the physical layer in terms of what it can physically interface with. For example, the physical layer deals with unshielded twisted-pair (UTP) and shielded twisted-pair (STP), coax, 10BASE-T (an Ethernet standard that allows the use of twisted-pair to support 10Mbps to the desktop, largely for legacy systems), 100BASE-T (the standard enterprises currently favor),multimode fiber and single-mode fiber, xDSL, ISDN, and the various capacities in PDH (e.g., DS-1/DS-3, E-1/E-3) and SDH/SONET (e.g., OC-1 through OC-192) networks.
