Previous | Table of Contents | Next |
The Application Layer is responsible for providing the interface between lower layers and the user’s application programs. It is rich with application programming interface (API) function calls. The Application Layer utilizes API function calls to pass data and control information to and from the lower layers.
Some Application Layer responsibilities include:
Some Application Layer specifications include:
There are data communications protocols that exist to provide the orderly exchange of information in a network. One such protocol is the Transmission Control Protocol/Internet Protocol (TCP/IP). TCP/IP was originally developed by the Department of Defense to connect, or network, Department of Defense computers with university computers. TCP/IP is a set of rules used by software programmers who write networking code. There are other network protocols, usually proprietary.
Every protocol must specify how the network components will identify data and control information. A fundamental component of network protocols is the grouping of data and control information into clearly defined and therefore manageable buckets called frames. The most basic component of a frame is the simple binary bit. Bits are grouped into bytes which consist of eight bits. The bytes are then grouped together into frames. Frames are grouped together to form packets.
The position of the bits in a byte and the position of the bytes in the frame determine if the network components will interpret the bits as data or as control information. The bits are transmitted serially, that is, one after the other. Depending upon the network software, the first bit in a byte is either interpreted as the most significant bit (big endian) or as the least significant bit (little endian). And the network software also must interpret each transmitted or received byte as either a big endian or little endian. The choice of big endian or little endian is not significant as long as all the network components interpret the bits and bytes in the same fashion.
Why do we care about upper layer protocols when ATM is itself a lower layer (layers 1 to 3) protocol? Because ATM/ADSL must interwork with all protocols for ATM/ADSL to realize their full potential as unifying global technologies. Too much capital is invested in legacy systems for them to be replaced solely for migration to an ATM network platform. We must understand the interworking relationships between ATM/ADSL and the legacy protocols that we might maximize the opportunities ATM/ADSL provide. A beginning to the understanding follows.
Transmission Control Protocol/Internet Protocol (TCP/IP) is a software protocol for the configuration of networks. Included in the protocol is the ability to provide remote logon services, remote file transfers, and electronic e-mail. In reference to our OSI layering model, TCP/IP protocol makes use of the Network Layer, Transport Layer, and Application Layer.
IPX/SPX is Novell’s proprietary network software. IPX/SPX protocol provides for the ability to set the framing type used by network adapter drivers. The following framing specifications are supported by the IPX/SPX protocol: IEEE 802.2, IEE 802.3, Ethernet II, Ethernet SNAP, Token Ring, and Token SNAP. Specifically, a network administrator has the ability to choose the particular framing used by the network.
One of the key components of a network protocol is the ability to group electrical signals into precise, meaningful units. Meaningful for whom or what? Well, eventually, meaningful to the end user which may or may not be a human (how about automatic feeders for livestock?), and certainly meaningful to the hardware and software that acts upon or reacts to the individual signals. Electrical signals that are without a precise, known ordering based upon some defined relationship are not much more useful than noise. The typical bit arrangement of modern networks is called a frame. Each protocol must define the bit framing used to pass information from source to destination. The following is a more in-depth look at bit framing.
The objective of a communications network is to reliably transfer information from source to destination within some specified performance criteria, such as speed and bandwidth utilization. In order to accomplish the purpose of communication networks, communications protocols specify how a system will segment and package the data, called a Protocol Data Unit (PDU). Segmentation and packaging of the user data is necessary to maximize the use of the available bandwidth due to the bursty nature of data communications. Each data package, or PDU, is called a frame. Contained within the PDU is not only the user data but also routing and frame control information. The routing and frame control data is called the PDU header. The data is called, interestingly enough, the data unit. In terms of the OSI communications model, a frame is a group of data at the Data Link Layer while a group of frames forms a packet at the Network Layer.
A LAN connecting several computers in an office setting represents a communications network that does not need to be switched to perform its intended function. Each computer is hard-wired to the server. However, a server connected to another computer through the public switched telephone network in a dial-up mode is switched. An example of such a network is an office LAN whose server provides Internet access to the office client computers by dialing up the Internet.
Communication networks may or may not be switched between source and destination, depending upon the application and the geographical location of the system elements. WANs, GANs, and the Internet are examples of networks that must pass data through switched intermediate networks. Also, any particular user may have the ability to connect to more than one user. If a network connection is not switched, it is referred to as a permanent virtual circuit (PVC). When a network connection is switched, it is referred to as a switched virtual circuit (SVC).
Previous | Table of Contents | Next |