Street children and their overall strategy of survival

Data net Vs Network

By. Kh. Atiar Rahman

The network is the computer’s most apposite portrayal of client/server configuring where the users would like to feel that somewhere on the network, the services they need are available and are accessible based on a criteria and right of access, without regard to the technologies involved. When ready to move beyond personal productivity stand-alone applications and into client/server applications, organizations must address the issues of connectivity. Initially, most users discover their need to access a printer that is not physically connected to their client workstation. It is observed that sharing data files among non-networked individuals in the same office can be handled by hand-carrying diskettes, but printing is more awkward. The first LANs installed are usually basic networking services to support this printer-sharing requirement. Now a printer anywhere in the local area can be authorized for shared use. The physical medium to accomplish this connection is the LAN cabling. Each workstation is connected to a cable that routes the transmission either directly to the next workstation on the LAN or to a hub point that routes the transmission to the appropriate destination. There are two primary LAN topologies that use Ethernet (bus) and Token Ring (ring).

Ethernet and Token Ring are put into practice on well-defined Institute of Electrical and Electronic Engineers (IEEE) industry principles. These principles identify the product requirement detail and afford a pledge to a fixed measurement. This standardization has encouraged hundreds of vendors to develop competitive products and in turn has caused the functionality, performance, and cost of these LAN connectivity products to improve spectacularly over the last five years. Older LAN installations that use nonstandard topologies will eventually require replacement. There is a basic functional difference in the way Ethernet and Token Ring topologies placed data on the cable. With the Ethernet protocol, the processor attempts to unload data onto the cable whenever it requires service. Workstations vie for the bandwidth with these attempts, and the Ethernet protocol includes the appropriate logic to resolve collisions when they occur. On the other hand, with the Token Ring protocol, the processor only attempts to put data onto the cable when there is capacity on the cable to accept the transmission. Workstations pass along a token that one after the other gives each workstation the right to put data on the network.

Latest developments in the capabilities of intelligent hubs have changed the way we design LANs. Hubs owe their success to the efficiency and robustness of the 10BaseT protocol, which enables the implementation of Ethernet in a star fashion over Unshielded Twisted Pair (UTP) wiring. Now commonly used, hubs provide integrated support for the different standard topologies such as Ethernet, Token Ring, and Fiber (specifically, the FDDI protocol) over different types of cabling. By repeating or amplifying signals where necessary, they enable the use of high quality UTP cabling in virtually every situation. Hubs have evolved to provide tremendous flexibility for the design of the physical LAN topologies in large office buildings or plants. Various design strategies are now available. They are also an effective vehicle to put management intelligence throughout the LANs in a corporation, allowing control and monitoring capabilities from a network management center. Newer token-passing protocols, such as Fiber Distributed Data Interface (FDDI) and Copper Distributed Data Interface (CDDI), will increase in use as higher performances LANs (particularly backbone LANs) are required. CDDI can be implemented on the same LAN cable as Ethernet and Token Ring if the original selection and installation are done carefully according to industry recommendations. FDDI usually appears first as the LAN-to-LAN Bridge between floors in large buildings. Wireless LANs offer an substitute to wiring. Instead of cabling, these LANs use the airwaves as the communications medium. Motorola provides a system—Altair—that supports standard Ethernet transmission protocols and cards. The Motorola implementation cables workstations together into micro cells using standard Ethernet cabling. These micro cells communicate over the airwaves to similarly configured servers. Communications on this frequency do not pass through outside walls, so there is little problem with interference from other users. Wireless LANs are attractive when the cost of installing cabling is high. Costs tend to be high for cabling in old buildings, in temporary installations, or where workstations move frequently. NCR provides another implementation of wireless LAN technology using publicly accessible frequencies in the 902-MHz to 928-MHz band. NCR provides proprietary cards to provide the communications protocol. This supports lower-speed communications that are subject to some interference, because so many other devices, such as remote control electronic controllers and antitheft devices use this same frequency.

It is now a well-accepted fact that LANs are the preferred vehicle to provide overall connectivity to all local and distant servers. WAN connectivity should be provided through the interconnection of the LANs. Router and bridges are devices that perform that task. Routers are the preferred technology for complex network topologies, generating efficient routing of data packets between two systems by locating and using the optimal path. They also limit the amount of traffic on the WAN by efficiently filtering and by providing support for multiple protocols across the single network. WAN bandwidth for data communications is a critical issue. In terminal-to-host networks, traffic generated by applications could be modeled, and the network would then be sized accordingly, allowing for effective use of the bandwidth. With LAN interconnections, and applications that enable users to transfer large files (such as through e-mail attachments) and images, this modeling is much harder to perform. WAN services that have recently emerged, such as Frame Relay, SMDS (Switched Multimegabit Data Service), and imminent ATM (Asynchronous Transfer Mode) services, enable the appropriate flexibility inherently required for these applications. Frame Relay uses efficient statistical multiplexing to provide shared network resources to users. Each access line is shared by traffic destined for multiple locations. The access line speed is typically sized much higher than the average throughput each user is paying for. This enables peak transmissions (such as when a user transmits a large file) that are much faster because they use all available bandwidth. SMDS is a high-speed service that uses cell relay technology, which enables data, voice, and video to share the same network fabric. Available from selected RBOCs as a wide-area service, it supports high speeds well over 1.5 Mbps. ATM is an emerging standard and set of communication technologies that span both the LAN and the WAN to create a seamless network. It provides the appropriate capabilities to support all types of voice, data, and video traffic. Its speed is defined to be 155 Mbps, with variations and technologies that may enable it to run on lower speed circuits when economically appropriate. It will operate both as a LAN and a WAN technology, providing full and transparent integration of both environments. ATM will be the most significant connectivity technology after 1995. ATM provides the set of services and capabilities that will truly enable the "computing anywhere" concept, in which the physical location of systems and data is made irrelevant to the user. It also provides the network managers with the required flexibility to respond promptly to business change and new applications. Interoperability between distributed systems is not guaranteed by just providing network-based connectivity. Systems need to agree on the end-to-end handshakes that take place while exchanging data, on session management to set up and break conversations, and on resource access strategies. Network Management is an integral part of every network. The Simple Network Management Protocol (SNMP) is a well-accepted standard used to manage LANs and WANs through the management capabilities of hubs, routers, and bridges. It can be extended to provide basic monitoring performance measurements of servers and workstations. Full systems management needs much more functionality than SNMP can offer. The OSI management protocol, the Common Management Information Protocol (CMIP), which has the flexibility and capability to fully support such management requirements, will likely compete with an improved version of SNMP, SNMP V2. The existence of heterogeneous LAN environments in large organizations makes interoperability a practical reality. Organizations need and expect to view their various workgroup LANs as an integrated corporate-wide network. Citicorp, for example, is working to integrate its 100 independent networks into a single global net.1 The OSI model provides the framework definition for developers attempting to create interoperable products.2 Because many products are not yet OSI-compliant, there often is no direct correspondence between the OSI model and reality. The OSI model defines seven protocol layers and specifies that each layer be insulated from the other by a well-defined interface.

In view of the above it is evident that the physical layer is the lowest level of the OSI model and defines the physical and electrical characteristics of the connections that make up the network. It includes such things as interface specifications as well as detailed specifications for the use of twisted-pair, fiber-optic, and coaxial cables. Standards of interest at this layer for client/server applications are IEEE 802.3 (Ethernet), and IEEE 802.5 (Token Ring) that define the requirements for the network interface card (NIC) and the software requirements for the media access control (MAC) layer. Other standards here include the serial interfaces EIA232 and X.21. The data link layer defines the basic packets of data expected to enter or leave the physical network. Bit patterns, encoding methods, and tokens are known to this layer. The data link layer detects errors and corrects them by requesting retransmission of corrupted packets or messages. This layer is actually divided into two sub layers: the media access control (MAC) and the logical link control (LLC). The MAC sublayer has network access responsibility for token passing, collision sensing, and network control. The LLC sublayer operates above the MAC and sends and receives data packets and messages. Ethernet, Token Ring, and FDDI define the record format of the packets (frames) being communicated between the MAC layer and Network layer. The internal formats are different and without conversion workstations cannot interoperate with workstations that operate with another definition. And in this connection the network layer is responsible for switching and routing messages to their proper destinations. It coordinates the means for addressing and delivering messages. It provides for each system a unique network address, determines a route to transmit data to its destination, segments large blocks of data into smaller packets of data, and performs flow control. When a message contains more than one packet, the transport layer sequences the message packets and regulates inbound traffic flow. The transport layer is responsible for ensuring end-to-end error-free transmission of data. The transport layer maintains its own addresses that get mapped onto network addresses. Because the transport layer services process on systems, multiple transport addresses can share a single network address. Indeed, the session layer provides the services that enable applications running at two processors to coordinate their communication into a single session. A session is an exchange of messages—a dialog between two processors. This layer helps create the session, inform one workstation if the other drops out of the session, and terminate the session on request. The presentation layer is responsible for translating data from the internal machine form of one processor in the session to that of the other. The application layer is the layer to which the application on the processor directly talks. The programmer codes to an API defined at this layer. Messages enter the OSI protocol stack at this level, travel through the layers to the physical layer, across the network to the physical layer of the other processor, and up through the layers into the other processor application layer and program.

Connectivity and interoperability between the client workstation and the server are achieved through a combination of physical cables and devices, and software that implements communication protocols. One of the most important and most unnoticed parts of LAN implementation today is the physical cabling plant. A corporation's investment in cabling is significant. For most though, it is viewed strictly as a tactical operation, a necessary expense. Implementation costs are too high, and maintenance is a no budgeted, nonexistent process. The results of this shortsightedness will be seen in real dollars through the life of the technology. Studies have shown that over 65 percent of all LAN downtime occurs at the physical layer. It is important to provide a platform to support robust LAN implementation, as well as a system flexible enough to incorporate rapid changes in technology. The trend is to standardize LAN cabling design by implementing distributed star topologies around wiring closets, with fiber between wiring closets. Desktop bandwidth requirements can be handled by copper (including CDDI) for several years to come; however, fiber between wiring closets will handle the additional bandwidth requirements of a backbone or switch-to-switch configuration. Obviously, fiber to the desktop will provide extensive long-term capabilities; however, because of the electronics required to support various access methods in use today, the initial cost is significant. As recommended, the design will provide support for Ethernet, 4M and 16M Token Ring, FDDI, and future ATM LANs. Wiring standards include RG-58 A/U coaxial cable (thin-wire 10Base2 Ethernet), IBM Type 1 (shielded, twisted pair for Token Ring), unshielded twisted pair (UTP for 10BaseT Ethernet or Token Ring) and Fiber Distributed Data Interface (FDDI for 10BaseT or Token Ring). Motorola has developed a wireless Ethernet LAN product—Altair—that uses 18-GHz frequencies. NCR's Wave LAN provides low-speed wireless LAN support. Wireless LAN technology is useful and cost-effective when the cost of cable installation is high. In old buildings or locations where equipment is frequently moved, the cost of running cables may be excessive. In these instances wireless technology can provide an attractive alternative. Motorola provides an implementation that uses standard Ethernet NICs connecting a group of closely located workstations together with a transmitter.

The transmitter communicates with a receiver across the room to provide the workstation server connection. Recent reductions in the cost of this technology make it attractive for those applications where the cost of cabling is more than $250 per workstation. Wireless communication is somewhat slower than wired communication. Industry tests indicate a performance level approximately one-half that of wired 10-Mbps UTP Ethernet. NCR's alternative wireless technology, Wave LAN, is a slow-speed implementation using proprietary communications protocols and hardware. It also is subject to interference by other transmitters, such as remote control electronics, antitheft equipment, and point-of-sale devices. Ethernet is the most widely installed network topology today. Ethernet networks have a maximum throughput of 10 Mbps. The first network interface cards (NICs) developed for Ethernet were much cheaper than corresponding NICs developed by IBM for Token Ring. Until recently, organizations who used non-IBM minicomputer and workstations equipment had few options other than Ethernet. Even today in a heterogeneous environment, there are computers for which only Ethernet NICs are available. The large market for Ethernet NICs and the complete definition of the specification have allowed over 100 companies to produce these cards.3 Competition has reduced the price to little more than $100 per unit. 10BaseT Ethernet is a standard that enables the implementation of the Ethernet protocol over telephone wires in a physical star configuration (compatible with phone wire installations). Its robustness, ease of use, and low cost driven by hard competition have made 10BaseT the most popular standards-based network topology. Its pervasiveness is unrivaled: In 1994, new laptop computers will start to ship with 10BaseT built in. IBM is now fully committed to support Ethernet across its product line. IBM uses the Token Ring LAN protocol as the standard for connectivity in its products. In an environment that is primarily IBM hardware and SNA connectivity, Token Ring is the preferred LAN topology option. IBM's Token Ring implementation is a modified ring configuration that provides a high degree of reliability since failure of a node does not affect any other node. Only failure of the hub can affect more than one node. The hub isn't electric and doesn't have moving parts to break; it is usually stored in a locked closet or other physically secure area. Token Ring networks implement a wire transmission speed of 4 or 16 Mbps. Older NICs will support only the 4-Mbps speed, but the newer ones support both speeds. IBM and Hewlett-Packard have announced a technical alliance to establish a single 100Mbps standard for both Token Ring and Ethernet networks. This technology, called 100VG-AnyLAN, will result in low-cost, high-speed network adapter cards that can be used in PCs and servers running on either Token Ring or Ethernet LANs. The first Any LAN products are expected in early 1994 and will cost between $250 and $350 per port. IBM will be submitting a proposal to make the 100VG-AnyLAN technology a part of IEEE's 802.12 (or 100Base-VG) standard, which currently includes only Ethernet.

The Ethernet technique mechanism may function well when the cable is lightly loaded but, because of rear-ender that occur when an attempt is made to put data onto a busy cable, the technique provides poor performance when the LAN utilization exceeds 50 percent. To recover from the collisions, the sender retries, which puts additional load on the network. Ethernet users avoid this problem by creating subnets that divide the LAN users into smaller groups, thus keeping a low utilization level. In spite of  the prevalent implementation of Ethernet, Token Ring installations are mounting at a fast rate for client/server applications. IBM's commitment to Ethernet may slow this success, because Token-Ring will always cost more than Ethernet. Figure 5.3 presents the results of a recent study of installation plans for Ethernet, Token Ring, and FDDI. The analysis predicts a steady increase in planned Token Ring installations from 1988 until the installed base is equivalent in 1996. However, this analysis does not account for the emergence of a powerful new technology which has entered the marketplace in 1993, Asynchronous Mode, or ATM. It is likely that by 1996 ATM will dominate all new installations and will gradually replace existing installations by degrees.