A modem is like a telephone for a computer to communicate with other computers over telephone lines. Broadly, a modem’s performance depends on the characteristics of the modem and those of the telephone line it is working with. The condition of telephone lines varies widely across the country. So how your modem works will depend to a large extent on the telephone line.
There’re several different types of noise that can happen either individually or in combination in a telephone line. Different modems react differently to these. And there’s no way you can identify which of these conditions affect your line. So nobody can correctly predict which modem will work the best on your line. Test it on your phone line There’s no other way out. You’ll have to convince your vendor to test the modem at your place, and to take it back if it does not work properly.
Internal or external? If you’re buying a modem separately, go for an external fax-modem. External ones are easier to install. They cost Rs 2,000 upward. Buy the cheapest one that works okay with your line, giving you a steady high-speed connect for at least an hour. If you get an internal modem card pre-installed when you buy a new PC, that’s okay, if it’s a 56 kbps fax-modem. An internal card does have one advantage: it reduces clutter. Again, it helps to check that it’s working okay on your line, else you may need to change it Other modem features True plug-and-play with USB
If you’re looking for swift installs and convenience, go for a USB modem. They cost about Rs 4,000. The answering machine modem If you’re looking for a modem that’s also a digital answering machine and a fax machine, go for a message modem. Message modems can store voice messages and faxes even while the PC is switched off. Once the computer is switched on, these messages are transferred to it and can then be heard or viewed. Some also let you retrieve the messages from a remote location—all you have to do is call up your message modem.
Cable is the medium through which information usually moves from one network device to another. There are several types of cable which are commonly used with LANs. In some cases, a network will utilize only one type of cable; other networks will use a variety of cable types. The type of cable chosen for a network is related to the network's topology, protocol, and size. Understanding the characteristics of different types of cable and how they relate to other aspects of a network is necessary for the development of a successful network. The following sections discuss the
types of cables used in networks and other related topics.
Unshielded Twisted Pair (UTP) Cable
Twisted pair cabling comes in two varieties: shielded and unshielded. Unshielded twisted pair (UTP) is the most popular and is generally the best option for school networks (See fig. 1).
Fig.1. Unshielded twisted pair
The quality of UTP may vary from telephone-grade wire to extremely high-speed cable. The cable has four pairs of wires inside the jacket. Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs and other electrical devices. The tighter the twisting, the higher the supported transmission rate and the greater the cost per foot. The EIA/TIA (Electronic Industry Association/Telecommunication Industry Association) has established standards of UTP and rated five categories of wire.
Categories of Unshielded Twisted Pair
Type
Use
Category 1
Voice Only (Telephone Wire)
Category 2
Data to 4 Mbps (LocalTalk)
Category 3
Data to 10 Mbps (Ethernet)
Category 4
Data to 20 Mbps (16 Mbps Token Ring)
Category 5
Data to 100 Mbps (Fast Ethernet)
Buy the best cable you can afford; most schools purmyoung Category 3 or Category 5. If you are designing a 10 Mbps Ethernet network and are considering the cost savings of buying Category 3 wire instead of Category 5, remember that the Category 5 cable will provide more "room to grow" as transmission technologies increase. Both Category 3 and Category 5 UTP have a maximum segment length of 100 meters. In Florida, Category 5 cable is required for retrofit grants. 10BaseT refers to the specifications for unshielded twisted pair cable (Category 3, 4, or 5) carrying Ethernet signals. Category 6 is relatively new and is used for gigabit connections.
Unshielded Twisted Pair Connector
The standard connector for unshielded twisted pair cabling is an RJ-45 connector. This is a plastic connector that looks like a large telephone-style connector (See fig. 2). A slot allows the RJ-45 to be inserted only one way. RJ stands for Registered Jack, implying that the connector follows a standard borrowed from the telephone industry. This standard designates which wire goes with each pin inside the connector.
Fig. 2. RJ-45 connector
Shielded Twisted Pair (STP) Cable
A disadvantage of UTP is that it may be susceptible to radio and electrical frequency interference. Shielded twisted pair (STP) is suitable for environments with electrical interference; however, the extra shielding can make the cables quite bulky. Shielded twisted pair is often used on networks using Token Ring topology.
Coaxial Cable
Coaxial cabling has a single copper conductor at its center. A plastic layer provides insulation between the center conductor and a braided metal shield (See fig. 3). The metal shield helps to block any outside interference from fluorescent lights, motors, and other computers.
Although coaxial cabling is difficult to install, it is highly resistant to signal interference. In addition, it can support greater cable lengths between network devices than twisted pair cable. The two types of coaxial cabling are thick coaxial and thin coaxial.
Thin coaxial cable is also referred to as thinnet. 10Base2 refers to the specifications for thin coaxial cable carrying Ethernet signals. The 2 refers to the approximate maximum segment length being 200 meters. In actual fact the maximum segment length is 185 meters. Thin coaxial cable is popular in school networks, especially linear bus networks.
Thick coaxial cable is also referred to as thicknet. 10Base5 refers to the specifications for thick coaxial cable carrying Ethernet signals. The 5 refers to the maximum segment length being 500 meters. Thick coaxial cable has an extra protective plastic cover that helps keep moisture away from the center conductor. This makes thick coaxial a great choice when running longer lengths in a linear bus network. One disadvantage of thick coaxial is that it does not bend easily and is difficult to install.
Coaxial Cable Connectors
The most common type of connector used with coaxial cables is the Bayone-Neill-Concelman (BNC) connector (See fig. 4). Different types of adapters are available for BNC connectors, including a T-connector, barrel connector, and terminator. Connectors on the cable are the weakest points in any network. To help avoid problems with your network, always use the BNC connectors that crimp, rather than screw, onto the cable.
Fig. 4. BNC connector
Fiber Optic Cable
Fiber optic cabling consists of a center glass core surrounded by several layers of protective materials (See fig. 5). It transmits light rather than electronic signals eliminating the problem of electrical interference. This makes it ideal for certain environments that contain a large amount of electrical interference. It has also made it the standard for connecting networks between buildings, due to its immunity to the effects of moisture and lighting.
Outer insulating jacket is made of Teflon or PVC.
Kevlar fiber helps to strengthen the cable and prevent breakage.
A plastic coating is used to cushion the fiber center.
Center (core) is made of glass or plastic fibers.
Fiber Optic Connector
The most common connector used with fiber optic cable is an ST connector. It is barrel shaped, similar to a BNC connector. A newer connector, the SC, is becoming more popular. It has a squared face and is easier to connect in a confined space.
Ethernet Cable Summary
Specification
Cable Type
Maximum length
10BaseT
Unshielded Twisted Pair
100 meters
10Base2
Thin Coaxial
185 meters
10Base5
Thick Coaxial
500 meters
10BaseF
Fiber Optic
2000 meters
100BaseT
Unshielded Twisted Pair
100 meters
100BaseTX
Unshielded Twisted Pair
220 meters
Wireless LANs
Not all networks are connected with cabling; some networks are wireless. Wireless LANs use high frequency radio signals, infrared light beams, or lasers to communicate between the workstations and the file server or hubs. Each workstation and file server on a wireless network has some sort of transceiver/antenna to send and receive the data. Information is relayed between transceivers as if they were physically connected. For longer distance, wireless communications can also take place through cellular telephone technology, microwave transmission, or by satellite.
Wireless networks are great for allowing laptop computers or remote computers to connect to the LAN. Wireless networks are also beneficial in older buildings where it may be difficult or impossible to install cables.
The two most common types of infrared communications used in schools are line-of-sight and scattered broadcast. Line-of-sight communication means that there must be an unblocked direct line between the workstation and the transceiver. If a person walks within the line-of-sight while there is a transmission, the information would need to be sent again. This kind of obstruction can slow down the wireless network.
Scattered infrared communication is a broadcast of infrared transmissions sent out in multiple directions that bounces off walls and ceilings until it eventually hits the receiver. Networking communications with laser are virtually the same as line-of-sight infrared networks.
Wireless LANs have several disadvantages. They provide poor security, and are susceptible to interference from lights and electronic devices. They are also slower than LANs using cabling.
Installing Cable - Some Guidelines
When running cable, it is best to follow a few simple rules:
Always use more cable than you need. Leave plenty of slack.
Test every part of a network as you install it. Even if it is brand new, it may have problems that will be difficult to isolate later.
Stay at least 3 feet away from fluorescent light boxes and other sources of electrical interference.
If it is necessary to run cable across the floor, cover the cable with cable protectors.
Label both ends of each cable.
Use cable ties (not tape) to keep cables in the same location together.
In a hub, a frame is passed along or "broadcast" to every one of its ports. It doesn't matter that the frame is only destined for one port. The hub has no way
of distinguishing which port a frame should be sent to. Passing it along to every port ensures that it will reach its intended destination. This places a lot of traffic on the network and can lead to poor network response times.
Additionally, a 10/100Mbps hub must share its bandwidth with each and every one of its ports. So when only one PC is broadcasting, it will have access to the maximum available bandwidth. If, however, multiple PCs are broadcasting, then that bandwidth will need to be divided among all of those systems, which will degrade performance.
For easy of use, both the FastEthernet card and such Dual-Speed hubs are usually configured
for automatic configuration : They are supposed to detect the type of hub or network card and
then select automatically the proper connection speed.
That works in most installations very well, but (like usual) NOT always:
sometimes the hub and the network card do not work properly together and the automatic
configuration fails: the network does not work (as it can be tested via the TCP/IPcommand).
As a first diagnostics, have a look at the indicators on your hub:
A switch is a device that performs switching. Specifically, it forwards and filters OSI layer 2 datagrams (chunk of data communication) between ports (connected cables) based on the Mac-Addresses in the packets. This is distinct from a hub in that it only forwards the datagrams to the ports involved in the communications rather than all ports connected. Strictly speaking, a switch is not capable of routing traffic based on IP address (layer 3) which is necessary for communicating between network segments or within a large or complex LAN. Some switches are capable of routing based on IP addresses but are still called switches as a marketing term. A switch normally has numerous ports with the intention that most or all of the network be connected directly to a switch, or another switch that is in turn connected to a switch. Switches is a marketing term that encompasses routers and bridges, as well as devices that may distribute traffic on load or by application content (e.g., a Web URL identifier). Switches may operate at one or more OSI layers, including physical, data link, network, or transport (i.e., end-to-end). A device that operates simultaneously at more than one of these layers is called a multilayer switch. Overemphasizing the ill-defined term "switch" often leads to confusion when first trying to understand networking. Many experienced network designers and operators recommend starting with the logic of devices dealing with only one protocol level, not all of which are covered by OSI. Multilayer device selection is an advanced topic that may lead to selecting particular implementations, but multilayer switching is simply not a real-world design concept. Switch 5530-24TFD is a next-generation stackable 10/100/1000/10000 Mbps Ethernet Layer 3 routing switch designed to provide high-density Gigabit desktop connectivity and Gigabit and 10 Gigabit fiber connectivity for aggregation for mid-size and large enterprise customers’ wiring closets. It combines higher flexibility of deployment using Gigabit copper or fiber connections coupled with exceptional performance utilizing dual 10 Gigabit uplinks.
The Ethernet Routing Switch 5530-24TFD provides with 24 10/100/1000BASE-T RJ-45 ports, 12 shared Small Form-factor Pluggable (SFP) slots, and 2 slots for 10 Gigabit Ethernet Small Form-factor Pluggable (XFP) modules. The switch includes two built-in stacking ports in a compact 1 rack-unit high design. The Ethernet Routing Switch 5530-24TFD may be utilized in standalone mode, or may be stacked together in a mixed stack of 8 units with existing Ethernet Routing Switch 5510-24T/48T or 5520-24T/48T-PWR devices.
Routers are networking devices that forward data packets between networks using headers and forwarding tables to determine the best path to forward the packets. Routers work at the network layer of the TCP/IP model or layer 3 of the OSI model. Routers also provide interconnectivity between like and unlike media (RFC 1812). This is accomplished by examining the Header of a data packet, and making a decision on the next hop to which it should be sent (RFC 1812) They
use preconfigured static routes, status of their hardware interfaces, and routing protocols to select the best route between any two subnets. A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP's network. Some DSL and cable modems, for home (and even office) use, have been integrated with routers to allow multiple home/office computers to access the Internet through the same connection. Many of these new devices also consist of wireless access points (waps) or wireless routers to allow for IEEE 802.11b/g wireless enabled devices to connect to the network without the need for a cabled connection it has some new Wi-Fi products on the market as part of the new LinkSys RangePlus family which promises to increase the range of your home wireless network at affordable prices. The new products include a wireless router (WRT100), notebook adapter (WPC100), PCI adapter (WMP100) and USB Notebook adapter (WUSB100).
The WRT100 router has dropped the Linksys blue color and has taken the flat black look more common of itsCisco parent. The family of RangePlus products uses Multiple Input, Multiple Output (MIMO) technology to provide better coverage in a larger area with fewer dead spots. Although Linksys would like you to buy all the products as they are designed to work together, they will likely support the standard 802.11 b/g. The products do however feature some added
features unique to Linksys that make setup a little easier.
The Linksys Easy Link Advisor on the WRT100 router is a software-based “wizard” that steps users through illustrated instructions for setting up a home network. Also, something called WiFi Protected Setup which simply comes down to a button on the different devices that can be pressed to automatically connect them together.
The WRT100 router and WPC100 notebook adapter are both available now for $99.99 each, and the other RangePlus pricing and products will be coming shortly with availability before the end of the year.
A WAN is a data communications network that covers a relatively broad geographic area (i.e. one city to another and one country to another country) and that often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer.
A Wide Area Network ( WAN) is a computer network covering multiple distance areas, which may spread across the entire world. WANs often connect multiple smaller networks, such as local area networks (LANs) or metro area networks (MANs). The world's most popular WAN is the Internet. Some segments of the Internet are also WANs in themselves. The key difference between WAN and LAN technologies is scalability C WAN must be able to grow as needed to cover multiple cities, even countries and continents.
Both packet switching and circuit switching technologies are used in the WAN. Packet switching allows users to share common carrier resources so that the carrier can make more efficient use of its infrastructure. In a packet switching setup, networks have connections into the carrier's network, and many customers share the carrier's network. The carrier can then create virtual circuits between customers' sites by which packets of data are delivered from one to the other through the network.
Circuit Switching allows data connections to be established when needed and then terminated when communication is complete. This works like a normal telephone line works for voice communication. Integrated Services Digital Network (ISDN) is a good example of circuit switching. When a router has data for a remote site, the switched circuit is initiated with the circuit number of the remote network.
Global Area Network (GAN)
Global area networks (GAN) specifications are in development by several groups, and there is no common definition. In general, however, a GAN is a model for supporting mobile communications across an arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge in mobile communications is "handing off" the user communications from one local coverage area to the next. In IEEE Project 802, this involves a succession of terrestrial Wireless local area networks (WLAN).
MAN is a bigger version of a LAN and uses similar technology. It uses one or two cables but does not contain switching elements. It covers an entire city and may be related to the local cable TV network. A MAN standard - DQDB (Distributed Queue Dual Bus) IEEE 802.6, as shown in Figure 4: • Two unidirectional buses. • Each bus has a head-end, which initiates transmission activity. • Traffic to the right uses the upper bus. • Traffic to the left uses the lower bus.
In recent years, SONET/SDH-based transport networks have come to be considered as too inflexible, inefficient, and overly complex for the purposes of data communication. As the importance of data communication has increased, a search has begun for a replacement for SONET/SDH. However, developing such a replacement is neither easy nor straightforward. On the contrary, many useful functions provided by the SONET/SDH appear to be too complicated to reinvent in other way. Furthermore, long-established telecommunications companies have already invested billions of euros in their SDH networks, and would therefore prefer to utilise the existing infrastructure.
Fortunately, a new solution based on proven SDH/SONET technology is evolving, and promises to turn SONET/SDH into an efficient multi-service transport network that is easy to manage and provision. 'Data over SONET/SDH' (DoS) is based on three new features in SONET/SDH networks: VC (Virtual Concatenation), LCAS (Link Capacity Adjustment Scheme), and GFP (Generic Framing Procedure). VC and LCAS together enable fine-grained capacity allocation and management. Efficient framing and link-layer statistical multiplexing is achieved using GFP, which provides a unified method to multiplex packets from diverse sources. Furthermore, DoS-capable equipment can be mixed with older, inflexible SDH equipment to provide a reasonable evolutionary upgrade path for 'traditional' network operators.
The ability to provide optical connections rapidly and dynamically while making optimal use of network resources is also important. This can be achieved by adding intelligence to a traditional optical transport network and updating it to an Automatic Switched Transport Network (ASTN).
Through the OAN project, a network evaluation platform was developed. The objective was to design and implement the electrical parts of the feeder network, including all the networking activities starting from the link layer.
OAN Platform
The OAN network acts as a feeder network, connecting multiple subscribers to a core network. The test network prototype (Figures 2 and 3) exploits two counter-rotating 2.5 Gbit/s SDH rings in the transport network side, and two Gigabit Ethernet links together with one 2.5 Gbit/s SDH link in the access network side. The network functionality is implemented in Field Programmable Gate Arrays (FPGAs), which enable flexible network design and system testing. The physical equipment of the OAN node is fitted into a standard CompactPCI–frame, making the node easy to move and install. The CompactPCI-frame includes a Central Processing Unit (CPU) card, which is used by the management software.
Figure 2: Access node architecture.
Figure 3: An OAN prototype node is fitted into a standard CompactPCI-frame.
The access network and transport network interfaces are implemented into separate interface and router cards respectively. Data between the interface and router cards is transported using a dedicated bus system and specific transport control. Flexible design of the system allows the interconnection of multiple router and interface cards. By developing new interface card versions, multiple technologies can be supported in the access network side.
To obtain both efficient optical protection and a cost-effective structure, the OAN network is physically a ring and logically a star. The OAN prototype network consists only of three to four nodes, providing low network complexity. One node acts as a hub, connecting other nodes to the core network and to each other. The connections are established using WDM (Wavelength Division Multiplexing) technology.
Furthermore, a network-monitoring system for the OAN network was designed. In the design the usability of Simple Network Management Protocol (SNMP), standardised Management Information Bases (MIBs), and ready-to-use management software was considered.
Next-Generation SONET/SDH in Use The next-generation SONET/SDH enables new types of services with more efficient network usage to be easily implemented by utilising existing infrastructure.
Corporations require diverse services (eg voice, VPN, data storage, and Internet connection services) from operators. Traditionally the different services are provided through technology-specific transport pipes. However, the next-generation SDH enables the simultaneous transport of heterogeneous services over one wavelength, thereby saving network-building and maintenance costs.
Usually a virtual private connection (VPN) is used to bridge operators' access points. In some applications however, it is desirable to transport the native network signal without extracting packets or frames. Normally the datacom protocols rely on 8B/10B coding, which causes a 25 percent increase in bandwidth. Using the next-generation SDH, which maps 8B/10B-coded data into 64B/65B-coded sequences, the required bandwidth is substantially decreased.
A LAN is a high-speed data network that covers a relatively small geographic area. It typically connects workstations, personal computers, printers, servers, and other devices. LANs offer computer users many advantages, including shared access to devices and applications, file exchange between connected users, and communication between users via electronic mail and other applications.
Protocols and the OSI Reference Model
LAN protocols function at the lowest two layers of the OSI reference model, as discussed in Chapter 1, "Internetworking Basics," between the physical layer and the data link layer. Figure 2-2 illustrates how several popular LAN protocols map to the OSI reference model.
Figure 2-2 Popular LAN Protocols Mapped to the OSI Reference Model
Media-Access Methods
Media contention occurs when two or more network devices have data to send at the same time. Because multiple devices cannot talk on the network simultaneously, some type of method must be used to allow one device access to the network media at a time. This is done in two main ways: carrier sense multiple access collision detect (CSMA/CD) and token passing.
In networks using CSMA/CD technology such as Ethernet, network devices contend for the network media. When a device has data to send, it first listens to see if any other device is currently using the network. If not, it starts sending its data. After finishing its transmission, it listens again to see if a collision occurred. A collision occurs when two devices send data simultaneously. When a collision happens, each device waits a random length of time before resending its data. In most cases, a collision will not occur again between the two devices. Because of this type of network contention, the busier a network becomes, the more collisions occur. This is why performance of Ethernet degrades rapidly as the number of devices on a single network increases.
In token-passing networks such as Token Ring and FDDI, a special network frame called a token is passed around the network from device to device. When a device has data to send, it must wait until it has the token and then sends its data. When the data transmission is complete, the token is released so that other devices may use the network media. The main advantage of token-passing networks is that they are deterministic. In other words, it is easy to calculate the maximum time that will pass before a device has the opportunity to send data. This explains the popularity of token-passing networks in some real-time environments such as factories, where machinery must be capable of communicating at a determinable interval.
For CSMA/CD networks, switches segment the network into multiple collision domains. This reduces the number of devices per network segment that must contend for the media. By creating smaller collision domains, the performance of a network can be increased significantly without requiring addressing changes.
Normally CSMA/CD networks are half-duplex, meaning that while a device sends information, it cannot receive at the time. While that device is talking, it is incapable of also listening for other traffic. This is much like a walkie-talkie. When one person wants to talk, he presses the transmit button and begins speaking. While he is talking, no one else on the same frequency can talk. When the sending person is finished, he releases the transmit button and the frequency is available to others.
When switches are introduced, full-duplex operation is possible. Full-duplex works much like a telephone—you can listen as well as talk at the same time. When a network device is attached directly to the port of a network switch, the two devices may be capable of operating in full-duplex mode. In full-duplex mode, performance can be increased, but
not quite as much as some like to claim. A 100-Mbps Ethernet segment is capable of transmitting 200 Mbps of data, but only 100 Mbps can travel in one direction at a time. Because most data connections are asymmetric (with more data traveling in one direction than the other), the gain is not as great as many claim. However, full-duplex operation does increase the throughput of most applications because the network media is no longer shared. Two devices on a full-duplex connection can send data as soon as it is ready.
Token-passing networks such as Token Ring can also benefit from network switches. In large networks, the delay between turns to transmit may be significant because the token is passed around the network.
Transmission Methods
LAN data transmissions fall into three classifications: unicast, multicast, and broadcast.
In each type of transmission, a single packet is sent to one or more nodes.
In a unicast transmission, a single packet is sent from the source to a destination on a network. First, the source node addresses the packet by using the address of the destination node. The package is then sent onto the network, and finally, the network passes the packet to its destination.
A multicast transmission consists of a single data packet that is copied and sent to a specific subset of nodes on the network. First, the source node addresses the packet by using a multicast address. The packet is then sent into the network, which makes copies of the packet and sends a copy to each node that is part of the multicast address.
A broadcast transmission consists of a single data packet that is copied and sent to all nodes on the network. In these types of transmissions, the source node addresses the packet by using the broadcast address. The packet is then sent on to the network, which makes copies of the packet and sends a copy to every node on the network.
Topologies
LAN topologies define the manner in which network devices are organized. Four common LAN topologies exist: bus, ring, star, and tree. These topologies are logical architectures, but the actual devices need not be physically organized in these configurations. Logical bus and ring topologies, for example, are commonly organized physically as a star. A bus topology is a linear LAN architecture in which transmissions from network stations propagate the length of the medium and are received by all other stations. Of the three
most widely used LAN implementations, Ethernet/IEEE 802.3 networks—including 100BaseT—implement a bus topology, which is illustrated in Figure 2-3.
Figure 2-3 Some Networks Implement a Local Bus Topology
A ring topology is a LAN architecture that consists of a series of devices connected to one another by unidirectional transmission links to form a single closed loop. Both Token Ring/IEEE 802.5 and FDDI networks implement a ring topology. Figure 2-4 depicts a logical ring topology.
Figure 2-4 Some Networks Implement a Logical Ring Topology
A star topology is a LAN architecture in which the endpoints on a network are connected to a common central hub, or switch, by dedicated links. Logical bus and ring topologies are often implemented physically in a star topology, which is illustrated in Figure 2-5.
A tree topology is a LAN architecture that is identical to the bus topology, except that branches with multiple nodes are possible in this case. Figure 2-5 illustrates a logical tree topology.
A Logical Tree Topology Can Contain Multiple Nodes
Devices
Devices commonly used in LANs include repeaters, hubs, LAN extenders, bridges, LAN switches, and routers.
The answering machine modem
