Section 1.3: Networks

A network is defined as a group of two or more computers linked together for the purpose of communicating and sharing information and other resources, such as printers and applications. Most networks are constructed around a cable connection that links the computers, however, modern wireless networks that use radio wave or infrared connections are also becoming quite prevalent. These connections permit the computers to communicate via the wires in the cable, radio wave or infrared signal. For a network to function it must provide connections, communications, and services.

  • Connections are defined by the hardware or physical components that are required to connect a computer to the network. This includes the network medium, which refers to the hardware that physically connects one computer to another, i.e., the network cable or a wireless connection; and the network interface, which refers to the hardware that attaches a computer to the network medium and is usually a network interface card (NIC).
  • Communications refers to the network protocols that are used to establish the rules governing network communication between the networked computers. Network protocols allow computers running different operating systems and software to communicate with each.
  • Services define the resources, such as files or printers, that a computer shares with the rest of the networked computers.

1.3.1: Network Definitions

Computer networks can be classified and defined according to geographical area that the network covers. There are four network definitions: a Local Area Network (LAN), a Campus Area Network (CAN), a Metropolitan Area Network (MAN), and a Wide Area Network (WAN). There are three additional network definitions, namely the Internet, an intranet and an Internetwork. These network definitions are discussed in Table 1.2.

Table 1.2: Network Definitions

Definition Description
Local Area Network (LAN) A LAN is defined as a network that is contained within a closed environment and does not exceed a distance of 1.25 mile (2 km). Computers and peripherals on a LAN are typically joined by a network cable or by a wireless network connection. A LAN that consists of wireless connections is referred to as a Wireless LAN (WLAN).
Campus Area Network (CAN) A CAN is limited to a single geographical area but may exceed the size of a LAN
Metropolitan Area Network (MAN) A MAN is defined as a network that covers the geographical area of a city that is less than 100 miles.
Wide Area Network (WAN) A WAN is defined as a network that exceeds 1.25 miles. A WAN often consists of a number of LANs that have been joined together. A CAN and a MAN is also a WAN. WANs typically connected numerous LANs through the internet via telephone lines, T1 lines, Integrated Services Digital Network (ISDN) lines, radio waves, cable or satellite links.
Internet The Internet is a world wide web of networks that are based on the TCP/IP protocol and is not own by a single company or organization.
Intranet An intranet uses that same technology as the Internet but is owned and managed by a company or organization. A LAN or a WAN s usually an intranet.
Internetwork An internetwork consists of a number of networks that are joined by routers. The Internet is the largest example of an internetwork.

Of these network definitions, the most common are the Internet, the LAN and the WAN.

1.3.2: Types of Networks

These network definitions can be divided into two types of networks, based on how information is stored on the network, how network security is handled, and how the computers on the network interact. These two types are: Peer-To-Peer (P2P) Networks and Server/Client Networks. The latter is often also called Server networks.

  • On a Peer-To-Peer (P2P) Network, there is no hierarchy of computers; instead each computer acts as either a server which shares its data or services with other computers, or as a client which uses data or services on another computer. Furthermore, each user establishes the security on their own computers and determines which of their resources are made available to other users. These networks are typically limited to between 15 and 20 computers. Microsoft Windows for Workgroups, Windows 95, Windows 98, Windows ME, Windows NT Workstation, Windows 2000, Novell's NetWare, UNIX, and Linux are some operating systems that support peer-to-peer networking.
  • A Server/Client Network consists of one or more dedicated computers configured as servers. This server manages access to all shared files and peripherals. The server runs the network operating system (NOS) manages security and administers access to resources. The client computers or workstations connect to the network and use the available resources. Among the most common network operating systems are Microsoft's Windows NT Server 4, Windows 2000 Server, and Novell's NetWare. Before the release of Windows NT, most dedicated servers worked only as hosts. Windows NT allows these servers to operate as an individual workstation as well.

1.3.3: Network Topologies

The layout of a LAN design is called its topology. There are three basic types of topologies: the star topology, the bus topology, and the ring topology. Hybrid combinations of these topologies also exist.

  • In a network based on the star topology, all computers and devices are connected to a centrally located hub or switch. The hub or switch collects and distributes the flow of data within the network. When a hub is used, data from the sending host are sent to the hub and are then transmitted to all hosts on the network except the sending host. Switches can be thought of as intelligent hubs. When switches are used rather than hubs, data from the sending host are sent to the switch which transmits the data to the intended recipient rather than to all hosts on the network.

    Figure 1.3: The Star Topology

  • In a network based on the bus topology, all computers and devices are connected in series to a single linear cable called a trunk. The trunk is also known as a backbone or a segment. Both ends of the trunk must be terminated to stop the signal from bouncing back up the cable. Because a bus network does not have a central point, it is more difficult to troubleshoot than a star network. Furthermore, a break or problem at any point along the bus can cause the entire network to go down.

    Figure 1.4: The Bus Topology

  • In a network based on a ring topology, all computers and devices are connected to cable that forms a closed loop. On such networks there are no terminating ends; therefore, if one computer fails, the entire network will go down. Each computer on such a network acts like a repeater and boosts the signal before sending it to the next station. This type of network transmits data by passing a "token" around the network. If the token is free of data, a computer waiting to send data grabs it, attaches the data and the electronic address to the token, and sends it on its way. When the token reaches its destination computer, the data is removed and the token is sent on. Hence this type of network is commonly called a token ring network.

    Figure 1.5: The Ring Topology

Of these three network topologies, the star topology is the most predominant network type and is based on the Ethernet standard.

1.3.4: Network Technologies

Various network technologies can be used to establish network connections, including Ethernet, Fiber Distribution Data Interface (FDDI), Copper Distribution Data Interface (CDDI), Token Ring, and Asynchronous Transfer Mode (ATM). Of these, Ethernet is the most popular choice in installed networks because of its low cost, availability, and scalability to higher bandwidths. Ethernet

Ethernet is based on the Institute of Electrical and Electronics Engineers (IEEE) 802.3 standard and offers a bandwidth of 10 Mbps between end users. Ethernet is based on the carrier sense multiple access collision detect (CSMA/CD) technology, which requires that transmitting stations back off for a random period of time when a collision occurs.

Coaxial cable was the first media system specified in the Ethernet standard. Coaxial Ethernet cable comes in two major categories: Thicknet (10Base5) and Thinnet (10Base2). These cables differed in their size and their length limitation. Although Ethernet coaxial cable lengths can be quite long, they susceptible to electromagnetic interference (EMI) and eavesdropping.

Table 1.3: Coaxial Cable for Ethernet

Cable Diameter Resistance Bandwidth Length
Thinnet (10Base2) 10 mm 50 ohms 10 Mbps 185 m
Thicknet (10Base5) 5 mm 50 ohms 10 Mbps 500 m

Today most wired networks use twisted-pair media for connections to the desktop. Twisted-pair also comes in two major categories: Unshielded twisted-pair (UTP) and Shielded twisted-pair (STP). One pair of insulated copper wires twisted about each other forms a twisted-pair. The pairs are twisted top reduce interference and crosstalk. Both STP and UTP suffer from high attenuation, therefore these lines are usually restricted to an end-to-end distance of 100 meters between active devices. Furthermore, these cables are sensitive to EMI and eaves dropping. Most networks use 10BaseT UPT cable.

An alternative to twisted-pair cable is fiber optic cable (10BaseFL), which transmits light signals, generated either by light emitting diodes (LEDs) or laser diodes (LDs), instead of electrical signals. These cables support higher transmission speeds and longer distances but are more expensive. Because they do not carry electrical signals, fiber optic cables are immune to EMI and eavesdropping. They also have low attenuation which means they can be used to connect active devices that are up to 2 km apart. However, fiber optic devices are not cost effective while cable installation is complex.

Table 1.4: Twisted-Pair and Fiber Optic Cable for Ethernet

Cable Technology Bandwidth Cable Length
Twisted-Pair (10BaseT) 10 Mbps 100 m
Fiber Optic (10BaseFL) 10 Mbps 2,000 m Fast Ethernet

Fast Ethernet operates at 100 Mbps and is based on the IEEE 802.3u standard. The Ethernet cabling schemes, CSMA/CD operation, and all upper-layer protocol operations have been maintained with Fast Ethernet. Fast Ethernet is also backward compatible with 10 Mbps Ethernet. Compatibility is possible because the two devices at each end of a network connection can automatically negotiate link capabilities so that they both can operate at a common level. This negotiation involves the detection and selection of the highest available bandwidth and half-duplex or full-duplex operation. For this reason, Fast Ethernet is also referred to as 10/100 Mbps Ethernet.

Cabling for Fast Ethernet can be either UTP or fiber optic. Specifications for these cables are shown in Table 1.5.

Table 1.5: Fast Ethernet Cabling and Distance Limitations

Technology Wiring Type Pairs Cable Length
100BaseTX EIA/TIA Category 5 UTP 2 100 m
100BaseT2 EIA/TIA Category 3,4,5 UTP 2 100 m
100BaseT4 EIA/TIA Category 3,4,5 UTP 4 100 m
100BaseFX Multimode fiber (MMF) with 62.5 micron core; 1300 nm laser 1 400 m (half-duplex)
2,000 m (full-duplex)
  Single-mode fiber (SMF) with 62.5 micron core; 1300 nm laser 1 10,000 m Gigabit Ethernet

Gigabit Ethernet is an escalation of the Fast Ethernet standard using the same IEEE 802.3 Ethernet frame format. Gigabit Ethernet offers a throughput of 1,000 Mbps (1 Gbps). Like Fast Ethernet, Gigabit Ethernet is compatible with earlier Ethernet standards. However, the physical layer has been modified to increase data transmission speeds: The IEEE 802.3 Ethernet standard and the American National Standards Institute (ANSI) X3T11 FibreChannel. IEEE 802.3 provided the foundation of frame format, CSMA/CD, full duplex, and other characteristics of Ethernet. FibreChannel provided a base of high-speed ASICs, optical components, and encoding/decoding and serialization mechanisms. The resulting protocol is termed IEEE 802.3z Gigabit Ethernet.

Gigabit Ethernet supports several cabling types, referred to as 1000BaseX. Table 1.6 lists the cabling specifications for each type.

Table 1.6: Gigabit Ethernet Cabling and Distance Limitations

Technology Wiring Type Pairs Cable Length
1000BaseCX Shielded Twisted Pair (STP) 1 25 m
1000BaseT EIA/TIA Category 5 UTP 4 100 m
1000BaseSX Multimode fiber (MMF) with 62.5 micron core; 850 nm laser 1 275 m
  Multimode fiber (MMF) with 50 micron core; 1300 nm laser 1 550 m
1000BaseLX/LH Multimode fiber (MMF) with 62.5 micron core; 1300 nm laser 1 550 m
  Single-mode fiber (SMF) with 50 micron core; 1300 nm laser 1 550 m
  Single-mode fiber (SMF) with 9 micron core; 1300 nm laser 1 10 km
1000BaseZX Single-mode fiber (SMF) with 9 micron core; 1550 nm laser 1 70 km
  Single-mode fiber (SMF) with 8 micron core; 1550 nm laser 1 100 km

1.3.5: Network Addressing

Network addressing identifies either individual devices or groups of devices on a LAN. A pair of network devices that transmit frames between each other use a source and destination address field to identify each other. These addresses are called unicast addresses, or individual addresses, because they identify an individual network interface card (NIC).

The IEEE defines the format and assignment of network addresses by requiring manufacturers to encode globally unique unicast Media Access Control (MAC) addresses on all NICs. The first half of the MAC address identifies the manufacturer of the card and is called the organizationally unique identifier (OUI).

1.3.6: Bridging

Bridging is used to connect two network segments. This alleviates congestion problems on a single Ethernet segment and extends allowed cabling distances because the segments on each side of the bridge conformed to the same distance limitation as a single segment. This bridge is called "transparent bridging" because the end-point devices do not need to know that the bridge exists.

Transparent bridges forward frames only when necessary and, thus, reduces network overhead. To accomplish this, transparent bridges learning MAC addresses by examining the source MAC address of each frame received by the bridge; decides when to forward a frame or when to filter a frame, based on the destination MAC address; and creates a loop-free environment with other bridges by using the Spanning-Tree Protocol.

Generally, broadcasts and multicast frames are forwarded by the bridge in networks that use bridges. In addition, transparent bridges perform switching of frames using Layer 2 headers and Layer 2 logic and are Layer 3 protocol-independent. Store-and-forward operation, which means that the entire frame is received before the first bit of the frame is forwarded, is also typical in transparent bridging devices. However, the transparent bridge must perform processing on the frame, which also can increase latency.

A transparent bridge operates in the following manner:

  • The bridge has no initial knowledge of the location of any end device; therefore, the bridge must listen to frames coming into each of its ports to figure out on which network a device resides.
  • The bridge constantly updates its bridging table upon detecting the presence of a new MAC address or upon detecting a MAC address that has changed location from one bridge port to another. The bridge is then able to forward frames by looking at the destination address, looking up the address in the bridge table, and sending the frame out the port where the destination device is located.
  • If a frame arrives with the broadcast address as the destination address, the bridge must forward or flood the frame out all available ports. However, the frame is not forwarded out the port that initially received the frame. Hence, broadcasts are able to reach all available networks. A bridge only segments collision domains but does not segment broadcast domains.
  • If a frame arrives with a destination address that is not found in the bridge table, the bridge is unable to determine which port to forward the frame to for transmission. This is known as an unknown unicast. In this case, the bridge treats the frame as if it was a broadcast and forwards it out all remaining ports. After a reply to that frame is received, the bridge will learn the location of the unknown station and add it to the bridge table.
  • Frames that are forwarded across the bridge cannot be modified.

1.3.7: LAN Switching

An Ethernet switch uses the same logic as a transparent bridge, but performs more functions, has more features, and has more physical ports. Switches use hardware to learn MAC addresses and to make forwarding and filtering decisions, whereas bridges use software.

A switch listens for frames that enter all its interfaces. After receiving a frame, a switch decides whether to forward a frame and out which port(s). To perform these functions, switches perform three tasks:

  • Learning, which means that the switch learns MAC addresses by examining the source MAC address of each frame the bridge receives. Switches dynamically learn the MAC addresses in the network to build its MAC address table. With a full, accurate MAC address table, the switch can make accurate forwarding and filtering decisions. Switches build the MAC address table by listening to incoming frames and examining the frame's source MAC address. If a frame enters the switch, and the source MAC address is not in the address table, the switch creates an entry in the table. The MAC address is placed in the table, along with the interface in which the frame arrived. This allows the switch to make good forwarding choices in the future. Switches also forward unknown unicast frames, which are frames whose destination MAC addresses are not yet in the bridging table, out all ports, which is called flooding, with the hope that the unknown device will be on some other Ethernet segment and will reply. When the unknown device does reply, the switch will build an entry for that device in the address table.
  • Forwarding or filtering, which means that the switch decides when to forward a frame or when to filter it, i.e., not to forward it, based on the destination MAC address. Switches reduce network overhead by forwarding traffic from one segment to another only when necessary. To decide whether to forward a frame, the switch uses a dynamically built table called a bridge table or MAC address table. The switch looks at the previously learned MAC addresses in an address table to decide where to forward the frames.
  • Loop prevention, which means that the switch creates a loop-free environment with other bridges by using Spanning-Tree Protocol (STP). Having physically redundant links helps LAN availability, and STP prevents the switch logic from letting frames loop around the network indefinitely, congesting the LAN.

Frames sent to unicast addresses are destined for a single device; frames sent to a broadcast address are sent to all devices on the LAN. Frames sent to multicast addresses are meant for all devices that care to receive the frame. Thus, when a switch receives a frame, it checks if the address is a unicast address, a broadcast address or a multicast address. If the address is unicast, and the address is in the address table, and if the interface connecting the switch to the destination device is not the same interface on which the frame arrived, the switch forwards the frame to the destination device. If the address is not in the address table, the switch forwards the frame on all ports. If the address is a broadcast or multicast address, the switch also forwards the frame on all ports.

The internal processing on a switch can decrease latency for frames. Switches can use store-and-forward processing as well as cut-through processing logic. With cut-through processing, the first bits of the frame are sent out the outbound port before the last bit of the incoming frame is received. However, because the frame check sequence (FCS) is in the Ethernet trailer, a cut-through forwarded frame might have bit errors that the switch will not notice before sending most of the frame.

1.3.8: Wireless Networks

Conventional Ethernet networks require cables connected computers via hubs and switches. This has the effect of restricting the computer's mobility and requires that even portable computers be physically connected to a hub or switch to access the network. An alternative to cabled networking is wireless networking. The first wireless network was developed at the University of Hawaii in 1971 to link computers on four islands without using telephone wires. Wireless networking entered the realm of personal computing in the 1980s, with the advent to networking computers. However, it was only in the early 1990s that wireless networks started to gain momentum when CPU processing power became sufficient to manage data transmitted and received over wireless connections.

Wireless networks use network cards, called Wireless Network Adapters, that rely radio signals or infrared (IR) signals to transmit and receive data via a Wireless Access Point (WAP). The WAP uses has an RJ-45 port that can be attached to attach to a 10BASE-T or 10/100BASE-T Ethernet hub or switch and contains a radio transceiver, encryption, and communications software. It translates conventional Ethernet signals into wireless Ethernet signals it broadcasts to wireless network adapters on the network and performs the same role in reverse to transfer signals from wireless network adapters to the conventional Ethernet network. WAP devices come in many variations, with some providing the Cable Modem Router and Switch functions in addition to the wireless connectivity.

Note: Access points are not necessary for direct peer-to-peer networking, which is called ad hoc mode, but they are required for a shared Internet connection or a connection with another network. When access points are used, the network is operating in the infrastructure mode. Wireless Network Standards

In the absence of an industry standard, the early forms of wireless networking were single-vendor proprietary solutions that could not communicate with wireless network products from other vendors. In 1997, the computer industry developed the IEE 802.11 wireless Ethernet standard. Wireless network products based on this standard are capable of multivendor interoperability.

The IEEE 802.11 wireless Ethernet standard consists of the IEEE 802.11b standard, the IEEE 802.11a standard, and the newer IEEE 802.11g standard.

Note: The Bluetooth standard for short-range wireless networking is designed to complement, rather than rival, IEEE 802.11-based wireless networks.

  • IEEE 802.11 was the original standard for wireless networks that was ratified in 1997. It operated at a maximum speed of 2 Mbps and ensured interoperability been wireless products from various vendors. However, the standard had a few ambiguities allowed for potential problems with compatibility between devices. To ensure compatibility, a group of companies formed the Wireless Ethernet Compatibility Alliance (WECA), which has come to be known as the Wi-Fi Alliance, to ensure that their products would work together. The term Wi-Fi is now used to refer to any IEEE 802.11 wireless network products that have passed the Wi-Fi Alliance certification tests.
  • IEEE 802.11b, which is also called 11 Mbps Wi-Fi, operates at a maximum speed of 11 Mbps and is thus slightly faster than 10BASE-T Ethernet. Most IEEE 802.11b hardware is designed to operate at four speeds, using three different data-encoding methods depending on the speed range. It operates at 11 Mbps using quatenery phase-shift keying/complimentary code keying (QPSK/CCK); at 5.5 Mbps also using QPSK/CCK; at 2 Mbps using differential quaternary phase-shift keying (DQPSK); and at 1 Mbps using differential binary phase-shift keying (DBPSK). As distances change and signal strength increases or decreases, IEEE 802.11b hardware switches to the most suitable data-encoding method.

    Wireless networks running IEEE 802.11b hardware use the 2.4 GHz radio frequency band that many portable phones, wireless speakers, security devices, microwave ovens, and the Bluetooth short-range networking products use. Although the increasing use of these products is a potential source of interference, the short range of wireless networks (indoor ranges up to 300 feet and outdoor ranges up to 1,500 feet, varying by product) minimizes the practical risks. Many devices use a spread-spectrum method of connecting with other products to minimize potential interference.

    IEEE 802.11b networks can connect to wired Ethernet networks or be used as independent networks.

  • IEEE 802.11a uses the 5 GHz frequency band, which allows for much higher speeds, reaching a maximum speed of 54 Mbps. The 5 GHz frequency band also helps avoid interference from devices that cause interference with lower-frequency IEEE 802.11b networks. IEEE 802.11a hardware maintains relatively high speeds at both short and relatively long distances.

    Because IEEE 802.11a uses the 5 GHz frequency band rather than the 2.4 GHz frequency band used by IEEE 802.11b, standard IEEE 802.11a hardware cannot communicate with 802.11b hardware. A solution to this compatibility problem is the use of dual-band hardware. Dual-band hardware can work :with either IEEE 802.11a or IEEE 802.11b networks, enabling you to move from an IEEE 802.11b wireless network at home or at Starbucks to a faster IEEE 802.11a office network.

  • IEEE 802.11g is also known as Wireless-G and combines compatibility with IEEE 802.11b with the speed of IEEE 802.11a at longer distances. This standard was ratified in mid-2003, however, many network vendors were already selling products based on the draft IEEE 802.11g standard before the final standard was approved. These early IEEE 802.11g hardware was slower and less compatible than the specification promises. In some cases, problems with early-release IEEE 802.11g hardware can be solved through firmware upgrades. Wireless Network Modes

Wireless networks work in one of two modes that are also referred to as topologies. These two modes are ad-hoc mode and infrastructure mode. The mode you implement depends on whether you want your computers to communicate directly with each other, or via a WAP.

  • In ad-hoc mode, data is transferred to and from wireless network adapters connected to the computers. This cuts out the need to purchase a WAP. Throughput rates between two wireless network adapters are twice as fast as when you use a WAP. However, a network in ad-hoc mode cannot connect to a wired network as a WAP is required to provide connectivity to a wired network. An ad-hoc network is also called a peer-to-peer network.
  • In infrastructure mode, data is transferred between computers via a WAP. Because a WAP is used in infrastructure mode, it provides connectivity with a wired network, allowing you to expand a wired network with wireless capability. Your wired and wirelessly networked computers can communicate with each other. In addition, a WAP can extend your wireless network's range as placing a WAP between two wireless network adapters doubles their range. Also, some WAPs have a built-in router and firewall. The router allows you to share Internet access between all your computers, and the firewall hides your network. Some of these multifunction access points include a hub with RJ-45 ports. Security Features

Because wireless networks can be accessed by anyone with a compatible wireless network adapter, most models of wireless network adapters and WAPs provide for encryption options. Some devices with this feature enable you to set a security code known as an SSID on the wireless devices on your network. This seven-digit code prevents unauthorized users from accessing your network and acts as an additional layer of security along with your normal network authentication methods, such as user passwords. Other wireless network adapters and WAPs use a list of authorized MAC addresses to limit access to authorized devices only.

All Wi-Fi products support at least 40-bit encryption through the wired equivalent privacy (WEP) specification, but the minimum standard on newer products is 64-bit WEP encryption. Many vendors also offer 128-bit or 256-bit encryption on some of their products. However, the WEP specification is insecure. It is vulnerable to brute-force attacks at shorter key lengths, and it is also vulnerable to differential cryptanalysis attacks, which is the process of comparing an encrypted text with a known portion of the plain text and deriving the key by computing the difference between them. Because WEP encrypts TCP headers, hackers know what the headers should contain in many cases, and they can attempt to find patterns in a large body of collected WEP communications in order to decrypt the key. The attack is complex and difficult to automate, so it is unlikely to occur for most networks, especially at key lengths greater than 128 bits. Furthermore, WEP does not prevent an intruder from attaching a hidden WAP on the network and using it to exploit the network.

New network products introduced in 2003 and beyond now incorporate a new security standard known as Wi-Fi Protected Access (WPA). WPA is derived from the developing IEEE 802.11i security standard, which will not be completed until mid-decade. WPA-enabled hardware works with existing WEP-compliant devices, and software upgrades might be available for existing devices.