Saturday 31 March 2012

NETWORK


In computer networking, topology refers to the layout of connected devices. This article introduces the standard topologies of networking.

TOPOLOGY IN NETWORK DESIGN

Think of a topology as a network's virtual shape or structure. This shape does not necessarily correspond to the actual physical layout of the devices on the network. For example, the computers on a home LAN may be arranged in a circle in a family room, but it would be highly unlikely to find a ring topology there.

INTRODUCTION TO NETWORK TYPES


One way to categorize the different types of computer network designs is by their scope or scale. For historical reasons, the networking industry refers to nearly every type of design as some kind of area network. Common examples of area network types are:
  • LAN - Local Area Network
  • WLAN - Wireless Local Area Network
  • WAN - Wide Area Network
  • MAN - Metropolitan Area Network
  • SAN - Storage Area Network, System Area Network, Server Area Network, or sometimes Small Area Network
  • CAN - Campus Area Network, Controller Area Network, or sometimes Cluster Area Network
  • PAN - Personal Area Network
  • DAN - Desk Area Network

LAN and WAN were the original categories of area networks, while the others have gradually emerged over many years of technology evolution.
Note that these network types are a separate concept from network topologies such as bus, ring and star.

 

LAN - Local Area Network

A LAN connects network devices over a relatively short distance. A networked office building, school, or home usually contains a single LAN, though sometimes one building will contain a few small LANs (perhaps one per room), and occasionally a LAN will span a group of nearby buildings. In TCP/IP networking, a LAN is often but not always implemented as a single IP subnet. In addition to operating in a limited space, LANs are also typically owned, controlled, and managed by a single person or organization. They also tend to use certain connectivity technologies, primarily Ethernet and Token Ring.

SUBNET
Definition: A subnet is a logical grouping of connected network devices. Nodes on a subnet tend to be located in close physical proximity to each other on a LAN. Network designers employ subnets as a way to partition networks into logical segments for greater ease of administration. When subnets are properly implemented, both the performance and security of networks can be improved. In Internet Protocol (IP) networking, devices on a subnet share contiguous ranges of IP address numbers. A mask (known as the subnet mask or network mask) defines the boundaries of an IP subnet. The correspondence between subnet masks and IP address ranges follows defined mathematical formulas. IT professionals use subnet calculators to map between masks and addresses.

Also Known As: subnetwork

IP - Internet Protocol

Definition: IP (Internet Protocol) is the primary network protocol used on the Internet, developed in the 1970s. On the Internet and many other networks, IP is often used together with the Transport Control Protocol (TCP) and referred to interchangeably as TCP/IP. IP supports unique addressing for computers on a network. Most networks use the Internet Protocol version 4 (IPv4) standard that features IP addresses four bytes (32 bits) in length. The newer Internet Protocol version 6 (IPv6) standard features addresses 16 bytes (128 bits) in length. Data on an Internet Protocol network is organized into packets. Each IP packet includes both a header (that specifies source, destination, and other information about the data) and the message data itself. IP functions at layer 3 of the OSI model. It can therefore run on top of different data link interfaces including Ethernet and Wi-Fi.

Also Known As: Internet Protocol

TOKEN RING
Definition: Token Ring is a data link technology for local area networks (LANs). It operates at layer 2 of the OSI model.

How Token Ring Works

Unlike all other standard forms of LAN interconnects, Token Ring maintains one or more common data frames that continuously circulates through the network. These frames are shared by all connected devices on the network as follows:
  • a frame (packet) arrives at the next device in the ring sequence
  • that device checks whether the frame contains a message addressed to it. If so, the device removes the message from the frame. If not, the frame is empty (called a token frame).
  • the device holding the frame decides whether to send a message. If so, it inserts message data into the token frame and issues it back onto the LAN. If not, the device releases the token frame for the next device in sequence to pick up
  • the above steps are repeated continuosly for all devices in the token ring

Characteristics of Token Ring Networks

Token Ring was developed by IBM during the 1980s as an alternative to Ethernet. Starting in the 1990s, it significantly decreased in popularity and gradually was phased out of business networks as Ethernet technology began to dominate LAN designs. Standard Token Ring supports only up to 16 Mbps. In the 1990s, an industry initiative called High Speed Token Ring developed technology for extending Token Ring to 100 Mbps equal to Ethernet, but insufficient interest in the marketplace existed for HSTR products and the technology was abandoned.

TCP/IP - Transmission Control Protocol / Internet Protocol

TCP/IP is arguably the single most important computer networking technology. The Internet and most home networks support TCP/IP as the "language" computers use to find and connect with each other.

WAN - Wide Area Network

Definition: A WAN spans a large geographic area, such as a state, province or country. 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, like VPN-based extranets, are also WANs in themselves. Finally, many WANs are corporate or research networks that utilize leased lines. WANs generally utilize different and much more expensive networking equipment than do LANs. Key technologies often found in WANs include SONET, Frame Relay, and ATM. As the term implies, a WAN spans a large physical distance. The Internet is the largest WAN, spanning the Earth. A WAN is a geographically-dispersed collection of LANs. A network device called a router connects LANs to a WAN. In IP networking, the router maintains both a LAN address and a WAN address. A WAN differs from a LAN in several important ways. Most WANs (like the Internet) are not owned by any one organization but rather exist under collective or distributed ownership and management. WANs tend to use technology like ATM, Frame Relay and X.25 for connectivity over the longer distances.

LEASED LINE
Definition: A leased line connects two locations for private voice and/or data telecommunication service. Not a dedicated cable, a leased line is actually a reserved circuit between two points. Leased lines can span short or long distances. They maintain a single open circuit at all times, as opposed to traditional telephone services that reuse the same lines for many different conversations through a process called "switching." Leased lines most commonly are rented by businesses to connect branch offices, because these lines guarantee bandwidth for network traffic. So-called T1 leased lines are common and offer the same data rate as symmetric DSL (1.544 Mbps). Individuals can theoretically also rent leased lines for high-speed Internet access, but their high cost (often more than $1000 USD per month) deters most. Fractional T1 lines, starting at 128 Kbps, reduce this cost somewhat and can be found in some apartment buildings and hotels.

FRAME RELAY
Definition: Frame relay is a data link network protocol designed to transfer data on Wide Area Networks (WANs). Frame relay works over fiber optic or ISDN lines. The protocol offers low latency and to reduce overhead, does perform any error correction, which is instead handled by other components of the network. Frame relay has traditionally provided a cost-effective way for telecommunications companies to transmit data over long distances. Frame relay has decreased in popularity as companies are gradually migrating their deployments to Internet Protocol (IP) based solutions.

X.25
Definition: X.25 is a standard suite of protocols used for packet switching across computer networks. The X.25 protocols works at the physical, data link, and network layers (Layers 1 to 3) of the OSI model. Each X.25 packets contains up to 128 bytes of data. The X.25 network handles packet assembly at the source device, delivery, and then dis-assembly at the destination. X.25 packet delivery technology includes not only switching and network-layer routing, but also error checking and re-transmission logic should delivery failures occur. X.25 supports multiple simultaneous conversations by multiplexing packets and using virtual communication channels. X.25 was originally designed more than 25 years ago to carry voice over analog telephone lines (dial-up networks). Typical applications of X.25 today include automatic teller machine networks and credit card verification networks. X.25 also supports a variety of mainframe terminal/server applications. With the widespread acceptance of Internet Protocol (IP) as a standard for corporate networks, many X.25 applications are now being migrated to cheaper solutions using IP as the network layer protocol and replacing the lower layers of X.25 with Ethernet or ATM hardware.

ROUTERS
Definition: Routers are physical devices that join multiple wired or wireless networks together. Technically, a wired or wireless router is a Layer 3 gateway, meaning that the wired/wireless router connects networks (as gateways do), and that the router operates at the network layer of the OSI model. Home networkers often use an Internet Protocol (IP) wired or wireless router, IP being the most common OSI network layer protocol. An IP router such as a DSL or cable modem broadband router joins the home's local area network (LAN) to the wide-area network (WAN) of the Internet. By maintaining configuration information in a piece of storage called the routing table, wired or wireless routers also have the ability to filter traffic, either incoming or outgoing, based on the IP addresses of senders and receivers. Some routers allow the home networker to update the routing table from a Web browser interface. Broadband routers combine the functions of a router with those of a network switch and a firewall in a single unit.

LAN, WAN and Home Networking

Residences typically employ one LAN and connect to the Internet WAN via an Internet Service Provider (ISP) using a broadband modem. The ISP provides a WAN IP address to the modem, and all of the computers on the home network use LAN (so-called private) IP addresses. All computers on the home LAN can communicate directly with each other but must go through a central gateway, typically a broadband router, to reach the ISP.

Other Types of Area Networks

While LAN and WAN are by far the most popular network types mentioned, you may also commonly see references to these others:
  • Wireless Local Area Network - a LAN based on WiFi wireless network technology
  • Metropolitan Area Network - a network spanning a physical area larger than a LAN but smaller than a WAN, such as a city. A MAN is typically owned an operated by a single entity such as a government body or large corporation.
  • Campus Area Network - a network spanning multiple LANs but smaller than a MAN, such as on a university or local business campus.
  • Storage Area Network - connects servers to data storage devices through a technology like Fibre Channel.
  • System Area Network - links high-performance computers with high-speed connections in a cluster configuration. Also known as Cluster Area Network.

SAN

Definition: A storage area network (SAN) is a type of local area network (LAN) designed to handle large data transfers. A SAN typically supports data storage, retrieval and replication on business networks using high-end servers, multiple disk arrays and Fibre Channel interconnection technology. SAN technology is similar but distinct from network attached storage (NAS) technology. While SANs traditionally employ low-level network protocols for transfering disk blocks, a NAS device typically works over TCP/IP and can be integrated fairly easily into home computer networks. The term SAN can sometimes refer to system area networks instead of a storage area network. System area networks are clusters of high performance computers used for distributed processing applications requiring fast local network performance. Storage area networks, on the other, are designed specifically for data management.

Also Known As: Storage Area Network, System Area Network

ISP
Definition: An ISP is a company that supplies Internet connectivity to home and business customers. ISPs support one or more forms of Internet access, ranging from traditional modem dial-up to DSL and cable modem broadband service to dedicated T1/T3 lines. More recently, wireless Internet service providers or WISPs have emerged that offer Internet access through wireless LAN or wireless broadband networks. In addition to basic connectivity, many ISPs also offer related Internet services like email, Web hosting and access to software tools. A few companies also offer free ISP service to those who need occasional Internet connectivity. These free offerings feature limited connect time and are often bundled with some other product or service.

IP ADDRESS
Definition: An IP address is a logical address for a network adapter. The IP address uniquely identifies computers on a TCP/IP network. An IP address can be private - for use on a local area network (LAN) - or public - for use on the Internet or other wide area network (WAN). IP addresses can be determined statically (assigned to a computer by a system administrator) or dynamically (assigned by another device on the network on demand). Two IP addressing standards are in use today. The IPv4 standard is most familar to people and supported everywhere on the Internet, but the newer IPv6 standard is planned to replace it and starting to be deployed. IPv4 addresses consist of four bytes (32 bits). Each byte of an IP address is known as an octet. Octets can take any value between 0 and 255. Various conventions exist for the numbering and use of IP addresses.

Common IP (IPv4) Addresses -

BROADBAND MODEM
Definition: A broadband modem is a type of digital modem used with high-speed DSL or cable Internet service. Cable modems connect a home computer (or network of home computers) to residential cable TV service, while DSL modems connect to residential public telephone service. Like the television "set top" box, both cable and DSL modems are normally supplied by the Internet service provider and not a piece of equipment individuals need to shop for on their own. Most broadband modems supply a 10 Mbps Ethernet connection for the home LAN, although broadband Internet services rarely if ever perform at those speeds. The performance of a cable modem can vary depending on the utilization of the shared cable line in that neighborhood, and DSL modem speeds also vary, but typical data rates range anywhere from 500 Kbps to 3500 Kbps.

Also Known As: cable modem, DSL modem

FIBRE CHANNEL
Definition: Fibre Channel is a set of related physical layer networking standards. Fibre Channel technology handles high-performance disk storage for applications on many corporate networks. Fibre Channel supports data backups, clustering and replication. Like Ethernet, its main competitor, Fibre Channel can utilize copper wiring. However, copper limits Fibre Channel to a maximum recommended reach of 30 meters. When using more expensive fiber optic cables, Fibre Channel reaches to 10 kilometers. Fibre Channel operates at either 1 Gbps or 2 Gbps, with 10 Gbps versions in development. Fibre Channel networks have a reputation for being expensive to build, difficult to manage, and inflexible to upgrade due to incompatibilities between vendor products.

Also Known As: Fibre Channel Arbitrated Loop, FC-AL

Examples:
Many storage area network (SAN) solutions use Fibre Channel technology. Gigabit Ethernet has emerged, however, as a lower cost alternative for storage networks. Gigabit Ethernet can better take advantage of Internet standards for network management like SNMP.

Wi-Fi - Wireless Fidelity

Definition: Wi-Fi is the industry name for wireless LAN (WLAN) communication technology related to the IEEE 802.11 family of wireless networking standards. To some, the term Wi-Fi is synonymous with 802.11b, as 802.11b was the first standard in that family to enjoy widespread popularity. Today, however, Wi-Fi can refer to any of the established standards: 802.11a, 802.11b, 802.11g and 802.11n. The Wi-Fi Alliance (see sidebar) certifies vendor products to ensure 802.11 products on the market follow the various 802.11 specifications. Unfortunately, 802.11a technology is not compatible with 802.11b/g/n, so Wi-Fi product lines have been somewhat fragmented.

Also Known As: (incorrectly) wireless fidelity

Introduction to Wi-Fi Network Security

A consideration on any computer network, security is especially important on Wi-Fi wireless networks. Hackers can easily intercept Wi-Fi network traffic over open air and extract information like passwords and credit card numbers. Several Wi-Fi network security technologies have been developed to combat hackers, of course, although some of these technologies can be defeated more easily than others.

Network Data Encryption

Network security protocols usually use encryption technology. Encryption scrambles data sent over network connections to hide information from humans while still allowing computers to properly decipher the messages. Many forms of encryption technology exist in the industry.

Network Authentication

Authentication technology for computer networks verifies the identity of devices and people. Network operating systems like Microsoft Windows and Apple OS-X include built-in authentication support based on user names and passwords. Home network routers also authenticate administrators by requiring them to enter separate login credentials.

Ad Hoc Wi-Fi Network Security

Traditional Wi-Fi network connections go through a router or other wireless access point. Alternatively, Wi-Fi supports a mode called ad hoc wireless that allows devices to connect directly to each other in peer to peer fashion. Lacking a central connection point, the security of ad hoc Wi-Fi connections tends to be low. Some experts discourage the use of ad-hoc Wi-Fi networking for this reason.

Common Wi-Fi Security Standards

Most Wi-Fi devices including computers, routers and phones support several security standards. The available security types and even their names vary depending on a device's capabilities.

WEP
WEP stands for Wired Equivalent Privacy. It is the original wireless security standard for Wi-Fi and is still commonly used on home computer networks. Some devices support multiple versions of WEP security
  • WEP-64-bit key (sometimes called WEP-40)
  • WEP 128-bit key (sometimes called WEP-104)
  • WEP 256-bit key
and allow an administrator to choose one, while other devices only support a single WEP option. WEP should not be used except as a last resort, as it provides very limited security protection.

WPA
WPA stands for Wi-Fi Protected Access. This standard was developed to replace WEP. Wi-Fi devices typically support multiple variations of WPA technology. Traditional WPA, also known as WPA-Personal and sometimes also called WPA-PSK (for pre-shared key), is designed for home networking while another version, WPA-Enterprise, is designed for corporate networks. WPA2 is an improved version of Wi-Fi Protected Access supported by all newer Wi-Fi equipment. Like WPA, WPA2 also exists in Personal/PSK and Enterprise forms.

802.1X provides network authentication to both Wi-Fi and other types of networks. It tends to be used by larger businesses as this technology requires additional expertise to set up and maintain. 802.1X works with both Wi-Fi and other types of networks. In a Wi-Fi configuration, administrators normally configure 802.1X authentication to work together with WPA/WPA2-Enterprise encryption. 802.1X is also known as RADIUS.

NETWORK SECURITY KEYS AND PASSPHRASES

WEP and WPA/WPA2 utilize encryption keys, long sequences of hexadecimal numbers. Matching key values must be entered into a Wi-Fi router (or access point) and all client devices wanting to join that network. In network security, the term passphrase can refer to a simplified form of encryption key that only uses alphanumeric characters instead of hexadecimal values. However, the terms passphrase and key are often used interchangeably.

CONFIGURING WI-FI SECURITY ON HOME NETWORKS

All devices on a given Wi-Fi network must use matching security settings. On Windows 7 PCs, the following values must be entered on the Security tab of Wireless Network Properties for a given network:
  • Security type refers to authentication options including Open, Shared, WPA-Personal and –Enterprise, WPA2-Personal and –Enterprise, and 802.1X. The Open option utilizes no authentication, while Shared utilizes WEP for authentication.
  • Encryption type options available depend on the Security type chosen. Besides None, which can be only used with Open networks, the WEP option can be used with either WEP or 802.1X authentication. Two other options, called TKIP and AES, refer to specialized encryption technologies usable with the WPA family of Wi-Fi security standards.
  • An encryption key or passphrase can be specified in the Network security key field when required.
  • The Key Index, a value between 1 and 4, refers to the position of the matching key stored on the wireless router (access point). Many home routers allow four different encryption keys numbered 1 through 4 to be configured in order to support legitimate clients without forcing them to all use a common key.

WLAN

Definition: A WLAN provides wireless network communication over short distances using radio or infrared signals instead of traditional network cabling. A WLAN typically extends an existing wired local area network. WLANs are built by attaching a device called the access point (AP) to the edge of the wired network. Clients communicate with the AP using a wireless network adapter similar in function to a traditional Ethernet adapter. Network security remains an important issue for WLANs. Random wireless clients must usually be prohibited from joining the WLAN. Technologies like WEP raise the level of security on wireless networks to rival that of traditional wired networks.

Also Known As: wireless LAN

Examples:
For WLANs that connect to the Internet, Wireless Application Protocol (WAP) technology allows Web content to be more easily downloaded to a WLAN and rendered on wireless clients like cell phones and PDAs.

WEP - Wired Equivalent Privacy

Definition: WEP is a standard network protocol that adds security to 802.11 Wi-Fi networks at the data link layer (OSI model Layer 2). WEP was designed to give wireless networks the equivalent level of privacy protection as a comparable wired network. However, the underlying technology behind WEP has been demonstrated to be relatively insecure compared to newer protocols like WPA. WEP utilizes a data encryption scheme called RC4 with a combination of user- and system-generated key values. The original implementations of WEP supported encryption keys of length 40 bits and 24 additional bits of system-generated data (64 bits total). In an attempt to increase protection, these encryption methods were extended to support longer keys including 104-bit (128 bits of total data), 152-bit and 256-bit.When communicating over a Wi-Fi connection, the protocol encrypts the data stream using these keys so that it is no longer human readable but still can be processed by receiving devices. The keys themselves are not sent over the network but rather are generally stored on the wireless network adapter or in the Windows Registry.WEP represents just one element of an overall wireless network security strategy.

Also Known As: Wired Equivalent Privacy

NETWORK ADAPTER
Definition: A network adapter interfaces a computer to a network. The term "adapter" was popularized originally by Ethernet add-in cards for PCs. Modern network adapter hardware exists in several forms. Besides traditional PCI Ethernet cards, some network adapters are PCMCIA devices (also known as "credit card" or "PC Card" adapters) or USB devices. Some wireless network adapter gear for laptop computers are integrated circuit chips pre-installed inside the computer. Windows and other operating systems support both wired and wireless network adapters through a piece of software called a "device driver." Network drivers allow application software to communicate with the adapter hardware. Network device drivers are often installed automatically when adapter hardware is first powered on. A few network adapters are purely software packages that simulate the functions of a network card. These so-called virtual adapters are especially common in virtual private networking (VPN).

Also Known As: NIC, LAN card

Topology of Networking


Network topologies are categorized into the following basic types:
  • bus
  • ring
  • star
  • tree
  • mesh
More complex networks can be built as hybrids of two or more of the above basic topologies.

Bus Network Topology

This diagram illustrates the bus network topology. A bus topology such as 10Base-2 or 10Base-5 Ethernet uses a single communication backbone for all devices.

Bus Topology
Bus networks (not to be confused with the system bus of a computer) use a common backbone to connect all devices. A single cable, the backbone functions as a shared communication medium that devices attach or tap into with an interface connector. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message. Ethernet bus topologies are relatively easy to install and don't require much cabling compared to the alternatives. 10Base-2 ("ThinNet") and 10Base-5 ("ThickNet") both were popular Ethernet cabling options many years ago for bus topologies. However, bus networks work best with a limited number of devices. If more than a few dozen computers are added to a network bus, performance problems will likely result. In addition, if the backbone cable fails, the entire network effectively becomes unusable.

Ring Network Topology

This diagram illustrates the ring network topology. A ring topology such as FDDI or SONET sends messages clockwise or counterclockwise through the shared link.

Ring Topology

In a ring network, every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction (either "clockwise" or "counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network. To implement a ring network, one typically uses FDDI, SONET, or Token Ring technology. Ring topologies are found in some office buildings or school campuses.

Star Network Topology

This diagram illustrates the star network topology. A star topology typically uses a network hub or switch and is common in home networks.

 

Star Topology

Many home networks use the star topology. A star network features a central connection point called a "hub node" that may be a network hub, switch or router. Devices typically connect to the hub with Unshielded Twisted Pair (UTP) Ethernet. Compared to the bus topology, a star network generally requires more cable, but a failure in any star network cable will only take down one computer's network access and not the entire LAN. (If the hub fails, however, the entire network also fails.)

 

Tree Network Topology

This diagram illustrates the tree network topology. A tree topology integrates the star and bus topologies in a hybrid approach to improve network scalability.

 

Tree Topology

Tree topologies integrate multiple star topologies together onto a bus. In its simplest form, only hub devices connect directly to the tree bus, and each hub functions as the root of a tree of devices. This bus/star hybrid approach supports future expandability of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the number of hub connection points) alone.

Mesh Network Topology

This diagram illustrates the mesh network topology. A mesh topology provides redundant communication paths between some or all devices (partial or full mesh).

Mesh Topology

Mesh topologies involve the concept of routes. Unlike each of the previous topologies, messages sent on a mesh network can take any of several possible paths from source to destination. (Recall that even in a ring, although two cable paths exist, messages can only travel in one direction.) Some WANs, most notably the Internet, employ mesh routing. A mesh network in which every device connects to every other is called a full mesh. As shown in the illustration below, partial mesh networks also exist in which some devices connect only indirectly to others.

Summary

Topologies remain an important part of network design theory. You can probably build a home or small business computer network without understanding the difference between a bus design and a star design, but becoming familiar with the standard topologies gives you a better understanding of important networking concepts like hubs, broadcasts, and routes.