Introduction to the Computer Network Architecture

What Is a Network?
Here you will learn about computer network architecture, physical design, logical topology, protocols introduction, communication planning and basic communication technologies. To properly build, maintain and secure a network you should first know that what a compute network is and how data travels through the wired or wireless network. A person with a good networking skills will be able to tell that a network consist of a computer, cables, PRI lines, Routers, switches, NIC cards, PBXs, TIs, fiber optic and Ethernet cables.  A person who have the strong background in the data network must know that a network consists of a server, workstations, routers, hubs, WAN, LAN, fiber optic and Ethernet cables and devices. Both the telecommunication and data communication persons agree that the cabling is an essential part of any computer network.The simplest definition of the data network is to connect two more computer computers with each other to share data and resources. The network exists in different sizes and shapes, from home networks to WAN networks. Despite the different roles and various sizes of a network you can have some common similarities in all the networks such as protocols, architecture and topology/design.

  Computer Network Architectures
Basically computer network architectures are dividing into three basic types such as LAN (local area network), MAN (Metropolitan area network) and WAN (wide area network. A LAN can consist of two or more computers in the same room or building. Fiber optic or Ethernet cables are used to connect the computers in a LAN. Home networks, personal networks and office networks fall in LAN. A typical MAN consists of two or more computers at two different geographical locations in the same city.
A MAN can be wired (fiber optic cable) or wireless and a number of communication devices are used in a MAN. A WAN consists of two or more computers in two different geographical areas (different cities or countries) and there are different methods to connect the computers in a WAN such as leased lines (ISDN lines, radio waves, microwaves, dial-up connections and connectivity through satellite. The internet is a largest WAN in the world. With the invention of the wireless networking, mobile and optical technology the usage of the wires has been decreased. There are a number of the terms that describe the architecture of a network.

Computer Network Topologies
The topology or physical design is closely related to the architecture of a network. Topology defines that how the network is physically connected. There are three main types of the topologies.
• Star Topology: In the star topology the all the networking components are connected to the central point, which is a hub or a switch. The star topology is mostly in use in LAN.
• Bus Topology: In the Bush topology the networking components are connected to the same cable. This is also called linear bus or backbone.
• Ring Topology: In the ring topology the all the components are connected with each other in the form of a ring. A token continuously passes through the loop.

Network Architecture Terminology
• CAN (campus area network): CAN is a type of a network that connects the buildings/offices of a university, educational or office complex.
• Intranet: Intranet is a private network that belongs to an office, college or an organization and that is only accessible to the authorized users.
• Internet: The internet is a network of networks and connecting millions of computes with each other by different designs.
• MAN (metropolitan area network): MAN is a type of a network that is designed for a city. A MAN is larger than LAN but smaller than WAN.
• SAN (storage area network): SAN is a type of a network that is used to connect the storage related devices like RAID, file servers and tape systems.
• VLAN (virtual local area network): VLAN is a type of a network that allows computers on separate physical networks to communicate as if they were connected to the same network.
• Client-Server: Client- Server is a type of networking in which dedicated systems that provides services are called serves and the system that get these services are called work stations. The main services include file, printer, scanner, CD, Hard disk, processor, internet connection and other services.
• Peer-to-peer: This is a type of a networking where each computer shares the same functionalities. No centralized server is required in the Peer to peer networking.

Computer Network Protocols and the OSI Model
Protocol is one of the most important components of a computer network. Protocol means a set of rules, agreed upon ways or a communication language which all computer and devices understand. A protocol defines error checking, how data will be send and receive, and transmitting data between the systems. There are a large number of protocols and following is a list of the most commonly used protocol in the computer communications.
• AppleTalk: AppleTalk is a communication protocol that was developed by the Apple System to connect Macintosh computers to the printers.
• Asynchronous Transfer Mode (ATM): ATM is a type of protocol in which data travels in the form of fixed size packets. These fixed size packets provide high speed, data security, and video and voice communication over the same network.
• DECnet: DECnet is a protocol that was developed by the Digital Equipment Systems to connect the PDP and VAX systems.
• Ethernet: Ethernet is a LAN protocol that was developed by the Intel, Xerox and Digital Equipment System. Ethernet is a most widely used LAN communication standard.
• Fiber Distributed Data Interface (FDDI): FDDI is a protocol that is used to transmit the data over the fiber optic cables.
• Internet Protocols (IP): IP is a protocol transmitting data between the packet switched IP networks originally developed by the DOD (department of defense). TCP/IP (Transmission control protocol/Internet protocol) is a suite of protocol and FTP, HTTP, E-mail, Telnet are all IP protocols.
• Internetwork Packet Exchange (IPX): IPX is a networking protocol that is used by the Novell Netware operating systems.
• NetWare: Netware is a LAN protocol that is developed by the Novell Corporation.
• Signaling System 7 (SS7): SS7 is a telecommunication protocol that was developed by the International Telecommunication Union.
• Systems Network Architecture (SNA): SNA is a set of protocols that was developed by the IBM mainframe systems.
• Token Ring: Token Ring is a LAN protocol that was developed by the IBM where systems have the tokens before they transmit the data. Transmission Control Protocol/Internet Protocol (TCP/IP): TCP/IP is a suite of the protocols used to connect the computers on the internet. TCP/IP is a most commonly used protocol.
• X.25: X.25 is a protocol that was developed by CCITT for the packet switched network.
Protocols are combined with the OSI layers model. OSI model is an ISO standard for the communication system. There are seven layers in the OSI model and each layer performs the different functionalities. The seven layers are Application, Presentation, Session, Transport, Network, Data link and physical layers. Each layer know how to communication with the upper and lower layer. You can remember the name of all the layers by the following sentence.
“All people seems to need data processing”

Planning a Logical Network Design
When you plan a logical network design you can either start from scratch or upgrade an existing network. You should have the sufficient information about the networking components, hardware, protocols and topologies. You should analyze the traffic pattern, security needs, future expansion, and server capability, internet access to the clients, FTP and other things. You should also make a plan for the disaster recovery, data recovery and instant troubleshooting techniques.

What is DNS

DNS: Domain name service is an Internet service that translates domain names into IP addresses. Domain names are easy to remember because they are alphabetic. On the other end, the internet is based on the IP addresses i.e every computer on the internet is associated with a unique IP address.
The communication on the internet is carried out on the basis of IP addresses and not on the domain names. A Domain name server service translates the domain name into its corresponding IP address for example the domain name www.abc.com might be translated to 102.222.34.56. The DNS system is, in fact, forms its own network.
If one Domain name serverserve doesn’t know to translate a domain name, it looks for another one and so on until the exact IP address is returned. DNS translates between the internet names and internet addresses.

How DNS Works
DNS organizes the hostnames in the form of hierarchy. A domain is a collection of of the sites that are related because they form a network (all computers that are geographically close as well. Universities are commonly grouped in .edu domain with each university or college using a separate sub domain.
While most of the topic does not require much technical knowledge, there is one technical part of the Domain name server.
When you type a name like example.com in your internet browsers it finds a way to map that name to the internet IP numbers, by which the internet easily reach the example.com computer. For this purpose your computer uses DNS of your Internet service provider company. All internet traffic work on these numbers and the important factor is that the looking up for the name is done by Domain name server.
That computer has a list of the host names/IP address mapping, which is regularly updated by the root DNS. Root DNS servers are the master servers that can help you look up any name. All the root DNS servers copy their own data from the one master server, which is under the control of ICANN. The root servers usually have a list that where you can look for the top level domains like .com, .net, org and .info etc. The ISP sends request for the particular domain name to the root sever and root server directs the request to the master server. In this way, you get the answer with your requested domain name.
IP routing and root servers
The domain lookups go to and from the root servers because main routers on the internet, ISPs and backbones have the list that where to find it.
Domains Description
.edu This domain name is used by the educational institutes like colleges and universities
.com This domain name is used by the commercial institutes/organizations and companies.
.Org This domain is used by the non-commercial organizations.
.net Administrative hosts, gateways and other networks.
.mil This is used by the U.S. military institutions.
.Gov This is used by the government institutes.
.info Used by the Informative sites.
There is a common question that how large is the internet and how Domain name server works. Domain name server simply provides mapping between hostnames and the IP addresses. When you dial in your ISP number and access it how does it get the answer in the form of requested domain name for you. It’s most likely that ISP may not have stored the information (requested site). In this case first the ISP server will send the query to the root-servers. These are the set of very high-powered servers that know all about the top level domains like .com, .org, .net, .info and all the country domains.
So, the ISP’s name server first contact with the root-servers. If the root-servers don’t have the requested information then redirect the request to the GTLD servers for any kind of top level domain and the answers come back to the ISP’s name servers with the requested information. At each step, ISP’s name server caching all the information. The Domain name server is central to the internet because without a domain name systems (DNS) its impossible to communication on the internet.

An Introduction To The Frame Relay

Frame relay is a fastest growing network communication technology and is widely used in the corporate offices, multination companies and in the ISPs. It provides cost effective, fast and secure data communication between LAN and WAN. Frame relay is based on the X.25 technology, which was initially designed for analog data transmission. It is a fastest method of transmitting the information between two similar or different networks. Frame relay is a network communication technology that is used for the data transmission between local area network and wide area network

The data is transferred in frames and they are relayed on the data link layer. The network provides permanent and fast virtual connection PVC. Frame relay provides the service between ISDN and ATM. Over the leased telephone lines, frame relay is used to connect the LAN with the fiber optic backbones and with the WAN. The data is transmitted to the destination by means of permanent of switched virtual circuits. Many virtual circuits can exist across a transmission between because virtual circuits only consume the bandwidth when they transmit the data.

The frame relay circuits are the logical circuits that are created at the time of installation and are not created by the users at their systems. In device that is a part of the frame relay network can operate at a very high speed due to the availability of the more bandwidth. The high performance and the error handing techniques allow frame relay protocol to discard the time consuming error handling techniques. The standards of the Frame relay protocols have been developed by the ANSI and CCITT. The frame relay structure is based on the LAPD protocol.
The header of the frame has the data link connection identifier DLCI and congestion bits. When a network becomes congested i.e. it is not able to transmit any new data it begins to discard the frames and these discarded frames retransmitted so more congestion is produced. To avoid this problem several methods have been developed.
Two bits Forward Explicit Congestion Notification and Backward Explicit Congestion Notification in the header of the frame are used to signal the devices at the user end that congestion has been developed. When congestions occur the FECN is changed to 1 and in this way all the downstream nodes and the attached devices at the user end learn about the congestion in the line. The BECN is changed to 1 in the frame that traveling back to the source of the data transmission. So the source is notified of the congestion in the line and to slow down the transmission until the congestion in the line is minimized.
Frame relay works on the consumer hardware such as router and switches and each switch relays the data to the next switch in frame relay network. Frame relay also work with the meshed network because each endpoint can be connected to the different locations and so on. FR is a cost effective and standardized way of creating a wide area network. Frame relay provides error free high data transmission rate and the speed comes in a number of options such as 56 KB/s, 64 KB/s, 128 KB/s, 256 KB/s, 512 KB/s, 1.5 MB/s, and 2 MB per second.

Network Security : Brief Introduction

Network security refers to protecting computer network from the unauthorized access, hacker’s attacks, viruses, spyware and internal and external threats.  It also refers to protecting the network from the unauthorized modifications, data lost, destruction, physical attacks and the protection of the company’s assets. Computer networks are being used to conduct the transactions, communications, data and resources sharing, orders processing, voice communication among the businesses and the individuals.   

Every computer network without proper security mechanisms can be attacked by the hackers, viruses and intruders so it is the most important responsibility of the computer network administrators to implement the security solutions.

A personal computer, gateway computer and the laptops are the common point of attacks.  Unauthorized users can steal the company’s important data.  Being an administrator you need to know the common threats and implement the solution to deal with them.  The common network threats include the following.

Common Threats

• Viruses
• Hacking attacks
• Spyware
• Adware
• Malware
• Trojans
• Web Worms
• Rootkits
• Honeypots
• Identity lost
• Sniffers
• Physical Threats
• Dangerous Macros
• Eavsdropping
• Security Vulnerabilities
• Risks from the Naive employees.
• Denial of service attacks.
• Data Interception.

Just ask yourself that are you ready to deal with any of the above mentioned threats?  If your answer is NO then your network is at great security risk.  As the uptime and the security are the most important thing in the computer network so you need to implement the solutions to deal with these threats. A computer network can be kept secured by implementing the following techniques and using the tools.

• Install an up-to-dated antivirus program on all the computers and regularly scan them.
• Install an up-to-dated anti spyware program.
• Configure a software or hardware firewall on the gateway computer.
• Limit the rights of the users in your network.
• Use monitoring, diagnosing, troubleshooting and network management tools.
• Implement an intrusion detection system as it will determine that if your network is under attack or not.
• Use strong password, digital authentication keys, and security certifications to identify the users and control their activities.
• Monitory the online activities of the users through he monitoring software and block the suspicious and potentially risky online applications and the websites.
• Encrypt your messages and data while transmitting over the network.  Encryption ensures that your data cannot be intercepted or read by the unauthorized users.
• Apply security patches against the known vulnerabilities.
• Update your operating system regularly.
• Lock your server room and no unauthorized user should be allowed to enter in the server room.
• Keep inventory of all the devices including computers, hubs, switches, routers, cables, printers and scanners etc.
• Block the unwanted ports and services.
• Increase the security of your web browsers.
• Regularly take backup of your critical data.

Quick Understanding of OSI (For Interviews and Fast remembering )

The Open System Interconnection Model

The Open System Interconnection (OSI) model specifies how dissimilar computing devices such as Network Interface Cards (NICs), bridges and routers exchange data over a network by offering a networking framework for implementing protocols in seven layers. Beginning at the application layer, control is passed from one layer to the next. The following describes the seven layers as defined by the OSI model, shown in the order they occur whenever a user transmits information.

Layer 7: Application

This layer supports the application and end-user processes. Within this layer, user privacy is considered and communication partners, service and constraints are all identified. File transfers, email, Telnet and FTP applications are all provided within this layer.

Layer 6: Presentation (Syntax)

Within this layer, information is translated back and forth between application and network formats.  This translation transforms the information into data the application layer and network recognize regardless of encryption and formatting.

Layer 5: Session

Within this layer, connections between applications are made, managed and terminated as needed to allow for data exchanges between applications at each end of a dialogue.

Layer 4: Transport

Complete data transfer is ensured as information is transferred transparently between systems in this layer. The transport layer also assures appropriate flow control and end-to-end error recovery.

Layer 3: Network

Using switching and routing technologies, this layer is responsible for creating virtual circuits to transmit information from node to node. Other functions include routing, forwarding, addressing, internetworking, error and congestion control, and packet sequencing.

Layer 2: Data Link

Information in data packets are encoded and decoded into bits within this layer. Errors from the physical layer flow control and frame synchronization are corrected here utilizing transmission protocol knowledge and management. This layer consists of two sub layers: the Media Access Control (MAC) layer, which controls the way networked computers gain access to data and transmit it, and the Logical Link Control (LLC) layer, which controls frame synchronization, flow control and error checking.

Layer 1: Physical

This layer enables hardware to send and receive data over a carrier such as cabling, a card or other physical means. It conveys the bitstream through the network at the electrical and mechanical level. Fast Ethernet, RS232, and ATM are all protocols with physical layer components.

This order is then reversed as information is received, so that the physical layer is the first and application layer is the final layer that information passes through.

Ethernet Products

Ethernet Products

The standards and technology just discussed will help define the specific products that network managers use to build Ethernet networks. The following presents the key products needed to build an Ethernet LAN.

Transceivers

Transceivers are also referred to as Medium Access Units (MAUs). They are used to connect nodes to the various Ethernet media. Most computers and network interface cards contain a built-in 10BASE-T or 10BASE2 transceiver which allows them to be connected directly to Ethernet without the need for an external transceiver.

Many Ethernet devices provide an attachment unit interface (AUI) connector to allow the user to connect to any type of medium via an external transceiver. The AUI connector consists of a 15-pin D-shell type connector, female on the computer side, male on the transceiver side.

For Fast Ethernet networks, a new interface called the MII (Media Independent Interface) was developed to offer a flexible way to support 100 Mbps connections. The MII is a popular way to connect 100BASE-FX links to copper-based Fast Ethernet devices.

Network Interface Cards

Network Interface Cards, commonly referred to as NICs, are used to connect a PC to a network. The NIC provides a physical connection between the networking cable and the computer’s internal bus. Different computers have different bus architectures. PCI bus slots are most commonly found on 486/Pentium PCs and ISA expansion slots are commonly found on 386 and older PCs. NICs come in three basic varieties: 8-bit, 16-bit, and 32-bit. The larger the number of bits that can be transferred to the NIC, the faster the NIC can transfer data to the network cable. Most NICs are designed for a particular type of network, protocol, and medium, though some can serve multiple networks.

Many NIC adapters comply with plug-and-play specifications. On these systems, NICs are automatically configured without user intervention, while on non-plug-and-play systems, configuration is done manually through a set-up program and/or DIP switches.

Cards are available to support almost all networking standards. Fast Ethernet NICs are often 10/100 capable, and will automatically set to the appropriate speed. Gigabit Ethernet NICs are 10/100/1000 capable with auto negotiation depending on the user’s Ethernet speed. Full duplex networking is another option where a dedicated connection to a switch allows a NIC to operate at twice the speed.

Hubs/Repeaters

Hub:

Hubs/repeaters are used to connect together two or more Ethernet segments of any type of medium. In larger designs, signal quality begins to deteriorate as segments exceed their maximum length. Hubs provide the signal amplification required to allow a segment to be extended a greater distance. A hub repeats any incoming signal to all ports.

Ethernet hubs are necessary in star topologies such as 10BASE-T. A multi-port twisted pair hub allows several point-to-point segments to be joined into one network. One end of the point-to-point link is attached to the hub and the other is attached to the computer. If the hub is attached to a backbone, then all computers at the end of the twisted pair segments can communicate with all the hosts on the backbone. The number and type of hubs in any one-collision domain is limited by the Ethernet rules. These repeater rules are discussed in more detail later.

Repeater

A very important fact to note about hubs is that they only allow users to share Ethernet. A network of hubs/repeaters is termed a “shared Ethernet,” meaning that all members of the network are contending for transmission of data onto a single network (collision domain). A hub/repeater propagates all electrical signals including the invalid ones. Therefore, if a collision or electrical interference occurs on one segment, repeaters make it appear on all others as well. This means that individual members of a shared network will only get a percentage of the available network bandwidth.

Basically, the number and type of hubs in any one collision domain for 10Mbps Ethernet is limited by the following rules:

Network Type	Max Nodes Per Segment	Max Distance Per Segment
10BASE-T	2	100m
10BASE-FL	2	2000m