Types of Twisted-Pair Cables

1. A straight-through cable is used when devices at the both end, are not similar in nature of it's operation. for example:
                 a)   from host to a hub (or switch)
                 b)   from a router to a hub (or switch)
The wires on each end of a straight-through cable must be identical. The most common straight-through standard is wired as follows:

2. A cross-over cable is used, when connecting "like" devices. for example:
                          a) a host to a host
                          b) a hub to a hub
                          c) a switch to a switch
To make cross-over cable, we must swap pins 1 and 3, and pins 2 and 6 on the one end of the cable. The most common cross-over standard is as follows:

3. A roll-over cable is used to connect a PC into a Cisco Router's console or auxiliary port The pins are completely reversed on one end to make a rollover cable.

Note:  

1. pins 1, 2 3 and 6 are responsible for transmission of data, rest pins are being used for flow control, congestion control and noise control.
2. Connector RJ45 is used in twisted pair cables.

RJ45

  3. Roll-over cable is used to configure routers.         

TCP/IP Utilities

You can use several utilities to verify TCP/IP function on Windows workstations:

Using the arp utility:

1. Choose Start--->Run and enter cmd to open the MS-DOS Prompt window.
2. type "arp -a", you'll get.

The arp utility is primarily useful for resolving duplicate IP addresses. For example, your workstation receives its IP address from a DHCP server, but it accidentally receives the same address as another workstation. Your workstation is trying to determine the MAC address, and it can't do so because two machines are reporting that they have the sane IP address. To display the entire current ARP table, use arp command with -a switch.

Using the netstat Utility:

1. Open MS-DOS Prompt window
2. enter "netstat /?", it will show all its coll switches and its functions.

'netstat' with switch '-a' is normally used to get a list of open ports on your own system, i.e. on the local system. This can be particularly useful to check and see whether your system has a Trojan installed or not. Remember, port numbers over 1024 are used by your system to connect to remote computers.
Trojans:  Netbus uses port number 12345 (TCP) and Back Orifice uses port number 31337(UDP)
"netstat -n" is numerical form of -a, its shows ip address of local and remote systems.

Using the ping Utility: We use ping utility for two primary puurposes; to find out if you can reach a host, to find out if a host is responding.
open MS-DOS window
ping hostname or IP address


Using tracert utility : It will show you every router interface a TCP/IP packet passes through on its way to a destination.

1. open MS-DOS window
2. enter "tracert hostname( or host's ip address)"
3. It will respond with a list of DNS names and IP address of routers that the packet is passing through on its way.

 Using Telnet utility:: It  is used to open terminal sessions from remote system to server, it has evolved into a troubleshooting tool.

1. open MS-DOS, type telnet hostname(or host ip address), make sure this facility is installed in your system. If installed then jump to  4 , otherwise follow the steps.
2. Control Panel----> Programs-----> "Turn Windows features on or off", a dialog box will appear.
3. Select telnet client and server or any other service which you want to install.
4. It will ask for login name and password.
5. Once login name and password is being verified, a telnet session get established.
6. If the host supports SSH then use putty.
7. If the telnet service is not running at the default port number on the host, then use telnet hostname:port number command.

Using nslookup Utility: It allows you to query a name server and quickly find out which name resolves to which IP address.

1. open MS-DOS window.
2. enter nslookup example.com 

Configuring TCP/IP on Windows Workstations

To begin configuring TCP/IP, folloe these steps:

1. Start--> Settings--> Conrol Panel--> Network  and Dial-Ip connections to open the Network and Dial-Up Connections dialog box, which list all of the currently installed network components.
2. Right-click one of your network adapters and select properties to open the Properties dialog box for your adapter.
3. If you do not see Internet Protocol (TCP/IP) in the scrolling list, follow the next three steps. If you do see it, skip to step 7.
4. Click the install button, which brings up the select Network Component Type window.
5. Click Protocol in the list and click the Add button to bring up the Select Network protocol dialog box.
6. Select Microsoft from te left frame and Internet Protocol (TCP/IP) from the right frame and click the OK button.This take you to the Property dialog box for your adapter. Now Internet protocol (TCP/IP) should appear in the scrolling list.
7. Click Internet Protocol (TCP/IP) and the Properties button, which will oen Internet Protocol (TCP/IP) properties window.
8. Initially, you see the General tab, which is set to obtain your IP address, subnet mask, default gateway, and DNS server via DHCP.
9. Click the advance button, which brings up the Advance TCP/IP Settings dialog box.
10. Notice the IP settings, DNS, WINS, and Options tabs along the top of the Advanced TCP/IP settings dialog box.

 

        

Ports and Sockets

Ports
A port is numerical value that identifies the application associated with data. The source port number identifies the application that sent the data, and the destination port number identifies the application that receives the data. Each port is assigned a unique 16-bit number in the range of 0 through 65535. 
                     The well known port are those from 0 through 1023. This range or port is bound to the services running on them. for example, FTP runs on port 21 by default. 
                   The registered ports are those from 1024 through 49151. This range of port numbers is not bound to any specific service. Actually, networking utilities like your browser opens a random port within this range and starts a communication with remote server.
                    The dynamic or private ports are those from 49152 through 65535. this range is rarely used and is mostly used by Trojans. However some application do tend to use such high range port numbers.

Re-mapping Ports: A common technique employed by a number of system administrators is re-mapping ports. for example, normally the default port for HTTP is 80. However it can be re-mapped to port 8080. If that is the case then homepage hosted on that server would be at : http://domain.com:8080 
             The idea behind port re-mapping is that instead of running a service on a well-known port, where it can easily be exploited, it is better to run it on a not so well-known port, as any hacker, will find it more difficult to find that service.



Sockets
Let 10.0.0.1 wants to connect to the ftp daemon to download a file and at the same time, it wants to connect to 20.0.0.1's website, i.e. connect to its HTTP daemon. In such a scenario, 20.0.0.1 will have to initiate two distinct connections with 10.0.0.1 simultaneously. If there are two distinct connections existing between the same pair of systems. There has to be a way in which this system is able to distinguish between connection for FTP daemon and HTTP daemon. To solve such problems, we have ports. Thus, the 10.0.0.1 computer knows as to which port to connect to in order to download a FTP file. As a result, it will communicate with the 20.0.0.1 machine using what is known as the "socket pair", which is a combination of an ip address and a port. TCP/IP or over the Internet, all communication is done using this socket pair.

socket pair for HTTP port on local host : 127.0.0.1:80
socket pair for SMTP port on local host: 127.0.0.1:25

Appication Protocols

Application layer protocols are built on top of and ito the TCP/IP protocol suite and are available on most implementations. Some of application protocols have been discussed below:

File Transfer Protocol : It provides a mechanism for single or multiple file transfers between computer systems; when written in lower case as "ftp". The FTP package provides all the tools needed to look at files and directories, and transfer text and binary files from one system to another. FTP uses TCP to actually move files.
accessing files on remote machine:

1. ftp machine_name(or machine address)
2. FTP responds by asking login_name and password.
3. once authentication is being done, it will prompt
4. ftp> 
5. ftp> ? (help), "explore different commands by yourself".

Simple Mail Transfer Protocol: It allows for a simple e-mail service and is responsible for moving messages from one e-mail server to another.


Post Office Protocol (POP): It provides a storage mechanism for incoming mail, the latest version of the standard is known as POP3. When a client connects to a POP3 server, all the messages addressed to that client are downloaded; there is no way to download messages selectively.Once messages are downloaded, we can delete or modify without any interaction with server.
gmail e-mail pop server: pop.gmail.com (Port 995 with SSL) 
yahoo e-mail pop server: pop.mail.yahoo.com

Internet Message Access Protocol (IMAP): It allows users to download mail selectively, look at the message header, download just a part of a message, store message on the e-mail server in a hierarchical structure, and link to documents.The current version of IMAP is version 4.
gmail e-mail imap server: imap.gmail.com (port 993 with ssl)
yahoo e-mail imap server : imap.mail.yahoo.com (port 993 with ssl).


Telnet: Telnet is a terminal emulation protocol that provides a remote logon to another host over the network. It allows a user to connect to a remote host over a TCP/IP connection as if they were sitting at that host. Keystroke typed into a Telnet program will be transmitted over a TCP/IP network to the host and the visual reponses are sent back by the host to the client's screen.


Hypertext Transfer Protocol (HTTP): HTTP is the command and control protocol used to manage communications between a web browser and a web server. HTTP is the mechanism that opens the related document when you select a link, no matter where that document is actually located. Secure version of HTTP is known as HTTPS.


Lightweight Directory Access Protocol (LDAP): In large network, most administrator have set uo some kind of directory that keeps track of users and resources. LDAP allows client to perform object lookups with a directory using a standard method.

Network Protocols

Transmission Control Protocol (TCP):
TCP is the transport layer of the protocol and serves to ensure a reliable, verifiable data exchanged between hosts on a network. TCP breaks data into pieces, wraps the pieces with the information needed to identify it as a piece of the original message, and allow the pieces to be reassembled at the receiving end of the communications link. The wrapped and bundled pieces are called Datagrams (or segments).

• Source port and Destination port ensure that data is sent back and forth to the correct application process running on each machine.
• Segment Number allows datagrams to be rebuilt in the correct order in the receiving device, and checksum allows the protocol to check whether the data sent is same as data received.
• Acknowledgement number indicates that the data was received successfully, and Offset specifies the length of the header in 32-bit chunks.
• Reserved specifies variables that are set aside for future use, it must contain ZEROS, and Flags , these are six one-bit fields that indicate various things, like whether segment is end of higher-layer message, acknowledgement number is significant, sender is requesting that a virtual circuit, data in the segment is urgent.
• Window provides a way to increase the number of segments transmitted before the sender excepts an acknowledgement, and Urgent gives the location where the urgent data ends.
• Options and Padding communicates various parameters of TCP virtual circuit, like maximum segment size, and it also ensures that the header ends on a 32-bit boundary so that the offset field makes sense as a whole number.

Internet protocol (IP) :

The network layer portion of the DoD model is called the Internet layer. The main protocol at this layer is the Internet Protocol (IP). IP is considered connectionless; i.e; it does not swap control information in order to establish an end to end connection before starting a transmission. This is known as best effort transmission. and if a packet is lost in transmission, IP must rely on TCP to determine it the data did not arrive successfully.




Address Resolution Protocol (ARP) :

The network layer protocol, ARP, associates the physical hardware address of a network node to its already known IP address. Using ARP, an IP process construct a table that maps ip addresses to mac addresses of nodes on LAN. When a node needs to send a packet to a known IP address on local sub-net, it first checks the ARP cache to see it the physical address information is already present. If so, that address is used and network traffic is reduced.


Internet Control Message Protocol (ICMP) :
It works at the network layer and provides the functions used for Network layer management and control. Routers send ICMP messages to respond to undeliverable datagrams by placing an ICMP message in an IP datagram and then sending the datagram back to the original source. The PING command.

ICMP EXAMPLE

User Datagram Protocol (UDP) :

It is a transport layer connectionless protocol that does not provide the reliability services with TCP but provides best effort transmission services to application protocols.It gives applications a direct interface with IP and the ability to address a specific application protocol running on a host via a port number without setting up an end-to-end connection.

TCP/IP Model

Because TCP/IP is so central to working with the Internet and intranets, you should understand it in details. TCP/IP uses the Department of Defense (DoD) model, which describes communications in only four layers.

DoD model:

In late 1960s, Advanced Research Projects Agency (ARPA) carried out a research project to connect a number different networks designed by different vendors into a network of networks (the "Internet"). And the model proposed for transmission of data was known as DoD model, it describes the communication in four layers: 

• The Network Access Layer is responsible for delivering data over the particular hardware media in use. Different protocols are selected from this layer, depending on the type of physical network.
• The Internet Layer is responsible for delivering data across a series of different physical networks that interconnect a source and destination machine. Routing protocols are most closely associated with this layer, as is the IP Protocol, the Internet's fundamental protocol. 
• The Host-to-Host Layer handles connection rendezvous, flow control, retransmission of lost data, and other generic data flow management. The mutually exclusive TCP and UDP protocols are this layer's most important members.
• The Process Layer contains protocols that implement user-level functions, such as mail delivery, file transfer and remote login.

TCP/IP was first proposed in 1973 and was split into separate protocols, TCP and IP, in 1978. In 1983, TCP/IP became the official transfer mechanism for all connections tp ARPAnet, replacing the earlier Network Control Protocol. We will study different protocols of TCP/IP model in details in later posts.

DoD model is theoretically comparable with OSI model :

IP Addresses and MAC Addresses

MAC Addresses
The MAC (or Hardware) address is used by devices to communicate on the local network. Hardware addressing is a function of the Data-link layer of the OSI model. MAC address that is usually hard-coded on the network card, this means The MAC address cannot be altered; the MAC address is often stored in flash on the NIC, and thus can be changed with special utilities.MAC address is represented in two formats (either notation is acceptable):
                                       00:43:AB:F2:32::13
                                       0043:ABF2:3213
           MAC address is a 48 bit hexadecimal coded address. first 24 bits tell the information about manufacturer and rest 24 bits represent the machine's information.
for example: 00:43:AB   stores manufacturer's details and F2:32:13 stores system's detail.

Changing MAC Address:

1. open Network sharing center----> change adapter setting----> choose connection type (LAN or Wireless LAN)
2. go to properties---> check connection controller (optional) ---> configure -----> Advance
3. Now click on the Network Address, you will see that it is selected as "not present", select "value" and enter some value of MAC address.
4. Your MAC address has been changed for a while. Once you turn off your machine the value will be set to default.
5. to change value permanently, you will have to change value from "registry", it has been discouraged to change value from the registry if you don't have sound knowledge of registry, it may crash your system.                   

IP Address
IP Addressing is a function of the network layer of the OSI model. It helps in routing data packets hop by hop, node by node in the network. IP address is a 32 bit hexadecimal coded address. Few initial bits represent network id, and rest bit represent host address in the network.
for example:    ip address  192.168.65.133
                       sub-net mask: 255.255.255.0

so,   network address is 192.168.65.0
   and host location in the network is 133th position.

To Avoid wastage of logical addresses in the network sub-netting is done. To understand sub-netting, you will have to learn IP Addressing. Please watch videos shared below to understand IP addressing in detail. I recommend you to watch these videos, if you are interested in designing and managing networks.

IP Addressing Scheme:

IP Sub-netting And Addressing:

To view all network addresses of your machine:
  start--> run---> type "cmd" then enter-----> type command "ipconfig  /all".

Logical Topology

The Data Link layer also bars the responsibility of dictating the logical topology of a network, or the way the devices perceive the topology of the network. A logical topology differs from a physical topology in that the physical topology dictates the way the cables are laid out, whereas the logical topology dictates the way the information flows. The types of logical topologies are the same as the physical topologies except that the information flow specifies the type of logical topology in use.For example, in early days of Ethernet, networks were physically wired using a bus topology.Once twisted pair and fiber were introduced into the Ethernet Physical layer, hubs were designed to allow the network to be wired as a physical star while allowing the devices attached to the hub to continue to believe they were part of a bus, making this configuration a physical star/logical bus. Finally, the DLL can describe the method of media access.There are some media access methods, discussed below:

Contention: Every device "competes" with other devices for the opportunity to transmit, and each has an equal chance. If two devices transmit at the same time, a collision occurs and the devices try again.


Polling: A central device, called a controller, polls each devices in turn and asks if it has data to transmit. This type of media access virtually eliminates collisions.


Token Passing : This uses a special data packet called a token. When a device has the token, it can transmit. If it doesn't have the token, it can't transmit. This media access technology also eliminates collision problems.





Carrier Sense/Multiple Access with Collision Detection (CSMA/CD) : When a protocol that uses CSMA/CD has data to transmit, it first sense if a signal is already on the wire (a carrier), indicating that someone is transmitting currently. That's the "Carrier Sense" part. If no one is transmitting, it attempts a transmission. If someone else transmits at the exact same time, a collision occurs. Both senders "back off" and don't transmit until some random period of time has passed. That's the "Collision Detection" part. "Multiple Access" means that more than one station can be on the network at the same time.

Carrier Sense/Multiple Access with Collision Avoidance (CSMA/CA): It is identical to CAMA/CD, but instead of sending the whole data chunk and then listening to hear if it was transmitted, the sender transmits a request to send (RTS) packet and waits for a clear to send (CTS) before sending. When it receives the CTS, the sender sends the chunk.

Difference between CSMA/CD and CSMA/CA:
Say you want to cross a busy street and you want to use one of these protocols to cross it. If you are using CSMA/CD, you just cross the street. If you get hit, you go back to the curb and try again. If you're using CSMA/CA , you send your friend across. If he makes it, it's probably OK for you to go.

OSI Model

The OSI model was designed to promote interoperability by creating a guideline for network data transmission between computers and components that have different hardware vendors, software, operating system, and protocols.The OSI model consists of seven layers. Each Layers performs a specific function and then passes on the result to another layer.

Understanding Data Transfer using OSI model.

Application Layer: The highest layer; defines the manner in which applications interact with the network including databases, e-mail, and terminal-emulation programs using Application layer protocols similar to Lightweight Directory Access Protocol (LDAP), Simple Mail Transfer Protocol (SMTP), and telnet etc.
Presentation Layer: This layer is responsible for the formatting and code conversion of data being passed up to the Application layer.In this layer, character sets are converted (e.g., from ASCII to Unicode or EBCDIC) and data is encrypted. Data may also be compressed in this layer. Anything that is done to the data on the transmitting device must be undone on the receiving device. Presentation Layer is responsible for recognizing file types in an incoming data stream and performing any massaging to the dta to make a file presentable to the Application protocol.
Session Layer: It defines how two computers establish, synchronize, maintain and end a session. Its functions such as security authentication, connection ID establishment, data transfer, acknowledgements, and connection release take place here.
Transport Layer: It is responsible for checking that the data was delivered error-free. It is also used to divide a message that is too long into smaller segments and, in the reverse, take a series of short messages and combine them in to one longer segment.This layer also handles logical address/name resolution. Additionally, this layer can send an acknowledgement that it got the data packet.
Network Layer: It is responsible for logical addressing and translating logical addresses into physical addresses.A little-known function of the network layer is prioritizing data. Nobody is hurt it an e-mail message is delayed a fraction of a second but delaying audio or video data a fraction of a second could be disastrous to the message. This prioritization is known as quality of service (QoS). This layer controls congestion, routes data, build and tear down packets. Routers and manageable switches operates on this layer. Routing protocols (RIP, OSPF, EIGRP, BGP etc)are being implemented on this layer.

Data-Link Layer: It takes raw data from the physical layer and gives it a logical structure, known as a frame. in the opposite direction of flow, the Data Link layer hands frames down to the Physical layer for bit-level encoding onto the networking medium.Frames include information about where the data is meant to go, which device on the local link sent the data, and the overall validity of the bytes sent. It also controls functions of logical network topologies, physical addressing as well as data transmission synchronization and connections. Non-manageable switches and bridges operate at this layer.
Physical Layer: Physical layer is responsible for controlling the functional interface, such as transmission technique, encoding scheme, cable specifications, pin layout, and connector.
                 Physical layer concept involve a network component that is tangible or measurable. For example, when a protocol at the physical receives information from the upper layers, it translates all the data into signals that can be transmitted on a transmission medium.

Data Flow in a Network
 

Networking Devices

Now that you are familiar with the various types of media and connections, you should learn about some devices commonly found on today's network.

NIC: 

NIC

The network interface card, as its name suggests, is the expansion card you install in your computer to connect, or interface, your computer to the network. This device provides the physical, electrical, and electronic connections to the network media. Generally all NICs have one ot two light emitting diodes (LEDs) that help in diagnosing problems with their functionality.

HUB:

A hub is a devices that connects all segments of the network together.Every device in the network connects to the hub through the single cable. Any transmission received on one port will be sent out all the other portsin the hub, including the receiving pair for the transmitting device. A broadcast sent out by any device on the hub will be propagated to all devices connected to hub. Any two or more devices connected to hub have capability of causing collision with each other. The LED will turn orange from green on collision, and we will have to turn off the hub, and wait for a while, then restart the hub, now the hub will be work properly. This was the major drawback of hubs.

SWITCH:

Like a hub, a switch connects multiple segments of the network together, with one important difference. Whereas a hub broadcast anything it receives, a switch recognizes frame boundaries and pays attention to the MAC address of the incoming frame as well as the port on which it was received. A switch, when it is installed in the network, first time it broadcasts to store MAC address of the devices in the network in its table, whenever it receives a data packet then it matches MAC address from its MAC table and forward it to specific port. In this way, probability of collision is being reduced. There are two types of switches:

• Manageable: operates on Data Link Layer and Network Layer, it may be used to implement routing protocols in a small network.
• Non-manageable: operates on Data Link layer. It has MAC table, it does not support routing protocols.

switching methods:

• Cut-through Method: cut-through switching is a method for packet switching systems, wherein the switch starts forwarding a frame (or packet) before the whole frame has been received, normally as soon as the destination address is processed. This technique reduces latency through the switch, but decreases reliability.
• Store & Forward Method: information is sent to an intermediate station where it is kept and sent at a later time to the final destination or to another intermediate station. The intermediate station, or node in a networking context, verifies the integrity of the message before forwarding it.

BRIDGE:
A bridge, specifically a transparent bridge,is a network device that connects two similar network segments together. The primary function of a bridge is to keep traffic separated on both sides of the bridge. Bridge is used in bus topology. for example :

MSMQCLIENT1 wants to send data to MSMQCLIENT5, bus will broadcast the data in upper segment and try to broadcast in lower segment also, since MSMQCLIENT5 is in upper segment so it will be wastage of resource utilization by broadcasting data in lower segment. hence we use bridge, it will match MAC address of the data packet header with its MAC table, since MSMQCLIENT5 is not in lower segment so it will not allow data flow in lower segment.


ROUTER

Representation of a router

A router is a network device that connects multiple, often dissimilar, network segments into an internetwork. The router, once connected, can make intelligent decisions about how best to get network data to its destination based on network performance data that it gathers from the network itself. Router operates at Network layer. It decide the best route with the help of network layer address. We will study about router and routing protocols in detail later on. Router Vendors:
Cisco, Juniper, ZDNet etc


GATEWAY
A gateway is any hardware and software combination that connects dissimilar network environments. Gateways are the most complex of network devices because they perform translations at multiple layers of the OSI model. For example, a gateway is the device that connects a LAN environment to a mainframe environment. E-mail gateway translates LAN based mail messages into the SMTP format that Internet mail uses.

MODEM

A modem is a device that modulates digital data onto an analog carrier for transmission over an analog medium and then demodulates from the analog carrier to a digital signal again at the receiving end.


FIREWALLS

Its job is to protect LAN resources from attackers on the Internet. Similarly, it can prevent computers on the network from accessing various services on the Internet. It can be used to filter packets based on rules that the network administrator sets. Firewalls can be stand-alone "black boxes", or can be set up in software on a server or router.

Physical Media

Although it is possible to use several forms of wireless networking, such as radio frequency and infrared, the majority of installed LANs today communicate via some sort of cable.In following section, we'll look at three types of cables:

• Coaxial
• Twisted Pair
• Fiber Optic

Coaxial Cable:
Coaxial cable contains a center conductor, made of copper, surrounded by a plastic jacket, with a braided shield over the jacket.A plastic such as polyvinyl chloride (PVC) or fluoroethylenepropylene (FEP) covers this metal shield. The Teflon type covering is frequently referred to as a plenum-rated coating. That simply means that the coating doesn't begin burning until a much higher temperature, doesn't release as many toxic fumes as PVC when it does burn, and is rated for use in plenums that carry breathable air, usually as nonenclosed fresh-air return pathways that share space with cabling.

types of coaxial cables are: (RG= radio guide)

RG8 :

• segment length is 500 m.
• Interference is good compared to any copper cable.
• resistance 50 ohm.
• BNC-T or Vampire Tap connector is used to connect this cable.
• Used in 10base5 network

RG6:

• broadband quad-shield cable that offers an impedance of 75 ohm.
• lower attenuation
• Used in cable TV, CCTV, satellite dish antenna.

RG59:

• Better breakdown characteristics allows for high voltage use in scientific experiments.
• Characteristic impedance 75 ohm.
• Used in security camera, cable TV, home theater.

using Thin Ethernet: Thin Ethernet, also referred to as Thinet or 10base-2, is a thin coaxial cable.It is basically the sane as thick coaxial cable except that the diameter of the cable is smaller ( about 1/4" in diameter). Thin Ethernet coaxial cable is RG-58. With Thinnet cable, we use BNC ( BayoNet Connector or British Navel Connector) to attach stations to the network.

 

Twisted -Pair Cable:

Twisted-pair cable consists of multiple, individually insulated wires that are twisted together in pairs. Sometimes a metallic shield is placed around the twisted pairs. hence, the name shield twisted-pair (STP). More commonly, you see cable without outer shielding; it's called unshielded twisted-pair (UTP). UTP is commonly used in twisted-pair Ethernet ( 10Base-T, 10Base-TX, etc), star-wired networks. Let's take a look at why the wires in this cable type are twisted. When electromagnetic signals are conducted on copper wires that are in close proximity, some electromagnetic interference occurs. This interference is called crosstalk. Twisting two wires together as a pair minimizes such interference and also provides some protection against interference from outside sources.

 

UTP cables:

• Category 1: two twisted wire pairs. Voice grade (not rated for data communications).Oldest UTP, used in PSTN (Public Switched Telephone Network) .
• Category 2: Four twisted wire pairs. Suitable for up to 4 Mbps, with a frequency limitation of 10 MHz
• Category 3: Four twisted wire pairs with three twists per foot.Acceptable for transmissions up to 16 MHz.
• Category 4: Four twisted wire pairs and rated for 20MHz.
• Category 5: Four Twisted wire pairs and rated for 100MHz
• Category 5e: Four Twisted wire pairs and rated for 100MHz, but capable of handling the disturbance on each pair caused by transmitting on all four pairs at the same time.
• Category 6: Four twisted wire pairs and rated for 250MHz.

Fiber-Optic Cable :

A fiber optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages modulated onto light waves.Electrical signals are converted into light signals. TIR (Total Internal Reflection) phenomena is responsible for transmission of data. Because fiber optic cable transmits digital signals using light impulses rather than electricity, it is immune to Electromagnetic Interference (EMI) and Radio Frequency interference (RFI).

 

          The cable itself comes in two different styles: single-mode fiber (SMF) and multi-mode fiber (MMF). The difference between single-mode fibers and multi-mode fiber is in the number of light rays ( and thus the number of signals) they can carry. Multi-mode fiber is used for short distance application and single-mode fibers for longer distance.

Fiber-optic connectors: 

 

• Straight tip (ST) : joins individual fibers to optical device.
• Subscriber Connector (SC) : attaches two fibers to send and receive signal.
• Medium interface connector (MIC) : joins fibers to FDDI
• Sub Miniature Type A (SMA) : uses individual connectors for each fiber stand.

 

NOTE:      mBase-n

•  m indicates transmission speed in megabits
• n can have different meaning. for example n=5, means 500m segment length but for n=2, segment length is 185m. Only the IEEE committee knows for sure what this was about.

for examples:

• 10BASE-5 (Thickwire coaxial cable with a maximum segment length of 500 meters)
• 10BASE-F (optical fiber cable)
• 10BASE-T (ordinary telephone twisted pair wire)
• 10BASE-36 (broadband multi-channel coaxial cable with a maximum segment length of 3,600 meters)

 

 

 

Physical Topologies

A topology is basically a map of a network. The physical topology of a network describes the layout of the cables and workstations and th location of all network components. Often, physical topologies are compared to logical topologies, which define how the information or data flows within the network. The topologies are usually similar. It is important to note, however, that a network can have one type of physical topology and a completely different logical topology.
            physical vs. logical concepts: Generally speaking, when we're referring to the physical aspects of a      network, we're referring to some aspect of the network that you can touch or that has physical substance (like electrons, electrical pulses, or the way cables are runs). Logical concepts, on the other hand, are more imaginary and esoteric and deal with things like how data flows in a network.

The cables or connections in a physical topology are often referred to as network media (or physical media). Choosing how computers will be connected in a company's network is critical. A wrong decision in the physical topology makes the media difficult to correct because it is costly and disruptive to change an entire installation once it is in place. We'll look at 4 most common topologies:


1. Bus Topology: In a bus topology, all computers are attached to a single continuous cable (or trunk) that is terminated at both ends, which is the simplest way to create a physical network. Maximum segment length of the cable is 200 meters, there can be around (max.) 30 devices per segment. cable uses thin-net (coaxial cable used in Ethernet). The trunk has terminators of 50 ohm at both the ends to terminate the flow of data.
             When communicating on a network that uses a bus topology, all computers see the data on the wire, This does not create chaos, though, because the only computer that actually accepts the data is the one to which it is addressed.In the network with bus topology, data is being broadcasted to send it to the receiver.

2. Star Topology: unlike those in a bus topology, each computer in a star topology is connected to a central point by a separate cable or wireless connection. The central point is a device known by such names as hub, MAU, concentrator, switches and access point, depending on the underlying technology.Although this setup uses more cable than a bus. a star topology is much more fault tolerant than a bus topology. this means that if a failure occurs along ine of the cables connecting to the hub, only that portion of the network is affected, not the entire network. the network's most vulnerable point is the hub. If it fails, the whole system collapses. Fortunately, hub failures are extremely rare.

3. Ring Topology: In the ring topology, each computer is connected directly to two other computers in the network. Data moves down a one-way path from one computer to another, as shown below. The good news about laying out cable in a ring is that the cable design is simple. the bad news is that, as with bus topology, any break, such as adding or removing a computer, disrupts the entire network.Also, because you have to break the ring in order to add another station, it is very difficult to reconfigure without bringing down the whole network. To avoid such problems, we use concept of dual ring, in which there are two rings
         -primary ring (clock-wise data flow)
         -secondary ring (anti clock-wise data flow)
if we need to add any new computer (or workstation), there is any fault, then the transmission of data is done through secondary ring.

4. Mesh Topology: A physical mesh topology is another physical topology that isn't widely used in computer networks (except in special WAN cases). In a physical mesh topology, every computer is directly connected to every other computer in the network. The more computers there are on a mesh network, the more cables make up the network. If a mesh network has n computers, there will be (n-1)/2 cables. The bandwidth available in the mesh configuration is wasted.

Backbones and Segments: With complex networks, we must have a way of intelligently identifying which part of the network we are discussing. For this reason, we commonly break networks into backbones and segments.
              A backbone is the part of the network to which all segments and servers connect. A backbone provides the structure for a network and is considered the main part of any network. It usually uses a high-speed communications technology of some kind, such as Fiber Distributed Data interface (FDDI) or 1 or 10 Gb Ethernet.
                       Segment is general term for any short section of the network that is not part of the backbone. Just as servers connect to the backbone, workstation connect to the segments. segments are connected to the backbone to allow the workstations on them access to the rest of the network.

Networking Essentials

Networks are made up of lots of different components, but the three most common network entities are :-

Host:- A host is a computer connected to a computer network. A network host may offer information resources, services, and applications to users or other nodes on the network.

Server:- In the truest sense, a server does exactly what the name implies, it provides resources to the client on the network. Server are typically powerful computers that run the software that controls and maintain the network. This is also known as network operating software. Servers are often specialized to a single purpose.Here are some example of servers that are dedicated to a single task:
       
                        File Server: holds and distributes files
                        proxy Server: performs a function on behalf of other computers
                        Application Server: hosts a network application.
                        Web Server: holds and delivers web pages and other web content using HTTP.
                         Print Server: controls and manages one or more printers in the network.

Workstations:- In the network environment, the term workstation normally refers to any computer that is connected to the network and used by an individual to do work. Workstations can be clients, but not all clients are workstations. for example, a printer can request resources from the network, but it is a client, not a workstation.

Types of Networks :-

1. Local Area network (LAN):
By definition, a local area network (LAN) is limited to a specific area, usually an office, and cannot extend beyond the boundaries of a single building. The first LANs were limited to a range (from a central point to the most distant computer) of 185 meters (about 600 feet) and no more than 30 computers. Today's technology allows a larger LAN, but practical administration limitations require dividing it into small, logical areas called workgroups. A workgroup is collection of individuals (for ex, a HR department) who share the same files and databases over the LAN.Typically LANs are half-duplex, although some LANs are full duplex.

2. Wide Area Network (WAN):
A WAN is any network that crosses metropolitan, regional or national boundaries. Most networking professionals define a WAN as any network that uses routers and public network links. WANs use full-duplex communication. Internet is the largest WAN network on the planet earth.

In half-duplex communication, information is transferred in both the directions but only one direction at a time, while in full-duplex communication can be happen in both the directions simultaneously.

3. Metropolitan Area Network (MAN):
A MAN is optimized for a larger geographical area than a LAN, ranging from several blocks of buildings to entire cities. MANs can also depend on communications channels of moderate-to-high data rates. A MAN might be owned and operated by a single organization, but it usually will be used by many individuals and organizations. MANs might also be owned and operated as public utilities. They will often provide means for internetworking of local networks.

4. Campus Area Network (CAN):

It is used to inter-connect networks in limited geographical locality like university campus, military bases, or organizational campuses etc. It can be taken as the metropolitan network that has the specific settings at the small area just like a computer lab in the university.

5. Storage Area Network (SAN):

 A network which is created by attaching storage devices such as disk arrays and tapes to a common server so that the storage devices appear to be confined to  operating system is known as storage area network. Data in SAN can be stored using two basic modules as DAS(Direct Attached Storage) and NAS (Network Attached Storage). In DAS, device is directly attached to a server without any intermediary network involved. while in NAS , device is connected to a server using network components, it may be also possible that storage device physically away from the server, but logically it is directly attached to a server. NAS uses hubs, bridges, switches.

 One thing you must know:

The Internet, with a capital "I," refers to the network that began its life as the ARPAnet and continues today as, roughly, the confederation of all TCP/IP networks directly or indirectly connected to commercial U.S. backbones. Seen up close, it's actually quite a few different networks-commercial TCP/IP backbones, corporate and U.S. government TCP/IP networks, and TCP/IP networks in other countries-interconnected by high-speed digital circuits.

A lowercase internet, on the other hand, is simply any network made up of multiple smaller networks using the same internetworking protocols. An internet (little "i") isn't necessarily connected to the Internet (big "I"), nor does it necessarily use TCP/IP as its internetworking protocol. There are isolated corporate internets, for example.