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All types of copper cable are vulnerable to fire and electrical hazards. Fire hazards exist since cable insulation and sheaths may be flammable, or produce toxic fumes when heated or burned. Building authorities or organizations may specify related safety standards for cabling and hardware installations.
Electrical hazards are a potential problem because copper wires can conduct electricity in unwanted ways. This could focus on personnel and equipment to a range of electrical hazards. For example; a defective network device could conduct currents to the chassis of other network devices. as well; network cabling could present unwanted voltage levels when used to connect devices that have power sources with different ground potentials.
Such situations are possible when copper cabling is used to connect networks in different buildings or on separate floors that use disparate power facilities. Finally; copper cabling may conduct voltages caused by lightning strikes to network devices.
The result of unwanted voltages and currents can include harm to network devices and linked computers, or hurt to personnel. It is essential that copper cabling is installed correctly, and according to the relevant specifications and building codes; in order to avoid potentially dangerous and damaging situations.
During Installation of Copper Cable Use following safety precautions to avoid any incident and hazards.
Wear safety glasses:
Use common sense with ladders.
Wear protective clothing.
Don’t be careless when lifting.
Don’t use power tools unless you know how to use them.
Be worry about an electrical cable.
Know local code: building code may prohibit drilling holes in firewalls or ceilings.
Take care of what you touch.
Use the right tools.
Depending on your project use the right materials.
Mark junction box locations carefully
When pulling the cable through drop-in ceilings, you must be sure to prevent the cable from scraping along sharp edges.
Every rushed install will most likely have something wrong with it, So take your time on installation.
A network required physical media to connect its nodes. The physical media is where the data actually flows. You have most likely heard about the OSI reference model, which defines network hardware and services in terms of the functions they perform. The OSI reference model we already discussed in detail in Chapter 1, (“Network and their building blocks.”) Transmission media work at Layer 1(physical layer) of the OSI model.
The physical layer represents bits as voltages, radio frequencies; or light pulses so various standards organizations have contributed to the definition of the physical; electrical, and mechanical properties of the media available for different data communications. These characteristics guarantee; that cables and connectors will function as expected with different data link layer implementations. There are three main categories of physical media:
Copper cable Media
Types of cable include unshielded twisted-pair (UTP), shielded twisted-pair (STP), and coaxial cable. Copper-based cables are inexpensive and easy to work with compared to fiber-optic cables, but as you’ll learn when we get into the particulars, the main disadvantage of copper cable is that it offers a limited range that cannot handle the advanced applications.
The most used physical media for data communications is cabling (copper media); that uses copper wires to send data and also control bits between network devices. Cables use for data communications generally consists of a series of individual copper wires; that from circuits dedicated to specific signalling purposes. There are three main types of copper media used in networking:
Unshielded Twisted-Pair (UTP)
Shielded Twisted-Pair (STP)
The above cables interconnect computers on a LAN and other devices such as switches, routers, and wireless access points. Each type of connection and the associated devices has cabling requirements specified by physical layer standards. The physical layer standards also specify the use of different connectors for different. Physical layer standards also specify the mechanical dimensions of the connectors and the acceptable electrical properties of each type.
Unshielded Twisted-Pair Cable (UTP)
Unshielded twisted-pair (UTP) cabling is the most common networking media for voice as well as for data communications. UTP cable consists of four pairs of colour-coded wires that have been twisted
Together and then encased in a flexible plastic sheath that protects from minor physical damage. The twisting of wires helps to decrease electromagnetic and also radio-frequency interference induced from one wire to the other.
UTP cabling terminated with RJ-45 connectors for interconnecting network hosts with intermediate networking devices Just like switches and routers. In the figure, you can see that the colour codes find the individual pairs and wires and help in cable termination.
Shielded Twisted-Pair Cable(STP)
Shielded twisted-pair (STP) provides better noise protection than UTP cabling. However, compared to UTP cable, STP cable is more expensive and difficult to install. Like UTP cable, STP uses an RJ-45 connector.
The extra covering in shielded twisted pair wiring protects the transmission line from electromagnetic interference leaking into or out of the cable. STP cabling often used in Ethernet networks, especially fast data rate Ethernet.
Shielded twisted-pair (STP) cables also combine the techniques of shielding to counter EMI and RFI, and wire twisting to counter crosstalk. To put on the full advantage of the shielding STP cables also terminated with special shielded STP data connectors. If the cable is improperly grounded the shield may act as an antenna and pick up unwanted signals.
Coaxial cable is called “coaxial” because of the fact that there are two conductors that share the same axis. The outer channel serves as a ground. Many of these cables or pairs of coaxial tubes placed in a single outer sheathing and; with repeaters, can carry information for a great distance. As shown in the figure, the coaxial cable consists of:
A copper conductor used to send the electronic signals.
A layer of flexible plastic insulation surrounding a copper conductor.
The insulating material surrounded in a woven copper braid or metallic foil; that acts as the second wire in the circuit and as a shield for the inner conductor. This second layer or shield also reduces the amount of outside electromagnetic interference.
The entire cable has covered with a cable jacket to prevent minor physical damage.
There are different types of connectors uses with coax cable.
Although, UTP cable has essentially replaced the coaxial cable in modern Ethernet installations, the coaxial cable design:
There are many types of coax cable that we can use in different ways following is the table of different types of coax cable
Fiber optics Cable Media
Fiber Optic cable is another type of physical media. It offers huge data bandwidth, protection to many types of noise and interference, and enhanced security.
So, fiber provides clear communications and a comparatively noise-free environment. The disadvantage of fiber is that it is costly to purchase and it requires specialized equipment and techniques for installation.
Properties of Fiber-Optic Cabling
Fiber optic cable can send data over long distances with higher bandwidths than any other networking media. Optic fiber cable can send signals with less attenuation and it’s totally protected from EMI and RFI. OFC is generally used to connect network devices.
Fiber optic cable is a flexible, but very thin; a transparent strand of very pure glass, not bigger than a human hair. Bits are encoded on the fiber as light impulses. The fiber-optic cable acts as a waveguide; or “light pipe,” to send light between the two ends with the minimal loss of signal.
As an analogy, consider an empty paper towel roll with the inside coated like a mirror. It is a thousand meters in length, and a small laser pointer sends a Morse code signal at the speed of light. Essentially that is how a fiber-optic cable operates; except that it is smaller in diameter and uses advanced light technologies.
Fiber-optic cabling is now being used in four types:
Enterprise Networks: Used for backbone cabling and interconnecting infrastructure devices.
Fiber-to-the-Home: Used to give always-on broadband services to homes and small businesses.
Long-Haul Networks: The service providers use this type to connect countries and cities.
Submarine Networks: Used to give reliable high-speed; high-capacity solutions capable of surviving in harsh undersea environments up to transoceanic distances.
Fiber Optic Cable structure
The optical fiber is composed of two kinds of glass (core and cladding) and a protective outer shield (jacket) shows that figure 3-8.
The core is actually the light transmission element at the center of the optical fiber. This core is typically silica or glass. Light pulses travel through the fiber core.
Made from little different chemicals than those used to make the core. It tends to do like a mirror by reflecting light back into the core of the fiber. This keeps the light in the core as it travels down the fiber.
Used to help shield the core and cladding from damage.
Surrounds the buffer, prevents the fiber cable from being stretched out when it is being pulled. The material used is often the same material uses to manufacture bulletproof vests.
Typically a PVC jacket that protects the fiber against abrasion; moisture, and other contaminants. This outer jacket composition can vary depending on the cable usage.
Types of Fiber Media
Light pulses in lieu of the transmitted data as bits on the media generated by either:
Light emitting diodes (LEDs)
Electronic semiconductor devices called photodiodes detect the light pulses and then convert them to voltages. The laser light transmitted over fiber-optic cabling can damage the human eye. Care must be taken to avoid looking into the end of active optical fiber. Fiber-optic cables are mostly classified into two types:
Single-mode fiber (SMF): its core is very small uses very expensive laser technology to send a single ray of light; as shown in Figure Popular in long-distance situations spanning hundreds of kilometres; such as those required in long-haul telephony and cable TV applications. Following is single mode cable characteristics.
Use laser as the light source
Suited for long distance application
Commonly used with campus backbone for the distance of several thousand meters.
Multimode fiber (MMF): Its core is very large and this type of cable uses LED emitters to send light pulses. Specifically, light from a LED enters the multimode fiber at different angles; as shown in Figure 3-10. Popular in LANs because they powered by low-cost LEDs. It provides bandwidth up to 10 Gb/s over link lengths of up to 550 meters. Following is single mode cable characteristics.
Larger core than single-mode cable
Uses LEDs as the light source
Allows greater dispersion and therefore, loss of signal
Suited for long distance application; but shorter than single mode
Commonly used with LANs or distances of a couple of hundred meters within a campus network.
Wireless media include radio frequencies, microwave, satellite, and infrared. Deployment of wireless media is faster and less costly than the deployment of cable, mostly where there is no existing infrastructure. There are a few disadvantages associated with wireless. It supports much lower data rates than do wired media. Wireless is also greatly affected by external environments, such as the impact of weather, as a result, reliability can be difficult to guarantee. It carries data in the form of electromagnetic signals using radio or microwave frequencies.
Wireless media provides the best mobility options, and the number of wireless-enabled devices continues to increase. As network bandwidth options increase, wireless is quickly gaining in popularity in enterprise networks and it has some important point to consider before planning:-
Coverage area: Wireless data communication technologies work well in open environments. However, certain construction materials used in buildings and structures, and the local terrain, will limit the effective coverage.
Interference: Wireless is at risk to intrusion and can be disrupted by such common devices as household cordless phones, some types of fluorescent lights, microwave ovens, and other wireless communications.
Security: Wireless communication coverage requires no access to a physical strand of media. thus, devices and users, not authorized for access to the network, can gain access to the transmission. Network security is the main component of wireless network administration.
Shared medium: WLAN work in half-duplex, which means just one device can send or receive at a time. The wireless medium is shared among all wireless users. The more users need to access the WLAN simultaneously, results in less bandwidth for each user.
Types of Wireless Media
The IEEE and telecommunications industry standards for wireless data communications cover both the data link and physical layers. cellular and satellite communications can also provide data network connectivity. But, we are not discussing these wireless technologies here in this chapter. In each of these standards, physical layer specifications applied to areas that include:
The transmission power of transmission
Data to radio signal encoding
Signal reception and decoding requirements
Antenna design and construction
Wi-Fi is a trademark of the Wi-Fi Alliance. The certified product uses that belong to WLAN devices that are based on the IEEE 802.11 standards. Different standards are the following:-
WLAN technology commonly referred to as Wi-Fi. WLAN uses a protocol known as Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). The wireless NIC must first listen before transmitting to decide if the radio channel is clear. If another wireless device is transmitting, then the NIC must wait until the channel is clear. We discuss CSMA/CA later briefly.
Usually known as Worldwide Interoperability for Microwave Access (WiMAX), uses a point-to-multipoint topology to give wireless broadband access.
Wireless LAN (WLAN)
General wireless data implementation wireless LAN requires the following network devices:
Wireless Access Point (AP): In a wireless local area network (WLAN), an access point (AP) is a station that transmits and receives data. An access point also connects users to other users within the network and can serve as the point of interconnection between the WLAN and a fixed wire network. Each access point can serve multiple users within a defined network area, so when people move beyond the range of one access point, they are automatically handed over to the next one. A small WLAN may only need a single access point; the number required increases the function of the number of network users and the physical size of the network.
Wireless NIC adapters: Provide wireless communication ability to each network host.
As technology has developed, a number of WLAN Ethernet-based standards have emerged. So, more care needs to be taken in purchasing wireless devices to make sure compatibility and interoperability.
The benefits of wireless data communications technologies are clear, particularly the savings on costly premises wiring and the convenience of host mobility
In computer networks, bandwidth is the amount of data that can be carried from one point to another in a given time period; (generally a second). Network bandwidth is usually expressed in bits per second (bps); kilobits per second (kb/s), megabits per second (Mb/s), or gigabits per second (Gb/s). It is sometimes thought of as the speed that bits travel, however, this is not accurate.
For example, in both 100Mb/s and 1000Mb/s Ethernet, the bits are sent at the speed of electricity. The difference is the number of bits that are transmitted per second.
A combination of factors determines the practical bandwidth of a network.
The properties of the physical media.
The technologies chosen for signalling and detecting network signals.
Physical media properties, current technologies, and the laws of physics play a role in determining the available bandwidth.
Bandwidths connections can be symmetrical, which means the data capacity is the same in both directions to upload or download data, or asymmetrical, which means download and upload capacity are not equal. In asymmetrical connections, upload capacity is typically smaller than download capacity.
Modern networks support the transfer of huge numbers of bits per second. Instead of quoting speeds of 10,000 or 100,000 bps, networks normally express per second performance in terms like:
1Kbps = 1,000 bits per second
1Mbps = 1,000 Kbps
1Gbps = 1,000 Mbps
So, a network with a performance rate of units in Mbps is much faster than one rated in units of Kbps but slower than network performance of Gbps.
Examples of Performance Measurements
The standard bandwidth examples are the following:
Storage capacity like hard disks, USBs is normally measured in units of kilobytes, megabytes, and gigabytes. In this type of usage, K represents a multiplier of 1,024 units of capacity. The following table defines the mathematics behind these terms:
The measurement of the transfer of a bit across the media over a given period is called throughput. It is a measure of how many units of information a system can process in a given amount of time. Due to some factors, it generally does not match the specified bandwidth in physical layer implementations. Many factors manipulate it, including the following:
The type of traffic
The amount of traffic
The latency created by the number of network devices between source and destination
Latency is the amount of time, including delays; for data to travel from one given point to another. The networks with multiple segments, throughput can’t be faster than the slowest link in the path from source to destination. Even if all or most of the segments have high bandwidth; it will only take one segment in the path with low throughput to create a tailback to the throughput of the entire network.
The average transfer speed over a medium is often described as throughput. This measurement includes all the protocol overhead information; such as packet headers and other data that is included in the transfer process. It also includes packets that are retransmitted because of network conflicts or errors.
There is another measurement to evaluate the transfer of usable data as goodput. Goodput is the measure of usable data transferred over a given period of time. Goodput is throughput minus traffic overhead for establishing sessions, acknowledgements, and encapsulation. It only measures the original data.
Throughput vs Bandwidth
Bandwidth and throughput are both related to speed, but what’s the difference? bandwidth is the theoretical speed of data on the network, whereas throughput is the actual speed of data on the network. Throughput can only send as much as the bandwidth will allow, and it’s usually less than the bandwidth. Latency, irregularities in the signal (jitter), and error rate reduce the overall throughput.
The Physical Layer is the lowest layer of the OSI Model. It provides the resources to transport the bits; that make up a data link layer frame across the network media. This layer accepts a complete frame from the data link layer and encodes it as a series of signals that transmitted the local media as shown in the figure below. The encoded bits contain a frame received by either an end device or an intermediate device.
This layer also deals with the physical connection to the network and with transmission and reception of signals. This layer also defines electrical and physical details represented as 0 or a 1. It’s also decided when the data transmitted or not and how the data would be synchronized. The process that data travel from a source node to a destination node is following:
The user data segmented by the transport layer placed into packets by the network layer and further encapsulated into frames by the data link layer. Data link layer sent these frames to the physical layer.
It encodes the frames and creates the electrical, optical; or radio wave signals that represent the bits(0 and 1) in each frame.
These signals then transmitted to the media.
The destination node physical layer retrieves these signals from the media; restores them to their bit representations, and passes the bits up to the data link layer as a complete frame.
Key points of the physical layer
Line configuration: – This layer connects devices with the medium, Point to Point configuration and Multipoint configuration.
Topologies: – Devices can connect using the following topologies Mesh, Star, Ring, and Bus
Transmission Modes: – It also defines the direction of transmission between devices ( Simplex, Half Duplex, Full Duplex).
Many different types of media can be used for this layer. For example, telephone twisted pair cable, coax cable, shielded copper cable, unshielded copper cable and fiber optics are the main types used for LANs. Different transmission techniques generally categorized as baseband, or broadband transmission may apply to each of these media types. There are three basic types of media. The physical layer produces the representation and groupings of bits for each type of media as:
Copper cable: The signals are patterns of electrical pulses.
Fiber-optic cable: The signals are patterns of light signals.
Wireless: The signals are patterns of microwave transmissions.
To enable physical layer interoperability, all aspects of these functions governed by standards organizations.
Upper layer Protocol: – Protocols and operation of the upper OSI layers totally performed in software designed by software engineers and computer scientists. IETF (Internet Engineering Task Force ) is an organization which defined the services and protocol for TCP/IP suite.
This layer consists of electronic circuitry, media, and connectors. Therefore, it is suitable that the standards governing this hardware are defined by the relevant electrical and communications engineering organizations.
There are many different international and national organizations, regulatory government organizations, and private companies involved in establishing and maintaining physical layer standards. For example, the physical layer hardware, media, encoding, and signalling standards defined and governed by the following:-
Institute of Electrical and Electronics Engineers (IEEE)
Telecommunications Industry Association/Electronic Industries Association (TIA/EIA)
National telecommunications regulatory authorities including the Federal Communication Commission (FCC) in the USA and the European Telecommunications Standards Institute (ETSI)
Canadian Standards Association (CSA)
European Committee for Electrotechnical Standardization(CENELEC)
Component and Functions
Components of this layer are the electronic hardware devices; media, and other connectors such as NIC, cable, and connector that send and carry the signals. Hardware components such as NIC, interfaces and connectors, cable materials, and cable designs are all specified in standards associated with the physical layer. Physical layer deals with all the above physical components of the network. For example, the network cable, the female adapter of your NIC and the network interface card of a computer is a part of the physical layer. So, let’s go take a look into what all basic things physical layer does and what protocols are run at the physical layer.
Delivery of Packets
The physical layer takes frames from the data link layer then converts these frames into electrical, electromagnetic; optical signals through different line coding techniques. Transmit these signals through wired/wireless telecommunication links (cables/antennas) to next hop. The physical layer delivered those signals from a cable, a wifi router or an optical fiber.
Encoding is a technique of converting a stream of data bits into a predefined “code”. Codes are groupings of bits used to give a predictable pattern that can be recognized by both the sender and the receiver. In the case of networking, encoding is a pattern of voltage or current used to represent bits; the 0s and 1s.
The physical layer generates the electrical, optical, and also wireless signals that represent the “1” and “0” on the media. The method of representing the bits is signalling. The physical layer standards must define what type of signal represents a “1” and what type of signal represents a “0”. This can be as simple as a change in the level of an electrical signal or an optical pulse. For example, a long pulse might represent a 1 whereas a short pulse represents a 0. Modulation techniques are a common method to send data. Modulation is the process by which the character of one wave modifies another wave.
Comparison between TCP/IP and OSI models are very important because protocols that make up the TCP IP protocol suite can also be described in terms of the OSI reference model. Network access layer and Application layer of the TCP IP models further divided In the OSI model describe discrete functions that must occur at these layers. Both models are widely used networking models for communication. The important difference between both is that OSI is a conceptual model which is not practically used for communication, whereas, TCP IP is used; for establishing a connection and communicating through the network.
Network Access layer of TCP IP model does not specify which protocols to use when transmitting over a physical medium. It only describes the handover from the internet layer to the physical layer. On the other hand, OSI Layer physical and data link layer discuss the basic process to access the media and the physical means to send data over a network.
Network Layer of OSI and Internet Layer of TCP/IP Model
The network layer maps directly to the TCP/IP Internet layer. This layer explains protocols that address and route messages from end to end an internetwork.
The transport layer maps directly to the TCP IP Transport layer. This layer explains general services and functions that give ordered and reliable delivery of data between source and destination hosts.
TCP/IP Application Layer
The application layer of the TCP IP model includes a number of protocols that give specific functionality to a variety of end-user applications. The OSI Model Layers 5, 6, and 7 used as references for application software developers and vendors to produce products that operate on networks.
TCP/IP and OSI Models and Protocols comparison
Both the TCP IP and OSI models commonly used when referring to protocols at various layers. OSI is a generic, protocol-independent standard, acting as a communication gateway between the network and end user. TCP/IP model is based on standard protocols around which the Internet has developed. It is a communication protocol, which allows connection of hosts over a network. OSI is a reference model around which the networks built. Generally, it is a guidance tool.TCP IP model is, in a way implementation of the OSI model.
OSI has 7 layers and TCP has 4 Layer model. following is the comparison model of both layer.
Key Differences between TCP/IP and OSI Model
1.TCP IP is a client-server model, i.e. when the client requests for service it is provided by the server. Whereas, OSI is a conceptual model.
2.TCP IP is a standard protocol used for everywhere in networking and internet, whereas, OSI is not a protocol, but a reference model used for understanding and designing the system architecture.
3.TCP IP has a four-layer, whereas, OSI has seven layers model.
During the period of 1970, the Defense Advanced Research Projects Agency (DARPA) was underway to create an open standard network model. This network model came to be known as the TCP IP Model. By 1985, the TCP IP model started gaining more importance and support from vendors and ultimately replaced the OSI model. Currently, Internet Engineering Task Force (IETF) maintaining the TCP IP model and its related protocols.
The TCP IP model was on the path of development when the OSI standard published. The TCP IP model is not the same as the OSI model. OSI is a seven-layered standard, but TCP IP is a four-layered standard. The OSI model has been very important in the growth and development of the TCP IP standard, and that is why much OSI terminology applied to TCP IP.
Both models are open to standard networking models. However, the TCP IP model has found more acceptances today and the TCP/IP protocol suite is more commonly uses. Just like the OSI reference model, the TCP/IP model takes a layered approach. In this section, we will look at all the layers of the TCP/IP model and various protocols used in those layers.
The TCP/IP model is a reduced version of the OSI reference model consisting of the following 4 layers:
Network Access Layer
The functions of these four layers are comparable to the functions of the seven layers of the OSI model. Figure below illustrates the comparison between the layers of the two models.
The following sections discuss the four layers and protocols in those layers in detail.
As we can see from the above figure, the presentation and session layers are not there in the TCP/IP model. Also, note that the Network Access Layer in the TCP/IP model combines the functions of Datalink Layer and Physical Layer.
Application Layer of TCP IP model
The application layer is the topmost layer of the TCP/IP model. It is present on the top of the Transport layer. Application layer defines TCP/IP application protocols and how host programs interface with transport layer services to use the network.
The Application Layer of the TCP/IP Model consists of various protocols that do all the functions of the OSI model’s Application, Presentation, and Session layers. This includes interaction with the application, data translation and encoding, dialogue control and communication coordination between systems.
The application layer includes all the higher-level protocols like DNS (Domain Naming System), HTTP (Hypertext Transfer Protocol), Telnet, SSH, FTP (File Transfer Protocol), TFTP (Trivial File Transfer Protocol), SNMP (Simple Network Management Protocol), SMTP (Simple Mail Transfer Protocol) , DHCP (Dynamic Host Configuration Protocol), X Windows, RDP (Remote Desktop Protocol) etc.
Transport Layer of TCP IP model
Transport Layer is the third layer of the TCP/IP model. The position of the Transport layer is between the Application layer and the Internet layer. The purpose of the Transport layer is to permit devices on the source and destination hosts to carry on a conversation. Transport layer defines the level of service and status of the connection used when transporting data. The main protocols included at the Transport layer are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).
TCP/IP transport layer’s function is the same as the OSI layer’s transport layer. It concerned with end-to-end transportation of data and setups up a logical connection between the hosts.
TCP is a connection-oriented and reliable protocol that uses windowing to control the flow and provides ordered delivery of the data in segments. On the other hand, UDP simply transfers the data without the bells and whistles. Though these two protocols are different in many ways, they do the same function of transferring data and they use a concept called port numbers to do this.
Internet Layer of TCP IP model
Once TCP and UDP have segmented the data and have added their headers, they send the segment down to the Network layer. The destination host may reside in a different network far from the host divided by multiple routers. It is the task of the Internet Layer to make sure that the segment moved across the networks to the destination network
Internet layer pack data into data packets known as IP datagram, which contains source and destination address (logical address or IP address) information that forwards the datagram between hosts and across networks. The Internet layer is also responsible for the routing of IP datagram.
Packet switching network depends on a connectionless internetwork layer. This layer is known as the Internet layer. Its job is to allow hosts to insert packets into any network and have them to deliver independently to the destination. At the destination side data; packets may seem in a different order than they were sent. It is the job of the higher layers to rearrange them to deliver them to proper network applications operating at the Application layer.
The main protocols included at Internet layer are IP (Internet Protocol), ICMP (Internet Control Message Protocol), ARP (Address Resolution Protocol), RARP (Reverse Address Resolution Protocol) and IGMP (Internet Group Management Protocol).
The Internet layer of the TCP/IP model corresponds to the Network layer of the OSI reference model in function. It provides logical addressing, path determination, and forwarding.
Network Access Layer of TCP IP model
The Network Access Layer is the first layer of the TCP/IP model. Network Access Layer defines details of how data is physically sent through the network. It is also including how bits are electrically or optically signalled by hardware devices that interface directly with a network medium, such as coaxial cable, optical fiber, or twisted pair copper wire. The important protocols of Network Access Layer are Ethernet, Token Ring, FDDI, X.25 also Frame Relay.
LAN architecture is the most popular among those listed above is Ethernet. Ethernet uses an Access Method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection) to access the media when Ethernet operates in a shared media.
IN CSMA/CD Access Method, every host has equal access to the medium and can place data on the wire when the wire is free from traffic or in the idle position. When a host wants to place data on the wire. It will check the wire to find out whether another host is already using the medium.
If there is traffic already in the medium, the host will wait and if there is no traffic, it will place the data in the medium. But, if two systems place data on the medium at the same instance they will collide with each other, destroying the data. If the data destroyed during transmission. This data will need to re-transmit. After the collision, each host will wait for a small interval of time and again the data will re-transmit.
In the previous article, we have learned various layers of the OSI reference model. We have discussed the function of each layer and relationship with other layers as well as with remote end. For example, the session layer at the source will interact with the session layer of the destination. For this interaction, each layer adds a header in front of the data from the earlier layer. This header contains control information related to the protocol uses at that layer. This process is the encapsulation Process. The steps of the encapsulation process are:
Upper layers (Application layer, Presentation layer and Session layer) convert the message to data and send it to the Transport layer which is the heart of the OSI Model.
The Transport layer converts the data to segments and sends it to the Network layer.
The Network layer converts the segments to packets and sends these packets to the Data Link layer.
The Data Link layer converts the packets to frames and then sends frames to the Physical layer.
The Physical layer converts the frames to binary 1’s and 0’s in the shape of electrical or light signals and sends them across the network.
The figure below illustrates the data encapsulation process at each layer; where header information is added.
Data De-Encapsulation Process
At the receiving end, the process is reversed, with headers being stripped off at each layer. This reverse process is known as de-encapsulation. As shown in the figure below when Layer 1 takes the data and sends it to Layer 2. Here the Layer 2 header, as well as the trailer, examined and removed.
The resulting Layer 3 PDU, then sent to Layer 3. Layer 3, in turn, examines the header in the PDU and removes it. The resulting PDU sent to Layer 4. Similarly, each layer removes the header added by the corresponding layer at the source before sending the data to the upper layer. Finally, the Application layer removes the Layer 7 header and sends the data to the application.
As the importance of computers grows, vendors recognized the need for networking them. They produced a variety of protocols whose specifications not made public. Hence each vendor had different ways of networking computers and these ways were not compatible with each other. This means that computers of one vendor could not networked with another vendor’s computers. Slowly these specifications public and some inter-vendor compatibility created but this still represented too many complications. To resolve this compatibility issues the OSI model was introduced.
The OSI model has created to support communication between devices of various vendors. It also promotes communication between disparate hosts such as hosts using different operating platforms. Keep in mind that you are very unlikely to ever work on a system that uses protocols conforming to the OSI model. But it is essential to know the model and its terminology because other models such as the TCP/IP model often compared to the OSI model. Hence the discussion on this model compared to the discussion on the TCP/IP model.
The OSI model, like most other network models, divides the functions, protocols; and devices of a network into various layers. The OSI model has seven such layers that divided into two groups. The upper layers (Layers 7, 6 and 5) define how applications interact with the host interface, with each other, and the user. The lower four layers (Layers 4, 3, 2 and 1) define how data transmitted between hosts in a network. The figure below illustrates the seven layers and a summary of their functions.
The layered approach provides many benefits, some of which are:
Communication is divided into smaller and simpler components
Since it is a layered approach, the vendors write to a common input and output specification per layer. The guts of their products function in between the input and output code of that layer.
Changes in one layer do not affect other layers. Hence development in one layer is not bound by limitations of other layers. For example, wireless technologies are new but old applications run seamlessly over them without any changes.
It is easier to normalize functions when they divided into smaller parts like this.
It allows various types of hardware and software, both new and old to communicate with each other seamlessly
The following section describes the 7 layers in detail.
This Layer provides an interface between the software application on a system and the network. Remember that this layer does not include the application itself, but provides services that an application requires. One of the easiest ways to understand this layer’s function is to look at how a Web Browser such as Internet Explorer or Firefox works is the application.
When it needs to fetch a web page, it uses the HTTP protocol to send the request and receive the page contents. This protocol resides at the application layer used by an application such as IE or FF to get web pages from web servers across the network. On the other side, the web server application such as Apache or IIS interacts with the HTTP protocol on the Application layer to receive the HTTP request and send the response back. Read More
This layer presents data to the Application layer. The Presentation Layer is also responsible for data translation and encoding. It will take the data from the Application layer and translate it into a generic format for transfer across the network. At the receiving end, the Presentation layer takes in generically formatted data and translates into the format recognized by the Application layer. An example of this is a JPEG to ASCII translation. The OSI model has protocol standards that define how data should be formatted. This layer is also involved in data compression, decompression, encryption, and decryption.
In a host, different applications or even different instances of the same application might request data from across the network. It is the Sessions layer’s responsibility to keep the data from each session separate. It is responsible for setting up, managing and tearing down sessions. its also provides dialogue control and coördinates communication between the systems.
Where the upper layers related to applications and data within the host, the transport layer is concerned with the real end-to-end transfer of the data across the network. This layer establishes a logical connection between the two communicating hosts and provides reliable or unreliable data delivery and can give flow control and error recovery. Although not developed under the OSI Model and not strictly conforming to the OSI definition of the Transport Layer, typical examples of Layer 4 are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). Read More
To best understand what the Network layer does, consider what happens when you write a letter and use the postal service to send the letter. You put the letter in an envelope and write the destination address as well as your own address so that an undelivered letter back to you.
In network terms, this address is a logical address and is unique in the network. Each host has a logical address. When the post office receives this letter. it has to find out the best path for this letter to reach the destination. Similarly, in a network, a router needs to decide the best path to a destination address.
This path determination. Finally, the post office sends the letter out the best path and it moves from the post office to post office before finally delivered to the destination address. Similarly, data moved across network mainly by routers before being finally delivered to the destination.
All these three functions – logical addressing, path determination, and forwarding – done at the Network Layer. Two types of protocols used for these functions – routed protocols for logical addressing and forwarding while routing protocols for path determinations.
There are many routed protocols and routing protocols available. Some of the common ones are discussed in great detail later in the book. Routers function in this layer. Remember that routers only care about the destination network. They do not care about the destination host itself. The task of delivery to the destination host lies on the Data Link Layer. Read More
Data Link Layer
The Network layer deals with data moving across networks using logical addresses. On the other hand, the Data Link layer deals with data moving within a local network using physical addresses. Each host has a logical address and a physical address. The physical address is only locally significant and is not used beyond the network boundaries (across a router).
This layer also defines protocols that send and receive data across the media. You will remember from earlier in the chapter that only a single host can send data at a time in a collision domain or else packets will collide and cause a host to back off for some time. The Data Link layer determines when the media is ready for the host to send the data and detects collisions and other errors in received data. Switches function in this layer. Read More
This layer deals with the physical transmission medium itself. It activates, maintains and deactivates the physical link between systems (host and switch for example). This is where the connectors, pinouts, cables, electrical currents defined. Essentially this layer puts the data on the physical media as bits and receives it in the same way. Hubs work at this layer. Read More
We can characterize types of networks with the size of the area covered, the number of users connected, number and types of services available and area of responsibility. The most important and famous types of network are:
Local Area Network (LAN)
It is the first and most important in the types of network. It provides access to users and end devices in a small geographical area such as a home network, or small business networks and a network in the same building. Following are the key points for LANs:-
A limited geographic area such as home, office, and building.
Allow multi-access to high bandwidth media.
Full-time connectivity to local users.
Control the network under local administration privately.
Connected physically to adjacent devices.
Metropolitan Area Network (MAN)
Metropolitan Area Network (MAN) is second in the types of a network which spans a physical area larger than a LAN but smaller than a WAN (e.g., a city). MANs are typically operated by a single entity such as a large organization. MAN provides a link to the internet in the long run. A MAN usually incorporates a number of LANs to form a network. This large network MAN’s backbone comprises of an optical fiber set-up. It is a hybrid network between a LAN and a WAN. It also connects two or more LANs in the same geographic area.
A MAN might connect two different buildings or offices in the same city. However, whereas WANs typically provide low to medium-speed access, MANs provide high-speed connections, such as T1 (1.544Mbps) and optical services. The optical services also provided SONET (the Synchronous Optical Network standard) and SDH (the Synchronous Digital Hierarchy standard). With these services, carriers can provide high-speed services, including ATM and Gigabit Ethernet. Devices used to provide connections for MANs include high-end routers, ATM switches, and optical switches.
Wide Area Network (WAN)
It is the 3rd in the types of network providing access to other networks over a wide geographical area such as across cities, states and countries called Wide Area Network (WAN), which is typically owned and managed by a telecommunications service provider.
The WAN is opposite of the personal area networks (PANs), local area networks (LANs), campus area networks (CANs), or metropolitan area networks (MANs) which are usually limited to its area of responsibility. The easiest way to understand what a WAN is to think of the internet as a whole, which is the world’s largest WAN. The internet is a WAN because, through the use of ISPs, it connects lots of smaller local area networks (LANs) or metro area networks (MANs).
On a smaller scale, a business may have a WAN that’s comprised of cloud services, its headquarters, and smaller branch offices. The WAN, in this case, would be used to connect all of those sections of the business together. Computers connected to a wide-area network often connected through public networks, such as the telephone system. We can also connect them through leased lines or satellites. WAN gives access through a serial interface which is generally slow. Its provide full-time and part-time connectivity.
This type of network is similar to a Local Area Network but wirelessly interconnects users and endpoints in a small geographical area. Wireless access points, a network of wi-fi routers are the examples of WLAN.
Storage Area Network (SAN)
A network infrastructure designed to support file servers and furthermore its provide data storage, retrieval, and replication.
Personal Area Network (PAN)
A Personal Area Network (PAN) is a computer network for communication between computer devices, including telephones and personal digital assistants, in proximity to an individual’s body. The devices may or may not belong to the person in question. The reach of a PAN is typically a few meters.
Before starting the Cisco Internetworking, the network introduction is important for the student of networking to be aware of “what is network” and “what is network importance themselves”.
So, first of all, what is network?
In simple words, It is a collection of interconnected devices (such as computers, printers, etc.) in such a way that they can communicate with each other. To better understand, let us look at the example, how things worked before networks. For this, suppose a large international company that sells ABC products at a time when networks did not exist.
Let us call this company XYZ Inc. to see in your mind’s eye the amount of information such as sales, inventory, account, etc. required by the management of the company to make everyday decisions. To get this information they will need to call their local offices. Their local offices will need postal mail or faxing from your email for printed reports or even send media (floppies!) through the postal service. This is a long and time taken process. This job also increases chance error since large numbers of reports are manually processed. This is just one part. You also need reflecting on the information required by the local offices. They also need various data from the head office and other offices around the world.
Now think the same company, but in the present time with all their offices interrelated through the networks. They would use a single application around the world that takes advantage of their global networks. The data from all offices would be instantly stored at the central site, the administration team can see data from around the world in any format. This data would also be real-time. This means that they see it as it’s happening. After centralizing the data, any site office can see data of any location.
The cost, time and effort involved in transferring data were much higher without networks. So networks decrease cost, time and effort and thereby increase output. They also help in resource optimization by helping to share resources.
Now you are familiar with how beneficial networks are, it’s time to look at how networks work. The figure below shows the most basic form of a network. This figure shows two hosts directly connected to each other using a networking cable. Today every host has a NIC for connectivity.
One end of the cable connects to the NIC on a host A and another end to host B. At this stage do not worry about cables and how the hosts communicate across the network. We will discuss this in detail later in the chapter. At this stage, it is important to understand how hosts connect to a network.
In Figure below, the hosts are “networked” and can send information to each other. This network is successful, but not scalable. If you have more than 2 hosts to this “network”, it will not work without a separate NIC card for each connection and that is not scalable or realistic. For more than 2 hosts to be networked, you need a device such as a hub. The figure below shows three hosts connected to a hub.
In the figure above the hub will relay any information received from Host-A to both Host-B and C. This means that all the three hosts can communicate with each other. When network hosts connect using a hub, there are two problems that arise:
A hub repeats information received from one host to all the other hosts. To understand this, consider Host-A in the above network sending a unicast message to Host-B. When the hub receives this message; it will relay the message to both Host-B and Host-C.
A hub creates a shared networking medium where only a single host can send packets at a time. If another host attempts to send packets at the same time, a collision will occur. Then each device will need to resend their packets and hope not to have a collision again. This shared network medium is a single collision domain.
The problems related to hubs can slow the process. To overcome these, use switches instead of a hub. Similar to hubs, switches connect hosts to one another but switches break up collision domain by providing a single collision domain for each port. This means that every host gets its own collision domain thereby eliminating the collisions in the network. With switches, each host can send data anytime. Switches simply “switch” the data from one port to another in the switched network. Also, unlike hubs, switches do not flood every packet out all ports. They switch a unicast packet to the port where the destination host resides. They only flood out a broadcast packet. The figure below shows a switched network.
Remember that each host in Figure above is in its own collision domain and if Host-A sends a packet to Host-C, Host-B will not receive it. Communication between hosts connected to switch is three types:
Unicast – Communication from one host to another host only.
Broadcast – Communication from one host to all the hosts in the network.
Multicast– Communication from one host to a few hosts only
The figure below shows a network See if you can figure out how many collision domains exist in the network.
If you answered 6 then you are absolutely correct since each port of the Switches represents a single collision domain. If you answered more than 5 then you need to remember that a hub does not break collision domains.
Now that you know how a switch works, consider the one problem associated with a switched network. Earlier, you learned that hubs flood out all packets, even the unicast ones. A switch does not flood out unicast packets but it does flood out a broadcast packet. All hosts connected to a switched network are said to be in the same broadcast domain. All hosts connected to it will receive any broadcast sent out in this domain.
While broadcasts are useful and essential for network operations, in a large switched network too many broadcasts will slow down the networking process. To remedy this situation, networks are broken into smaller sizes and these separate sub-networks are interconnected using routers. Routers do not allow broadcasts to be transmitted across different networks it interconnects and hence effectively breaks up a broadcast domain.
In the network shown in Figure below, broadcasts from hosts connected to one switch will not reach to hosts connected to another switch. This is because the router will drop the broadcast on its receiving interface.
In addition to breaking up broadcast domains, routers also do the following essential functions in any network:
At the barest least, routers are like switches because they essentially switch packets between networks.
Routers can talk to each other to learn about all the networks connected to various routers and then select the best path to reach a network.
Routers can drop or send packets based on certain criteria like their source and destination. This is also discussed in detail later in the book.