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Networking - Physical Characteristics of a Network, Network Architectures, Network Protocols and Transport

Physical Characteristics of a Network
Transmission Media
Transmission media are actually located below the physical layer and directly controlled by the physical layer.
We can say that transmission media belong to layer zero.
Figure shows the position of transmission media in relation to the physical layer.
Computers and other telecommunication devices use signals to represent data. These signals are transmitted from one device to another in the form of electromagnetic energy. Which is propagated through transmission media.
Electromagnetic energy, a combination of electric and magnetic fields vibrating in relation to each other includes power, radio waves, infrared light, visible light, ultra-violet light, and X, gamma and cosmic rays. Each of this constitutes a portion of the electromagnetic spectrum.
Not all portions of the spectrum are currently usable for telecommunications; however.
The media to harness those that are usable are also limited to a few types.
For Telecommunications, transmission media can be divided into two broad categories: guided and unguided. Guided media include twisted pair cable, co-axial cable and fiber-optic cable. Unguided medium is usually air.
Guided Media-
Guided media, which are those that provide a physical medium or path from one device to another, there are three commonly used types of physical media:
1. Twisted pair cable
2. co-axial cable
3. fiber-optic cable
Cable characteristics include susceptibility to electrical interference, flexibility, ease of installation, range of signal transmission and transmission rates.
Transmission rates that can se supported on each of these cable types are measured in millions of bits per second (Mbps). Current cabling media transmission rates can vary from 9 to 90 Mbps and beyond.
Twisted-pair cable -
Twisted pair cable consists of two insulated strands of copper wire twisted together. A number of twisted pair wires are grouped together and enclosed in a protective sheath to form a cable.
One of the two strands is used to carry signals to the receiver and the other one is used only as a ground reference. The receiver uses the difference between the two levels.
If the two wires are parallel, the effect of the unwanted signals is not the same in both wires because they are at different locations relative to the noise or crosstalk sources (e.g. One closer and one farther).
This results in a difference at the receiver. By twisting the pairs, a balance is maintained.
For example suppose in one twist, one wire is closer to the noise source and the other farther. In the next twist, the reverse is true.
This calculates a zero difference between the two and receiver receives no unwanted signals.
This cabling medium is susceptible to electrical interference and can only carry a signal for 90 meters.
Co-axial cable -
Co-axial (coax) cable is a conductive center wire surrounded by an insulating layer, a layer of wire mesh (shielding) and a non-conductive outer layer.
The most popular coaxial cable for networking applications are commonly called Thinnet and Thicknet.
The outer wire mesh serves both as a shield against noise and as the second conductor, which completes the circuit.
This outer conductor is also enclosed in an insulating sheath, and a plastic cover protects the whole cable.
Coaxial cable is resistant to interference and signal weakening and is generally better than twisted pair cable for longer distances.
Fiber-optic cable -
Optical fibers are used to carry digital signals in the form of modulated pulses of light.
An optical fiber consists of an extremely thin cylinder of glass, called the core, surrounded by a concentric layer of glass, Known as the cladding.
There are two fibers per cable - one to transmit and one to receive.
"An optical fiber is a thin, transparent and flexible that consists of a core surrounded by cladding."
The core and the cladding of an optical fiber are made fron the same material (Silica) and they differ only in their refractive indexes.
The difference in refractive indexes can be achieved by doping silica with different dopants.
We applied a third layer of coating over the cladding to protect the entire structure. The coating material is different from the core and cladding material
Advantages of fiber optics over other cables
. Higher Bandwidth
The higher the carrier's frequency, the greater the channel bandwidth and the higher the information carrying capacity. As optical fiber uses light as the signal carrier and its frequency is very high (in between 914 to 915 Hz.).
. Small Size and Weight
Its thickness is same as the thickness of human's hair. And its weight is too much small than corresponding copper cable.
. Immunity to interference and crosstalk
Optical fiber form a dielectric waveguide and are therefore free from electromagnetic interference and radiofrequency interference and crosstalk is negligible.
. Low transmission loss
The use of optical fiber cable provide a low attenuation or transmission loss than the perfect copper conductor wire. This feature is the main advantage of optical fiber communication.
Unguided Media: Wireless -
Unguided media transport electromagnetic waves without using a physical conductor, this type if Communication is often referred to as wireless communication.
Signals are normally broadcast through air and thus are available to anyone who has a device capable of receiving them.
Unguided signals can travel from the source to destination in several ways.
Radio Waves -
Although there is no clear-cut demarcation between radio waves and microwaves, electromagnetic waves ranging in frequencies between 3 KHz and 1 GHz are normally called radio waves.
Radio waves, for the most part, are omni directional. When an antenna transmits radio waves, they are propagated in all directions.
This means that the sending and receiving antennas do not have to be aligned.
A sending antenna can send waves that can be received by any receiving antenna. The omni directional property has a disadvantage, too.
The radio waves transmitted by one antenna are susceptible to interference by another antenna that may send signals using the same frequency or band.
Radio waves, particularly those waves that propagate in the sky mode, can travel long distances.
This makes radio waves a good candidate for long-distance broadcasting such as AM radio.
Microwaves -
Electromagnetic waves having frequencies between 1 and 300 GHz are called microwaves.
Microwaves are unidirectional. When an antenna transmits microwave waves, they can be narrowly focused. This means that the sending and receiving antennas need to be aligned.
The unidirectional property has an obvious advantage. A pair of antenna can be aligned without interfering with another pair of antennas.
Microwave propagation is line-of-sight. Since the towers with the mounted antennas need to be in direct sight of each other, towers that are far apart need to be very tall.
The curvatures of the earth as well as other blocking obstacles do not allow two short towers to communicate using microwaves.
Repeaters are often needed for long distance communication.
Infrared -
Infrared signals, with frequencies from 300 GHz to 400 THz (wavelengths from 1 mm to 770 nm), can be used for short-range communication.
Infrared signals, having high frequencies, cannot penetrate walls.
This advantageous characteristic prevents interference between one system and another; a short -range communication system in one room cannot be affected by another system in the next room.
When we use our infrared remote control, we do not interfere with the use of the remote by our neighbors.
However, this same characteristic makes infrared signals useless for long-range communication.
In addition, we cannot use infrared waves outside a building because the sun's rays contain infrared waves that can interfere with the communication.
Transmission Techniques
Baseband transmission -
To transmit encoded signals over cable, we mainly use two techniques such as Baseband transmission and broadband transmission.
Baseband systems use digital signaling over a single frequency. Signals flow in the form of discrete pulses of electricity or light.
As the signal propagates along the cable, it gradually decreases in strength and can become distorted.
To overcome this problem Baseband system sometime s use repeaters.
Repeaters receive an incoming signal and retransmit it at its original strength and definition. Thereby increasing the practical length of a cable.
Broadband Transmission -
Broadband systems use analog signaling and a range of frequencies.
With analog transmission the signals employed are continuous and non discrete.
Signals travel across the physical medium in the form of electromagnetic or optical waves.
To regenerate analog signals broadband systems use amplifiers.
Because the signal flows in broadband transmission is unidirectional, there must be two paths for data flow.
The two common ways to accomplish this are
i) Mid-split broadband configuration divided the bandwidth into two channels; one is used for transmitting signals, the other is for receiving.
ii) In dual cable broadband configuration, each device is attached to two cables. One is used to send and the other to receive.
Topology is a way for arranging a number of systems such that they provide a better communication and response among each other.
Bus Topology -
A bus topology connects each computer to a signal cable segment.
At each end of the cable there is a terminator. On a bus network, if the connection to one station comes loose, or if a cable breaks, the entire cable segment loses its connectivity.
Local bus network
A regular bus network uses drop cables and external transceivers to connect each station to the main "backbone" cable.
Drop cables are connected to AUI connectors on the network adapter at one end, and the other end is connected to an external transceiver that attaches to the main backbone cable.
Regular bus network
Star topology -
A special unit called a hub is used in a network using a star configuration. The hub provides a common connection so that all of the workstations can communicate with one another.
By using a hub you can centralize network management. However if the hub fails, the network fails.
A star topology uses signal splitters in the hub to send out signals in different directions on the cable connections.
Both active and passive hub can send a stronger signal to feed a longer cable and more signal splitters.
Ring Topology -
On a ring network, stations are situated on a continuous network ring on which a token is passed from one station to the next.
A station must wait for a free token in order to transmit data.
When there is a cable failure on a ring network only a small number of stations are affected.
Additional station can be added to a ring network without a server drop in performance.
There are three primary cabling media that can be used with a LAN at the physical layer; Twisted pair cable, Coaxial cable and Fiber-optic cable.
Twisted pair cable consists of two insulated strands of copper wire twisted together.
Coaxial cable is a conducting center wire surrounded by an insulating layer, a layer of wire mesh and a non-conducting outer layer.
Two popular types of Coaxial cable are Thinnet and Thicknet. Optical fibers are used to carry digital data signals in the form of modulated pulses of light.
Encoded signals can be transmitted over cable two ways, broadband and Baseband transmission.
Broadband transmissions use analog signaling and Baseband transmissions use digital signaling.
Three widely used LAN topologies are bus, star and ring topologies.
A bus topology connects each computer to a single cable. In a star topology, each station is connected to a special unit called a hub.
On a ring network, stations are situated on a continuous network loop.
Network Architectures
Media Access Control
Media Access Control tends to the job of controlling access to the cable. When a data frame is ready to be transmit to other station, it is sent one bit at a time.
A signal representing the first bit, for example a 1 bit, is imposed on the cable the transmission of the 1 bit takes one bit time. The signal for the next bit, for example a 0 bit, is then imposed on the cable and then received and stored by the stations.
This process continues until all bits of the data frame have been sent. A signal could be either a voltage change in the case of twisted pair or co-axial cable or a light pulse on a fiber-optic cable.
A 1 bit must be clearly distinguishable from a 0 bit overlapping signals result in garbled data, so there needs to be a method of sharing the cable to prevent such errors and collisions.
The two most popular methods are carrier Sense Multiple Access with Collision Detection (CSMA/CD) and Token Passing.
Carrier Sense Multiple Access with Collision Detection
Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is a type of access method generally used with bus topologies.
Carrier Sense
Using CSMA/CD, a station listens to the cable to determine whether or not another station is currently transmitting a data frame.
If the medium is quiet, that is, if no other station is sending, the station sends its data.
Multiple access
Multiple access means that when a data frame is transmitted, it is sent to all stations on the network. As the data frame arrives, each receiving station check the attached destination address.
If the address applies to the station, the station receives and processes the data, otherwise pass it to the next.
Collision Detection
A collision of data results when two stations detect a free cable and begin simultaneously transmitting data.
To reduce the likelihood of another collision, each station generates a random number to determine how ling to wait before retransmitting.
After the appropriate period of time, each station tests the cable to see if it is able to sending data or not.
Token passing
With networks that employ a ring topology, the most common media access control method is token passing.
The sending station adds data to the token, along with its address and the address of the recipient.
The token is then passed around the ring, so that each station can check the token's destination address.
The token continuous around the ring until it reaches the address specified by the original sender. When the recipient has copied the information from the token, it returns the token to the originating station to verify that the data was received.
The original sender that passes the token to the next station on the ring, so that the station can send information over the network.
Comman LAN Architectures
Common LAN Architectures, by the term we want to state the overall design of a LAN, included in the LAN architecture is the media access method and the physical components.
The three most common LAN architectures are Ethernet, Token ring and Arcnet.
Ethernet, the basis for the institute of Electrical and Electronic Engineers Inc(IEEE) standard 802.3, coas developed by Xerox, Digital Equipment Corporation , and Intel Ethernet network topologies are most commonly distinguished by the cable used.
The three cabling possibilities are Twisted pair, Thinnet and Thicknet.
Token Ring
A token ring network is an implementation of IEEE standard 802.5, the standard for token ring LAN's.
The token passing access method, more than the physical cable layout, distinguishes token ring networks from other networks.
The Attached Resource Computer network (ArcNet) loosely maps to IEEE's standard 802.4.
This specifies the standards for token passing bus networks using broadband cable.
ArcNet, however is a baseband network and can have a star or bus topology.
Originally known as Alto Aloha Network, Ethernet is a widely used local-area network (LAN) protocol originally created by Xerox PARC in 1973 by Robert Metcalfe and others (U.S. Patent # 4,063,220).
Being the first network to provide Carrier Sense Multiple Access / Collision Detection (CSMA/CD), Ethernet is a fast and reliable network solution that is still widely used today.
Below is a listing of different standards of Ethernet and additional information about each of them.
Ethernet II / DIX / 802.3
Ethernet II is a revised version of Ethernet rewritten by with Digital Equipment Corp, Intel and Xerox. Ethernet II, also known as DIX, (Digital, Intel, and Xerox) and 802.3.
Fast Ethernet / 100BASE-T / 802.3u
Fast Ethernet is also referred to as 100BASE-T or 802.3u and is a communications protocol that enables computers on a local-area network to share information with one another at rates of 100 million bits per second instead of the standard 10 million BPS.
Fast Ethernet works over Category 5 twisted-pair wiring.
There are two available types of 100BASE-T standards.
The first standard known as 100BASE-T utilizes CSMA/CD.
The second standard, known as 100VG-AnyLAN or 802.12, is similar to the other standard; however, it utilizes a different type of Ethernet frame to send its data.
100BASE-T is available in three different types of cable technologies:
1. 100BASE-T4 =
Utilizes four pairs of telephone-grade twisted-pair wire and is used for networks that need a low-quality twisted-pair on a 100-Mbps Ethernet.
2. 100BASE-TX =
Developed by ANSI 100BASE-TX is also known as 100BASE-X, 100BASE-TX uses two wire data grade twisted-pair wire
3. 100BASE-FX =
Developed by ANSI, 100BASE-FX utilizes 2 stands of fiber cable.
Ethernet SNAP
Ethernet SNAP is short for Ethernet SubNetwork Access Protocol and is a type of Ethernet protocol that enabled old and new protocols to be encapsulated in a Type 1 LLC.
Gigabit Ethernet / 1000BASE-T / 802.3z / 802.ab
Gigabit Ethernet is also known as 1000BASE-T or 802.3z / 802.3ab is a later Ethernet technology that utilizes all four copper wires in a Category 5 (Cat 5 & Cat 5e) capable of transferring 1 Gbps
10 Gigabit Ethernet / 802.3ae
10 Gigabit Ethernet is also known as 802.3ae is a new standard that supports 10.000 Gb/s.
A copper cable gigabit Ethernet standard that is no longer used. This standard has been replaced by 1000BASE-T.
A fiber optic gigabit Ethernet standard that operates over single-mode fiber.
A fiber optic gigabit Ethernet standard that operates over multi-mode fiber, with typical distances of up to 550 meters (1804 feet)
Ethernet adapter
Ethernet adapter is a term used to describe an Ethernet network card used to connect a desktop computer to a network. If you are looking for network adapter
Ethernet: Thinnet (9 Base 2)
This topology is called Thinnet 9 Base 2.
The nickname derives from the size of the cable, which is roughly the size of a garden hose and too stiff to band with your hands.
A thin coaxial cable can transmit upto 9 Mbps roughly 200 meters over a baseband wire.
Thinnet networks generally use a local bus topology.
The cable used for this type of network is relatively inexpensive, and easy to install and configure.
As a result, this network is an economical way to support a small department or workgroup.
Ethernet: Thicknet (9 Base 5)
The main specification of the Ethernet topology commonly known as Thicknet 9 Base 5 are 9 Mbps, baseband and 500 meter segments.
Thicknet network generally use a bus topology. Thicknet was designed to support a backbone for a large department, or even an entire building.
Ethernet: Twisted-pair (9 Base T)
Twisted-pair 9 Base T, (9 Mbps, baseband, over-twisted-pair cable), is an Ethernet LAN that uses unshielded twisted pair cable to connect stations.
Most networks of this type are configured in a star pattern but internally use a bus signaling system like other Ethernet configurations.
Token Ring
Data on a token ring network is transmitted at either 4 or 16 Mbps, depending on the cable.
Network computers are connected by shielded and unshielded twisted pair cable to a wiring concentrator.
Each computer can be up to 90 meters from the multistation access unit (MAU) using shielded wire, or 45 meters using unshielded wire.
ArcNet is an easy to install and inexpensive baseband network that can have a star or a bus topology.
It typically uses co-axial cable and includes both active and passive hubs. Each work station is connected by cable to a hub.
The maximum cable length is from 120 meters to 606 meters depending on the type of cable and hub used.
LAN Architectures refers to the overall design of a LAN.
Two major components of LAN Architecture are the media access control and LAN topology.
Media access methods dictate how a station gains access to the physical medium.
The two primary methods for LAN's are CSMA/CD and Token passing. CSMA/CD is found on Ethernet LAN while Token Passing LANs rely on token passing.
Three common types of LAN architecture are Ethernet, Token passing and ArcNet. Ethernet networks, closely aligned with IEEE standard 802.3 are commonly distinguished by their cable; Thinnet, Thicknet, and Twisted-pair cable.
A token ring network is an implementation of IEEE 802.5, the standard for the token ring networks.
ArcNet was designed as a token passing bus architecture mapping loosely to the IEEE 802.4 standard.
This specifies the standards for token passing bus networks using broadband cable. ArcNet however is a baseband network and can have a star or bus topology.
Network Protocols and Transport
The Media Access Control Driver
The MAC driver (Media Access Control Driver)
The second layer of OSI (Data-link layer) is divided into two sublayers by the Institute of Electrical and Electronic Engineers Inc (IEEE) project 802-MAC and LLC sublayers.
A MAC driver is the device driver located at the MAC sublayer.
A MAC driver provides low level access to network adapters by providing data transmission support and some basic adapter management functions.
These drivers also pass data from the physical layer to network protocols at the network and transport layers
Protocol Drivers
A protocol is a set of rules that governs communications between two entities (Stations).
A protocol defines what is communicated how it is communicated.
The key elements of a protocol are syntax, semantics, and timing.
Syntax refers to the structure or format of the data, meaning the order in which they are presented.
Semantics shows the meaning of each section of bits. How is a particular pattern to be interpreted and what action is to be taken based on that interpretation.
Timing refers to two characteristics: when data should be sent.
For example if a sender produces data at 90 Mbps but the receiver can process data at only 1 Mbps, the transmission will overload the receiver and data will be largely lost.
Responsibilities of a protocol driver
The protocol driver is responsible for offering four or five basic services to other layers in the network, while hiding the details of how the service is actually implemented.
The services the protocol driver performs include session management, datagram service data segmentation and sequencing, acknowledgement and possibly routing across a wide area network (WAN).
Session Management
Session Management involves establishing, maintaining and terminating connections between stations on the network in order to transfer data.
A session is analogous to a telephone call.
The protocol drivers on the computer wanting to communication negotiate the creation of connections.
One station sends a message requesting a connection.
The second station checks whether it has the necessary resources to support another connection.
If it does, it configures itself and sends a unique identifier that the first station uses to send data through the newly established connection.
Data Gram Services
A datagram service is just like a basic messaging service. Unlike the complete service that session management provides, datagram data transfer services operate more like mailing a letter.
Datagram services are connectionless, no association exists between the sender and the receiver before the message is sent, thus, the status of the receiver is unknown.
Datagrams are unreliable in that no acknowledgements are sent, and no guarantee is provided that a datagram will or had reached its recipient.
Although a datagram can be addressed to a specific station, its main benefit is its ability to be sent to many stations simultaneously.
Data Segmentation & Sequensing
Typically, the protocol driver can accept relatively large messages for a transmission, but there are strict frame-size imposed by lower layers. Consequently, the protocol driver must break up or segment the message into smaller units.
The smaller units are sent separately along with control information. Control information is attached in the form of header that is sent along with the frame.
This information enables the protocol on the other end to recognize message boundaries.
In order to deal with the potential problems of misordered data frames, duplicated data frames, and missing data frames, the header contains sequencing information.
This enables the protocol driver on the receiving end to get the pieces back together in the right order.
Acknowledgement is the process used to guarantee reliable end to end data delivery.
A protocol driver at one station guarantees reliable communications by requiring acknowledgement for every data frame received.
The protocol detects and recovers from errors by retransmitting nonacknowledged data frames and handling the possibility for duplicate data frame receipt.
After waiting a set amount of time without any acknowledgement the protocol simply retransmits a data frame.
During this time the original data frame may have disappeared or may simply be held up somewhere in the network duplicate frames result when both the original and the retransmitted data frames arrive at the destination.
The receiving station ignores the duplicate data frame.
Routing across a WAN requires the addition of a network address field to each frame. Intermediate systems Use the network address like an area code to route the frame to the correct destination LAN.
The main devices used to provide wide area networking are; a bridge, a router, a brouter and a gateway.
Each LAN usually has one machine that acts as the entry point into the subnet upon receipt of a data frame; the network address is examined to see if it is one of the networks it is directly connected to.
if it is not , the frame is forwarded to another intermediate system in the subnet.
Comon Network Protocols
Upto this point, protocols have been discussed generically. This section describes some of the more popular network protocols.
NetBEUI is a small, fast and efficient protocol that is supplied with all Microsoft network products.
Advantages include its small stack size, its speed of data transfer on the network medium and its compatibility with all Microsoft based network.
The major disadvantage of NetBEUI is that it does not support routing.
NetBEUI is a simple network layer transport protocol that was developed to support NetBIOS networks. Like NetBIOS, NetBEUI is not routable, so it really has no place on an enterprise network.
NetBEUI is the fastest transport protocol available to NT. It’s great for fast transmission, but is not usable across routed networks.
Benefits of NetBEUI include:
1. Fast speed.
2. Good error protection.
3. Ease of implementation.
4. And low memory overhead.
Some disadvantages are:
1. It’s not routable.
2. It has very little support for cross platform applications.
3. And it has very few troubleshooting tools available.
Transmission control protocol/Internet protocol (TCP/IP) is a suite of protocols that provide communications between dissimilar end systems.
Interoperability among many different types of computers is the primary advantage of TCP/IP. TCP/IP also has the advantage of bring the native protocol of the internet.
The two primary disadvantages are its large size and slower speed of data transfer. TCP/IP is the most widely used protocol suite in networking today.
This is due in part to the wide growth of the global Internet. TCP/IP is able to span wide areas and is very flexible.
It provides cross-platform support, routing capabilities, as well as support for the Simple Network Management Protocol (SNMP), the Dynamic Host Configuration Protocol (DHCP), the Windows Internet Name Service (WINS), the Domain Name Service (DNS), and a host of other useful protocols.
However, TCP/IP’s rich set of features are provided at the expense of additional overhead, which may make it too cumbersome for some networks or applications.
1. Communications between dissimilar end systems.
2. Can support the native protocol of the internet.
1. Large size
2. Slower speed of data transfer
Xerox network systems (XNS) was developed by Xerox for their Ethernet LANs. XNS is the basis for Novell’s IPX/SPX, but it is seldom found in today’s networks.
It is a large, slow protocol (like TCP/IP) but produces more broadcasts, causing more network traffic.
IPX/SPX is a suite of protocols similar to NetBEUI in that it is a relatively small and fast protocol on a LAN.
But unlike NetBEUI, it does support routing.
IPX/SPX is a derivative of XNS. IPX/SPX is normally used to connect to operate with Novell Netware.
There are other proprietary network protocols designed and developed specifically for their network systems. These are like
1. AppleTalk
3. X.25
4. High-level Data Link Control (HDLC)
A MAC driver provides low level access to network adapters by providing data transmission support and some basic adapter management functions.
The protocol driver is responsible for offering four or five basic services to other layers in the network. Hiding details of how the services is actually implemented.
The services that the protocol driver performs include session management, Datagram service, Data segmentation and sequencing, acknowledgement and possibly routing across a WAN.
Popular network protocols include NetBEUI, a small, fast and efficient protocol that is supplied with all Microsoft network products.
TCP/IP, an industry standard protocol providing communications between dissimilar end systems,
XNS, a precursor to IPX/SPX; and IPX/SPX, a protocol stack that is used in Novell networks.



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