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WEEK-VII

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An Overview of LAN

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INTRODUCTION  

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• There are three common network types according to their coverage size.
• These are, LANs, MANs and WANs.
• LAN is the smallest sized network in these three network type. LAN is a small network that is built within a small geographical area.
• A LAN can be your home network, your company’s network or any similar network that is consisting of mostly with switches and the hosts connected to them.
• A LAN can also consist of different office equipments like printers. A LAN can consist of just two computers or it can consist of hundreds of devices.

Fig.: A LAN

• According to size of LANs, various central devices can be used. The different devices are simple switches, hubs, access points or home routers.
• If the Local Area Network is a big LAN or an enterprise LAN, then one can use any one of the devices that has various size and capacity as central devices

Fig.: Devices in a LAN

• To build a Local Area Network, Ethernet technology is used.
• LANs are the simplest network topologies. People use this simple network for many reasons. Some of these reasons are given below :

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• Fast communication between different hosts

• Sharing data inside their home, company, etc.

• Providing a centralized controlled network

• To reach all devices users have over one network

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Features of LAN are :

• Network size is limited to a small geographical area, presently to a few kilometers.
• Data transfer rate is generally high. They range from 100 Mbps to 1000 Mbps.
• A LAN uses only one type of transmission medium, commonly category 5 coaxial cables.
• A LAN is distinguished from other networks by their topologies. The common topologies are bus, ring, mesh, and star.
• The number of computers connected to a LAN is usually restricted. In other words, LANs are limitedly scalable.
• IEEE 802.3 or Ethernet is the most common LAN.

• They use a wired medium in conjuncture with a switch or a hub.
• Originally, coaxial cables were used for communications. But now twisted pair cables and fiber optic cables are also used.
• Ethernet’s speed has increased from 2.9 Mbps to 400 Gbps.

Fig.: An Ethernet LAN

TYPICAL SOHO LANS

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• One of the most common local-area deployments is the Small-Office / Home-Office (SOHO) LAN.
• It is a small computer network built of one Ethernet switch, one router, and one wireless access point.
• The LAN uses Ethernet cables to connect different end-devices to one of the switch ports.

• Figure shows a diagram of a SOHO Ethernet LAN with one switch, one router, and one access point.
• Some of the end devices are connected to the access switch with Ethernet cables and some of the mobile devices are connected via wireless.
• The Access point acts as an Ethernet switch with the only difference that the clients are connected with radio waves instead of cables, using the IEEE 802.11 standards.
• Typical SOHO users primarily consume public services such an email and social media, so the traffic pattern is primarily from the Internet to the end clients. 
• Although in figure, the switch, router, and AP are shown as separate devices, many networking vendors combine them in one integrated network device specifically built for the SOHO LAN market.

Figure: SOHO LAN with a single integrated network device

• These types of devices, shown in figure are referred to as a "wireless router", but they combine 4-port Ethernet switch, wireless access point, IP router, and a firewall into an all-in-one device.
• These types of devices are easy to set up and ready to go after unboxing, but the downside is that they have lower performance and availability and most importantly, they don't scale and the enterprise-grade dedicated devices.

TYPICAL ENTERPRISE LANs

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• An enterprise network can integrate all systems, including Windows and Apple computers and operating systems (OS), UNIX Systems, mainframes and related devices like smartphones and tablets.
• A tightly integrated enterprise network effectively combines and uses different device and system communication protocols.
• The enterprise network is the backbone of any modern enterprise’s business operations.
• The sheer size and diversity of an enterprise network makes it very difficult and costly to build, maintain, administer and secure.
• The enterprise network is the infrastructure chiefly responsible for enabling business processes.

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• An enterprise network consists of the following :
• Endpoints (PCs, laptops, mobile devices, servers etc.)
• Network devices (repeaters, bridges, routers, switches, firewalls, storage, etc.)
• Communications protocols
• LANs (local area networks)
• WANs (wide area networks)
• Other components whose collective purpose is to enable communications and information exchange among an enterprise’s users, business units and trading partners.

Fig.: An Enterprise LAN

• Most enterprise networks consist of WANs that span vast geographical areas—many of them even reaching other continents.
• A single branch in the enterprise network can have hundreds or even hundreds of thousands of endpoints running different operating systems and applications.
• Each of these applications communicates through various protocols over a wide range of ports.
• Traditionally, an enterprise network’s collection of endpoints used to be made up of PCs, laptops and physical servers.
• These endpoints now operate alongside smartphones, tablets, virtual machines and a bevy of Internet of things (IoT) devices.

THE VARIETY OF ETHERNET

• A local area network (LAN) is a computer network that is designed for a limited geographic area such as a building or a campus.
• A LAN can be used as an isolated network to connect computers in an organization for the purpose of sharing resources.
• Some LANs are linked to a wide area network (WAN) or the Internet.
• All types of LANs used a media-access method to solve the problem of sharing the media.
• The Ethernet used the CSMA/CD approach. The Token Ring, Token Bus, and FDDI (Fiber Distribution Data Interface) used the token-passing approach.

• Another LAN technology, ATM LAN, which used the high speed WAN technology (ATM), appeared in the market.
• Almost every LAN except Ethernet has disappeared from the market place because Ethernet was able to update itself to meet the needs of the time.
• The Ethernet protocol was designed for higher transmission rates.
• IEEE (Institute of Electrical and Electronics Engineers) Project 802 - The IEEE standard of the Computer Society of the IEEE started a project, called Project 802, to set standards to enable inter communication among equipments from a variety of manufacturers.
• Project 802 will not replace any part of the OSI model or TCP/IP protocol suite, but it is a way of specifying functions of the physical layer and the data-link layer of major LAN protocols.

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Ethernet Evolution - The Ethernet LAN developed in the 1970s has gone through four generations:

• Standard Ethernet (10 Mbps)
• Fast Ethernet (100 Mbps)
• Gigabit Ethernet (1 Gbps)
• 10 Gigabit Ethernet (10 Gbps)

PHYSICAL LAYER STANDARDS

• The physical layer standards are implemented in hardware and are governed by many organizations.
• The protocols and operations of the upper OSI layers are performed by software and are designed by software engineers.
• The services and protocols in the TCP/IP suite are defined by the Internet Engineering Task Force (IETF).
• Similar to technologies associated with the Data Link layer, the Physical layer technologies are defined by organizations such as:

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1. The International Organization for Standardization (ISO)

2. The Institute of Electrical and Electronics Engineers (IEEE)

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3. The American National Standards Institute (ANSI)

4. The International Telecommunication Union (ITU)

5. The Electronics Industry Alliance/Telecommunications Industry

Association (EIA/TIA)

6. National telecommunications authorities such as the Federal

Communication Commission (FCC) in the USA and the European

Telecommunications Standards Institute (ESTI)

CONSISTENT BEHAVIOUR OVER ALL LINKS USING THE ETHERNET DATA-LINK LAYER

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• Ethernet includes many physical layer standards, Ethernet acts like a single LAN technology because it uses the same data link layer standard over all types of Ethernet physical links.
• That standard defines a common Ethernet header and trailer.
• It is already known that the header and trailer are bytes of overhead data that Ethernet uses to do its job of sending data over a LAN.
• No matter whether the data flows over a UTP cable, or any kind of fibre cable and no matter the speed, the data-link header and trailer use the same format.
• The Ethernet data-link protocols focus on sending an Ethernet frame, whereas the physical layer standards focus on sending bits over a cable.

BUILDING PHYSICAL ETHERNET NETWORKS WITH UTP

Fig.: Physical Ethernet Networks with UTP

• It is important to know the physical links between any two Ethernet nodes.
• Before the Ethernet network as an entire can send Ethernet frames between user devices, each node must be ready and ready to send data over an individual physical link.
• Ethernet sends data over these links. Here it is necessary to know the three most ordinarily used Ethernet standards:
• 10BASE-T (Ethernet)
• 100BASE-T (Fast Ethernet, or FE)
• 1000BASE-T (Gigabit Ethernet, or GE)
• An UTP cable is used for sending data in both directions over the cable. It then examines the specific wiring of the UTP cables used for 10-Mbps, 100-Mbps, and 1000-Mbps Ethernet.

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TRANSMITTING DATA USING TWISTED PAIRS

• Ethernet sends data over UTP cables, the physical means to send the data using electricity that flows over the wires inside the UTP cable.
• To better understand how Ethernet sends data using electricity, break the thought down into two parts:
• how to create an electrical circuit?
• how to make that electrical signal communicate 1s and 0s?
• First, to make one circuit, Ethernet defines how to use the two wires inside one twisted pair of wires, as shown in Figure.
• The figure does not show a UTP cable between two nodes, but shows two individual wires that are inside the UTP cable.

• An electrical circuit requires an entire loop, so the two nodes, using circuitry on their Ethernet ports, connect the wires in one pair to finish a loop, allowing electricity to flow.

Fig.: Creating one Electrical Circuit over One Pair to Send in One

Direction

BREAKING DOWN A UTP ETHERNET LINK

• Ethernet link refers to any physical cable between two Ethernet nodes.
• To learn about how a UTP Ethernet link works, it helps to break down the physical link into those basic pieces, as shown in Figure. - the cable itself, the connectors on the ends of the cable, and the matching ports on the devices into which the connectors are going to be inserted.

Fig.: Basic components of an Ethernet Link

UTP CABLING PINOUTS FOR 10 BASE-T AND 100BASE-T

• The 10BaseT and 100BaseTX ports use standard RJ-45 connectors.
• The 10BaseT and 100BaseTX ports have their transmit (TD) and receive (RD) pairs internally crossed.

Fig.: RJ-45 10BaseT and 100BaseTX Connector

STRAIGHT-THROUGH CABLE PINOUT

• Straight-Through refers to cables that have the pin assignments on each end of the cable.
• Pin 1 connector A goes to Pin 1 on connector B, Pin 2 to Pin 2, etc. Straight-Through wired cables are most commonly used to connect a host to a client.
• The Cat5e patch cables, the Straight-Through wired Cat5e patch cable is used to connect computers, printers, and other network client devices to the router switch or hub (the host device in this instance).

Fig.: Straight- Through Cable Pinout

CROSSOVER CABLE PINOUT

• Crossover wired cables, commonly called crossover cables are very much like Straight-Through cables with the exception that TX and RX lines are crossed, that is, they are at opposite positions on either end of the cable.
• For example, using the 568-B standard - Pin 1 on connector A goes to Pin 3 on connector B. Pin 2 on connector A goes to Pin 6 on connector B, etc.
• Crossover cables are most commonly used to connect two hosts directly.
• Examples would be connecting a computer directly to another computer, connecting a switch directly to another switch, or connecting a router to a router.

Fig.: Crossover Cable Pinout

CHOOSING THE RIGHT CABLE PINOUTS

• “Pinout” is a term describing how an electrical cable is wired.
• Some cables do not have pinouts because they only contain a single internal wire, like coax cables.
• But if a cable has multiple pins on the end of the cable, it will have a pinout.
• Each type of multi-pin cable has a standard pinout or two, but these layouts are not set in stone.
• Some machines will require non-standard pinouts; this will require users to use a custom cable.

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EXAMPLES INCLUDE

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• ETHERNET PINOUTS
• TELEPHONE PINOUTS
• SERIAL (DB) PINOUTS
• VGA PINOUTS
• DVI PINOUTS
• 3.5MM PINOUTS
• XLR PINOUTS

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UTP CABLING PINOUTS FOR 1000BASE-T

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• 1000BASE-T (Gigabit Ethernet) differs from 10BASE-T and 100BASE-T with respective to cabling and pinouts.
• First, 1000BASE-T requires four wire pairs.
• Second, it uses more advanced electronics that allow both ends to transmit and receive simultaneously on each wire pair.
• But the wiring pinouts for 1000BASE-T work almost identically to the earlier standards, adding details for the additional two pairs.
• The straight-through cable connects each pin with the same numbered pin on the other side, but it does so for all eight pins—pin 1 to pin 1, pin 2 to pin 2, up through pin 8.
• It keeps one pair at pins 1 and 2 and another at pins 3 and 6, just like in the earlier wiring. It adds a pair at pins 4 and 5 and the final.

SENDING DATA IN ETHERNET NETWORKS

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• One of the key elements to the success of Ethernet is the concept of packet data transmission, where data, images, and even sound files are divided and reformatted into small units, transmitted over the network, and reassembled for use by the receiving computer.
• It is certainly possible to transmit a file in a complete stream from start to end from one computer to another.
• This type of transmission is often used when a system only transmits in one direction, from one device to another.
• Think about a typical analog CCTV setup, where one camera is connected to a single monitor.
• The camera sends its images in one uninterrupted stream, and the pictures are displayed on the monitor.

• The situation becomes more complex when multiple communication devices, such as computers, are connected to a common cable or segment, and each device needs the capability to speak with all of the others.
• If files were sent from one computer to another in one uninterrupted stream, all of the other devices would be unable to communicate during that time period.
• Because of these issues, Ethernet divides files to be transmitted into packets, which provide addressing, sequencing and other information, which allows the receiving network device to properly reassemble the file.
• These packets are also referred to as frames.

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Fig.: Ethernet Transmission Packet

Ethernet Packet Format and Routing

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• The 802.3 standards specify how Ethernet packets are assembled for transmission.
• The Destination Address is the address to which the packets are going.
• For obvious reasons, this is placed at the beginning of the packet stream.
• The Transmitter Address is needed, so that the destination computer knows where to send receipts for packets and possibly requests for the resending of packets that were garbled during transmission.

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• The Sequence Code is the number of the packet within the overall file being sent. If a file has 567 packets in its transmission, the sequence code says that this is packet #324 of 567 total, for example.
• The Data section is where the actual file packetized contents are located within the packet.
• This data field can be a minimum of 46 bytes or a maximum of 1500, with a byte being eight bits (1s and 0s).
• The Frame Check Sequence provides an error correction mechanism to ensure that the complete packet arrived without error.