LAN ARCHITECTURE

Module 4: LAN: LAN architecture, advantages and services, characteristics of LAN, LAN topologies, LAN access topologies, CSMA/CD- based LAN, token ring protocol.

A Local Area Network (LAN) is a private network that connects computers and devices within a limited area like a residence, an office, a building or a campus. On a small scale, LANs are used to connect personal computers to printers. However, LANs can also extend to a few kilometers when used by companies, where a large number of computers share a variety of resources like hardware (e.g. printers, scanners, audiovisual devices etc), software (e.g. application programs) and data.

LAN Definition

The personal computers and workstations in the offices are interconnected via LAN to share resources. The resources to be shared can be hardware like a printer or softwares or data. A LAN is a form of local (limited-distance), shared packet network for computer communications. In LAN all the machines are connected to a single cable.  The data rates for LAN range from 4 to 16 Mbps with the maximum of 100 Mbps.

The term LAN can also refer just to the hardware and software that allows you to connect all the devices together. In this sense, Local Talk is one kind of LAN, Ethernet is another. (AppleTalk is the protocol for Local Talk.) 

The components used by LANs can be divided into cabling standards, hardware, and protocols. Various LAN protocols are Ethernet, Token Ring: TCP/IP, 5MB, NetBIOS and NetBeui, IPX/SPX, Fiber Distributed Data Interchange (FDDI) and Asynchronous Transfer Mode (ATM).

LANs can be distinguished from different types of networks by their size, their transmission technology, and their topology.

A local-area network (LAN) restricted in size, which means that it can span in a relatively small area.

LANs use a transmission technology consisting of a cable. Traditional LANs technology transmits at speeds of 10 Mbps to 100 Mbps and makes very few errors. However, current LANs technology transmits at up to 10 Gbps.

Various network topologies are possible for broadcast LANs. In a linear cable network (Bus), at any given time, one client machine is the master machine, and that is only allowed to transmit. All other machines refrain from sending data. The second type of broadcast LANs is the ring. In it,  LAN operating at 4 and 16 Mbps. FDDI is another example of a ring network.

The distinguishing 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.

In general, 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.

Wireless LANs (WLAN)

Wireless LANs use high-frequency radio waves instead of cables for communications. They provide clutter free homes, offices and other networked places. They have an Access Point or a wireless router or a base station for transferring packets to and from the wireless computers and the internet. Most WLANs are based on the standard IEEE 802.11 or WiFi.

Virtual LANs (VLAN)

Virtual LANs are a logical group of computers that appear to be on the same LAN irrespective of the configuration of the underlying physical network. Network administrators partition the networks to match the functional requirements of the VLANs so that each VLAN comprise a subset of ports on a single or multiple switches. This allows computers and devices on a VLAN to communicate in the simulated environment as if it is a separate LAN.

What is LAN (Local Area Network)

It is a privately-owned network and stands for local area network. A network is a group of computers and other devices connected so they can pass information back and forth. The local area network (LAN) is a network which is designed to operate over a small physical area such as an office, factory or a group of buildings up to a few kilometers in size. LANs very widely used in a variety of computers to share resources (e.g., printers) and exchange information.

The Evolution of LAN

As there is a tremendous use of PC or desktop computers in the office environment, it became apparent that attaching a printer or FAX machine to each and every computer is highly expensive. Further, copying files to a disk and moving from one computer to another to print the file is also time consuming. Connecting computers so that they could share a printer and share files translated into big savings.

In 1982, 10 Mbps Ethernet cards came into existence and they were very expensive. By 1988, 10 Mbps Ethernet had acceptable performance for large LANs and was still good for small installations. By 1990, large installations were beginning to see congestion. Hence, alternatives to 10 Mbps cards were becoming popular. One of these alternatives was to install a switched architecture, rather than constructing architecture with hubs.

In 1996, 10Mbps switched LANs were providing acceptable service for smaller installations. The awesome technology at this point was 100Mbps shared Ethernet. By 1998 switched 10Mbps Ethernet was common in small LANs and switched 100 Mbps Ethernet was common in large LANs. Large LANs are beginning to see limitations of 100 Mbps when everyone on the LAN starts doing video conferencing.

Now, Gigabyte Ethernet cards are available for sale. The 10 Mbps cards are not used anymore. Now-i-days 10/100, 10/100/1000 auto-sensing cards are available in the market. Everything is moving to dual mode auto-sensing technology. Even though Gigabyte Ethernet is getting more popular now-a-days, Gigabyte LANs are 10 times faster than a 100Mbps LAN. Ethernet was evolved from a protocol called ALOHA. ALOHA was mainly used in packet radio network which communicate using satellites. In brief, an earth station sends some data, as soon as the data is ready it waits for an acknowledgement (ACK). If it fails to get an ACK, it would time out and sends the same thing again. The sender keeps trying until the transmission is successful. The lesson to be learned here is that it is important to limit the number of users on a shared network medium. Traffic analysis can help determine the maximum number of users to ensure a reasonable Quality of Service (QOS).

LAN Architecture

The architecture of a LAN can be considered as a set of layered protocol, each layer describes a set of functions. The protocols defined for a LAN primarily deals with transmission of a block of data within the LAN. In OSI terms, the higher layer protocols are totally independent of the LAN architectures. Hence, only lower order layers are considered for the design of the LAN architecture. The physical layer of the LAN architecture is equivalent to the physical layer of the OSI. The data link layer of the LAN is split into two layers. One is the Medium Access Control layer (MAC) and the other is the Logical Link Control layer (LLC).The IEEE 802 committee had formulated the standards for the LAN.

The OSI Model and LAN Access

Because there are different types of LANs, both physical topology and logical topology, different methods have been devised to access these networks. Layer 2 of the OSI model has to handle datagram from layer 3 and prepare them to enable Layer 1 to convert the bits of the frame to signals and send them out.

The job of layer 2, the data link layer, can be subdivided into two sub layers.

Logical link Control (LLC)

The top half of the data link layer is the LLC sub layer. This sub layer provides flow and error control functions and manages the flow of data to and from layer 3. LLC layer is concerned with the transmission of data between the source and destination stations without involving the intermediate nodes.

LLC provides the following services to the higher order layers.

• Connection-oriented services.

• Unacknowledged connectionless services.

• Acknowledged connectionless services.

Media Access Control (MAC)

This sub layer resides in the lower half of layer-2. It has the responsibility of knowing the rules of access to the medium and follows the rules. It has to assemble the frame by creating and attaching the header and trailer of each frame. It also has to disassemble incoming frames, check the address information and perform the error check.

MAC frame format MAC layer gets a block of data from the Logical Link Layer (LLC), which is above this layer. The MAC layer then forms frames, which are often called Protocol Data Units (POU). The most generic format of the MAC frame is shown below.

The control header contains the control information needed for the MAC protocol. The next two fields are used to indicate the source and the destination address. The next field called the LLC data contains the actual data obtained from the LLC layer. Finally, the CRC field contains the error check sequence.

LAN standards often specify both layer one and layer two characteristics. For example, Ethernet is explained in detail in the IEEE 802.3 standard. Token Ring access rules are explained in the Token Ring standards document, published by the IEEE a5 standard 802.5. FDDI is another layer 2 LAN protocol, which provides another set of access rules.

The meaning of these standards is different for the user. When a new computer is purchased for the construction of an Ethernet LAN, the computer must have a Network Interface Card (NIC) that will permit it to connect to an Ethernet LAN. The NIC has a physical connection port on it, where it is connected to an Ethernet LAN cable. The card is responsible for following the Ethernet access rules. If it is required to connect the computer to a Token Ring network, a Token Ring NIC is to be installed in place of the Ethernet NIC. It also requires installing new driver software to enable the system to recognize the Token Ring card and use it for network access. The Token Ring card would follow Token Ring network access rules and give you access to the Token Ring LAN. Hence, completely different sets of rules are followed for different LAN access methods.

Hence, in a LAN standard, the physical layer performs data encoding and decoding, preamble generation and removal for synchronization, and bits transmission and reception. The physical layer also includes the specification of transmission medium and the topology. The MAC layer performs similar functions of the data link layer of the OSI. It forms frames by assembling the data bits, governs the access to the media. The LLC layer provides interface to higher layers and performs error and flow control.

LAN TOPOLOGIES

What are topologies?

A network topology is the physical and logical arrangement of nodes and connections in a network. Nodes usually include devices such as switches, routers and software with switch and router features. Network topologies are often represented as a graph.

The pattern of interconnection between the nodes of the network.

There are three common topologies found in LANs: star, ring, and bus. In a star topology, all devices on the LAN are interconnected through some central device. In a ring topology, the nodes are connected by a set of point-to-point links that are organized in a circle.

Star Topology

In this network, all the nodes are connected to Central Node. The devices are not connected to each other and transmits the messages to the Central Node. The central node is responsible for transmitting the message to the required destination. It is the most widely used topology for LAN’s.

In a star topology, the central hub acts like a server and the connecting nodes act like clients. When the central node receives a packet from a connecting node, it can pass the packet on to other nodes in the network. A star topology is also known as a star network.

Star topologies are often used in home networks.

The benefits of a star network topology include the following:

Limits the impact of a single point of failure. In star networks, each connecting node is isolated from other connecting nodes. If one connecting node goes down, it will not impact the performance of other connecting nodes in the network.

Facilitates adding or removing individual components to and from a network. Star networks are usually kept small because network performance can suffer when too many devices compete for access to the central node.

Bus Topology

Alternatively called line topology, bus topology is a network setup where each computer and network device is connected to a single cable or backbone. Depending on the type of computer network card, a coaxial cable or an RJ-45 network cable is used to connect them together.

• Bus topology is shown in Fig. In this topology at any instant only one computer acts as master and it is allowed to transmit (broadcast). The others are supposed to listen.

• If two or more machines want to transmit simultaneously then an arbitration mechanism has to be used for resolving the conflict.

• It is possible to have a centralized or distributed type arbitration mechanism.

• The most popular example of bus topology is Ethernet (IEEE 802.3). It has a decentralized control and it operates at 10 or 100 Mbps.

• Computers on Ethernet can transmit whenever they want. If collision of their packets takes place, then they wait for a random time and retransmit their packets.

Bus network topologies are used when a small, cheap and often temporary network is needed that does not rely on very high data-transfer speeds. They may be used in locations such as a laboratory or office

Advantages of bus topology

It works well when you have a small network.

It's the easiest network topology for connecting computers or peripherals in a linear fashion.

It requires less cable length than a star topology.

Ring Topology

Ring topology is a type of network topology in which each device is connected to two other devices on either side via an RJ-45 cable or coaxial cable. This forms a circular ring of connected devices which gives it its name. Data is commonly transferred in one direction along the ring, known as a unidirectional ring.

A ring topology is a network configuration where device connections create a circular data path. Each networked device is connected to two others, like points on a circle. Together, devices in a ring topology are called a ring network.

In a ring network, packets of data travel from one device to the next until they reach their destination. Most ring topologies allow packets to travel only in one direction, called a unidirectional ring network. Others permit data to move in either direction, called bidirectional.

• This is another broadcast topology.

• In a ring each bit propagates around on its own without waiting for the rest of the packet to which it belongs.

• Since it is a broadcast system, some rules are essential for arbitrating the simultaneous access to the ring.

• An example of ring based LAN is IEEE 802.5 (IBM token ring) operating at 4 and 16 Mbps.

Static and dynamic broadcast networks:

The broadcast networks are further classified into two types namely,

1. Static networks and

2. Dynamic networks.

• This classification is based on how the channel is allocated.

• In static allocation, each machine is allowed to broadcast only in its allotted time slot.

• But static allocation wastes the channel capacity when a machine does not want to transmit in its allotted time slot.

• Hence most of the systems try to allocate the channel dynamically i.e. on demand.

Ring topologies may be used in either LANs (local area networks) or WANs (wide area networks). Depending on the network card used in each computer of the ring topology, a coaxial cable or an RJ-45 network cable is used to connect computers together.

LAN Applications and Benefits

LANs are used almost exclusively for data communications over relatively short distances such as within an office, office building or campus environment. LANs allow multiple workstations to share access to multiple host computers, other workstations, printers and other peripherals, and connections to other networks. LANs are also being utilized for imaging applications, as well. They are also being used for video and voice communications, although currently on a very limited basis.

LAN applications include communications between the workstation and host computers, other workstations, and servers.

The servers may allow sharing of resources. Resources could be information, data files, e-mail, voice mail, software, hardware (hard disk, printer, fax, etc.) and other networks.

LAN benefits include the fact that a high-speed transmission system can be shared among multiple devices in support of large number of active terminals and a large number of active applications in the form of a multi-user, multi-tasking computer network. LAN-connected workstations realize the benefit of decentralized access to very substantial centralized processors, perhaps in the form of mainframe host computer and storage capabilities (information repositories). Additionally, current technology allows multiple LANs to be inter-networked through the use of LAN switches, routers and the like.

Disadvantages of LANs include concern for security of files and accounts.

LAN Advantages and Services

Advantages and benefits of Local Area Network (LAN)

Resource sharing. ...

The relationship between the client and the server. ...

Internet sharing. ...

Sharing applications and software. ...

Data security. ...

Easy and fast communication. ...

Computer ID.

They work on higher operating speed than WAN and MAN.

They suit the requirements of a specific organization.

They are easy to install and maintain.

They exist as connected (wired) and wireless configurations.

LAN can provide the following services.

File-based services Transfer of files from one node to another within the LAN area. For example, in a typical LAN used for local banking, the file containing the detailed transactions of a specific customer is transferred from sever to the client, whenever new transactions are made on behalf of the customer. It also provides efficient ways of storing and retrieving the data. When multiple copies of the same file exist, it provides necessary synchronization in updating the files. LAN also provides backup for the critical data so that safe recovery is possible when a failure occurs. It also provides data encryption facility to control the access to certain data to only selected persons.

Print services there can be one or more printers, modems, and fax machines attached to a LAN and used by applications. A number of clients can share these devices. Print services allow many clients to safely share printers and other such devices.

All modem LANs provide these services.

Application-based services Applications run on a client may require higher computational capabilities. Servers are more powerful than the clients. In a LAN, more than one client can share the computational power of a server. Application servers are good examples for this.

Mail and message-based services Electronic-mail is an important service used for exchanging information between people on a network. Mails can be stored or forwarded to another user. Many free mail servers are available on the Internet to provide free mail services to its clients.

Database services Storing and retrieving of data in databases is another requirement of a client to control and manipulate the data. Database servers are the most popular ones that service the need of clients.

Distributed data services When data is distributed, it is possible for more than one client system to share the data. Though the databases appear physically distributed, there is a single logical view given by the database server. There are so many issues involved in sharing a single file by more than one client. Consistency issue is very seriously tackled when updating shared files.

Remote services Linking a LAN with a remote computer or a mainframe is another service required.

This feature is used to access remote databases available in large mainframes.

Characteristics of a lan

LAN Characteristics

LANs are grouped according to four key characteristics:

The types of transmission media over which they can operate

The transport technique they use to transmit data over the network (that is, broadband or baseband)

The access method, which is involved in determining who gets to use the network and when they get to use it

The topology, or mapping, of the network (that is, the physical and logical connections between the nodes on the network)

The following sections describe each of these four characteristics in detail.

LAN Transmission Media

 “Transmission Media: Characteristics and Applications,” describes the advantages and disadvantages of the media types, and this section talks specifically about the considerations ...

A LAN can be characterized by means of hardware and software components and a set of protocols.

Hardware components of a LAN are:

The Server

Server is a computer that provides services to other computers (workstations or clients) on the network. The primary goal of a LAN server is data management. It stores, retrieves, and protects the data. A server also sends data to the requesters on the network and also to authorized remote users. The type and configurations needed for the server, depends mainly on the purpose for which the LAN is being constructed. Servers may be classified, based on the type of service they provide. A single server may provide a number of services also. The following are the list of servers categorized, based on the type of service.

File server and disk server A file server makes the disk storage space (in the order of several Gigabytes), to various client PCs. The file server satisfies the request for data from application programs running in client workstations. It also keeps the consistency of data when more than one client makes simultaneous data requests. In a LAN, working with a file server, all application running in a workstation may request for a file with involving the local operating system. The workstation sends its file request to the server and the server processes the request and sends the required file directly to the workstation.

Disk servers are similar to file servers. The difference is that, in a file server when an application running in a workstation requests for a specific file, the local operating system, running on the workstation interacts with the file server and gets the information required. In a disk server, the application can directly access the required file without the intervention of the local operating system.

Database server These servers are a subset of the file server category. They provide access to huge databases for clients. Database information is stored in hard disk storage or CD ROM or optical disk drives. It contains the DBMS, which is more sophisticated than the basic file I/O access method. DBMS eliminates data redundancy and allows the user, transparent data distribution. The database server extracts only the relevant data and passes it to the requesting client, rather than passing the entire file like a file server.

Print server Every LAN has one or more printers shared by all the nodes or workstations. The role of a print server is to collect the information from several workstations, store them on the disk and send it to the printer. This processing is known as print spooling. Print services become a part of the file server nowadays. In many LAN architectures, any PC on the LAN can act as a print server.

Backup server Such servers provide backup in case the main server fails. Every network must have a back up server to keep all the information safe. Periodically, these servers must be updated so that reliability can be improved.

Gateway server A gateway server is used to provide connectivity to other networks. The GIAS (Gateway server of VSNL, India) is a gateway server, which provides connectivity to all other networks on the Internet for the Indian segment of the Internet. A gateway server also provides connectivity to dissimilar networks.

Communication server Communication servers are more diverse than the other servers. The main functions of a communication server are linking client workstations on the LAN with mainframe computers, sharing a pool of modems among the client stations, and communicating with other LANs as well.

Examples of the most popular LAN servers are Novell Netware LAN Server, Microsoft Windows NT Server or Windows 2000, IBMOS2 Server, etc.

Workstations

Workstations or nodes are the clients that use the services provided by the network server. Workstations are loaded with special software to interact with the server to access the services. Examples of workstations include the Microsoft Windows NT workstation, Windows 2000 workstation, IBM workstation, Sun workstation, etc.

The Transmission Media for LAN

Various transmission media are used for constructing a LANs. The most popular among them are twisted pair, coaxial cable, or optical fibers. LANs operated with any such transmission media is known as wired LAN. LANs can also be constructed without a transmission media or cable. Such LANs are known as wireless LANs.

Communication Equipments

Repeater A repeater operates at layer-I. It has just enough intelligence to find out the layer-l incoming signals are and then send out a clean stream of signals built from scratch. Noise is eliminated from the signals in this manner. A repeater has one incoming and one outgoing line. It extends the distance that a signal may be sent over a transmission media.

Hub A hub is a multi-port repeater. Any incoming signal is repeated on all other outgoing lines. A hub functions at layer-I.

Bridge A bridge has more intelligence than a hub or repeater. This device separates two segments of a single LAN. A bridge operates at layer-2 by looking at the destination address in the frame header. Consulting a table, the bridge will determine if the frame needs to pass on to the other segment. Only certain frames may pass those with the correct MAC address.

Switch A switch is a multi-port bridge. It performs its functions at layer two. It looks at MAC layer addresses just like a bridge, consults a table, and determines if a frame needs to be sent on one of the attached lines. Multiple connections can occur through a switch simultaneously as long as they don’t compete for the same line. A switch, just like a bridge, does not divide a LAN into two LANs. It merely reduces unnecessary traffic on LAN segments.

Router A router is more intelligent than bridges and switches. It functions at layer-3. At layer-2 all incoming frames are checked for errors and then they are delivered to layer-3. A layer-3 datagram has an address that permits it to be sent across internet works (interconnected networks). This requires a single global addressing scheme. The router finds the layer-3 address and consults a table that it keeps. There the router will learn which attached line to send the datagram on. A router provides facilities to the stations on the LAN, to access other networks.

Advantages and benefits of Local Area Network (LAN)

Resource sharing. ...

The relationship between the client and the server. ...

Internet sharing. ...

Sharing applications and software. ...

Data security. ...

Easy and fast communication. ...

Computer ID.

Advantages and benefits of Local Area Network (LAN)

In the following, we will get acquainted with some of the benefits and advantages of LAN network

Resource sharing

All resources are connected to one network, and if a computer needs a resource, it can be shared with another computer. These resources include DVD drives, printers, scanners, modems, and hardware drives. Therefore, there is no need to buy separate resources for each computer, and you can save costs.

The relationship between the client and the server

All data of connected computers can be stored on one server. If each computer (client) needs specific data, the user can easily log in and access the data from the server. For example, movies and songs can be stored on a server and accessed by any authorized user (user computer).

Internet sharing

In offices and Internet cafes, it can be seen that an Internet connection is shared between all computers. This technology is another type of LAN in which the main Internet cable is connected to a server and distributed through the operating system among the connected computers.

Sharing applications and software

Applications and software can be easily shared over a LAN. You can use licensed software so that any user can use it on the network. Purchasing a license for each user can cost the organization a lot of money, so subscribing to the software program is easy and at the lowest cost.

Data security

Data storage on the server is highly secure. If you want to modify or delete data, you can easily do so on a server computer, and other computers can access the updated data. On the other hand, you can allow access to specific users to access the data in the network.

Easy and fast communication

In local area networks, computers can exchange data and messages easily and quickly. This saves time and gets things done faster. LAN allows any user to share messages and data with any other user on the network. Users can access the network from any computer and access the data stored on the server.

Computer ID

In a LAN, each computer has its own MAC Address, which is temporarily stored on a switch or router when communicating. All computers on the LAN can be identified by a MAC address, which can send and receive messages and data. Of course, the MAC address is stored on the network adapter and the motherboard of each computer. On older computers, network adapters are not built into the motherboard, but they are made with motherboards on newer computers.

What are the services of LAN?

Applications running on the Local Area Network server provide services such as database access, document sharing, email, and printing. Devices on a peer-to-peer local area network share data directly to a switch or router without the use of a central server.

Benefits of Using Managed LAN Services?

Security: A managed LAN services provider can secure your network by installing firewalls, intrusion detection/prevention systems, and other security products.

Network Monitoring: A managed LAN services company can monitor your network 24 hours a day, seven days a week. It can alert you when there are problems with your network.

Improved Network Performance: managed LAN service providers usually offer faster speeds than you could get from a public Internet access point. You won’t have to wait as long to download large files.

Cost Savings: Managed LAN service providers offer lower rates than you would pay for similar services.

Reliability: Most managed LAN service providers guarantee 99% uptime. If they fail to deliver this level of reliability, they will reimburse you for the downtime.

24-Hour Availability: Most managed LAN service providers offer 24-hour availability. If an issue arises, they will resolve it immediately.

In conclusion, managed LAN services are beneficial because they help improve your business’s productivity and save you money by reducing the cost of maintaining your network infrastructure.

Characteristics of LAN

It is a network owned by a private owner.

Personal computers, printers, etc., are connected through LAN.

LANs are very easy to design and troubleshoot.

A central database is used to connect the LAN networks.

The data transfer rate of LAN is 10 Gbits/s.

LANs are limited to local areas.

What are the 7 characteristics of a network?

Basic characteristics of a Computer Network

Security. Security is one of the most essential characteristics of a computer network. ...

Reliability. ...

Scalability. ...

Flow of Data. ...

High performance. ...

Fault tolerance. ...

Quality of Service (QoS) ...

Compatible With Hardware and Software Components.

Types of Network Topology

The arrangement of a network that comprises nodes and connecting lines via sender and receiver is referred to as network topology. The various network topologies are:

Mesh Topology:

In a mesh topology, every device is connected to another device via a particular channel. In Mesh Topology, the protocols used are AHCP (Ad Hoc Configuration Protocols), DHCP (Dynamic Host Configuration Protocol), etc.

Figure 1: Every device is connected to another via dedicated channels. These channels are known as links. 
Suppose, the N number of devices are connected with each other in a mesh topology, the total number of ports that are required by each device is N-1. In Figure 1, there are 5 devices connected to each other, hence the total number of ports required by each device is 4. The total number of ports required=N*(N-1).

Suppose, N number of devices are connected with each other in a mesh topology, then the total number of dedicated links required to connect them is NC2 i.e. N(N-1)/2. In Figure 1, there are 5 devices connected to each other, hence the total number of links required is 5*4/2 = 10.

Advantages of this topology: 

Communication is very fast between the nodes.

It is robust.

The fault is diagnosed easily. Data is reliable because data is transferred among the devices through dedicated channels or links.

Provides security and privacy.

Problems with this topology: 

Installation and configuration are difficult.

The cost of cables is high as bulk wiring is required, hence suitable for less number of devices.

The cost of maintenance is high.

Star Topology:

In star topology, all the devices are connected to a single hub through a cable. This hub is the central node and all other nodes are connected to the central node. The hub can be passive in nature i.e., not an intelligent hub such as broadcasting devices, at the same time the hub can be intelligent known as an active hub. Active hubs have repeaters in them. Coaxial cables or RJ-45 cables are used to connect the computers. In Star Topology, many popular Ethernet LAN protocols are used as CD(Collision Detection), CSMA (Carrier Sense Multiple Access), etc.

Figure 2: A star topology having four systems connected to a single point of connection i.e. hub. 

Advantages of this topology: 

If N devices are connected to each other in a star topology, then the number of cables required to connect them is N. So, it is easy to set up.

Each device requires only 1 port i.e. to connect to the hub, therefore the total number of ports required is N.

It is Robust. If one link fails only that link will affect and not other than that.

Easy to fault identification and fault isolation.

Star topology is cost-effective as it uses inexpensive coaxial cable.

Problems with this topology: 

If the concentrator (hub) on which the whole topology relies fails, the whole system will crash down.

The cost of installation is high.

Performance is based on the single concentrator i.e. hub.

Bus Topology:

Bus topology is a network type in which every computer and network device is connected to a single cable. It is bi-directional. It is a multi-point connection and a non-robust topology because if the backbone fails the topology crashes. In Bus Topology, various MAC (Media Access Control) protocols are followed by LAN ethernet connections like TDMA, Pure Aloha, CDMA, Slotted Aloha, etc.

Figure 3: A bus topology with shared backbone cable. The nodes are connected to the channel via drop lines. 

Advantages of this topology: 

If N devices are connected to each other in a bus topology, then the number of cables required to connect them is 1, known as backbone cable, and N drop lines are required.

Coaxial or twisted pair cables are mainly used in bus-based networks that support up to 10 Mbps.

The cost of the cable is less compared to other topologies, but it is used to build small networks.

Bus topology is familiar technology as installation and troubleshooting techniques are well known.

Problems with this topology: 

A bus topology is quite simpler, but still, it requires a lot of cabling.

If the common cable fails, then the whole system will crash down.

If the network traffic is heavy, it increases collisions in the network. To avoid this, various protocols are used in the MAC layer known as Pure Aloha, Slotted Aloha, CSMA/CD, etc.

Adding new devices to the network would slow down networks.

Security is very low.

Ring Topology:

In this topology, it forms a ring connecting devices with exactly two neighboring devices.

A number of repeaters are used for Ring topology with a large number of nodes, because if someone wants to send some data to the last node in the ring topology with 100 nodes, then the data will have to pass through 99 nodes to reach the 100th node. Hence to prevent data loss repeaters are used in the network.

The data flows in one direction, i.e.., it is unidirectional, but it can be made bidirectional by having 2 connections between each Network Node, it is called Dual Ring Topology. In-Ring Topology, the Token Ring Passing protocol is used by the workstations to transmit the data.

Figure 4: A ring topology comprises 4 stations connected with each forming a ring. 

The most common access method of ring topology is token passing.

Token passing: It is a network access method in which a token is passed from one node to another node.

Token: It is a frame that circulates around the network.

The following operations take place in ring topology are : 

One station is known as a monitor station which takes all the responsibility for performing the operations.

To transmit the data, the station has to hold the token. After the transmission is done, the token is to be released for other stations to use.

When no station is transmitting the data, then the token will circulate in the ring.

There are two types of token release techniques: Early token release releases the token just after transmitting the data and Delayed token release releases the token after the acknowledgment is received from the receiver.

Advantages of this topology: 

The data transmission is high-speed.

The possibility of collision is minimum in this type of topology.

Cheap to install and expand.

It is less costly than a star topology.

Problems with this topology: 

The failure of a single node in the network can cause the entire network to fail.

Troubleshooting is difficult in this topology.

The addition of stations in between or the removal of stations can disturb the whole topology.

Less secure. 

Tree Topology :

This topology is the variation of the Star topology. This topology has a hierarchical flow of data. In Tree Topology, protocols like DHCP and SAC (Standard Automatic Configuration ) are used.

Figure 5: In this, the various secondary hubs are connected to the central hub which contains the repeater. This data flow from top to bottom i.e. from the central hub to the secondary and then to the devices or from bottom to top i.e. devices to the secondary hub and then to the central hub. It is a multi-point connection and a non-robust topology because if the backbone fails the topology crashes.

Advantages of this topology : 

It allows more devices to be attached to a single central hub thus it decreases the distance that is traveled by the signal to come to the devices.

It allows the network to get isolated and also prioritize from different computers.

We can add new devices to the existing network.

Error detection and error correction are very easy in a tree topology.

Problems with this topology :  

If the central hub gets fails the entire system fails.

The cost is high because of the cabling.

If new devices are added, it becomes difficult to reconfigure.

Hybrid Topology :

This topological technology is the combination of all the various types of topologies we have studied above. It is used when the nodes are free to take any form. It means these can be individuals such as Ring or Star topology or can be a combination of various types of topologies seen above. Each individual topology uses the protocol that has been discussed earlier.

Hybrid Topology

Figure 6: The above figure shows the structure of the Hybrid topology. As seen it contains a combination of all different types of networks.

Advantages of this topology : 

This topology is very flexible.

The size of the network can be easily expanded by adding new devices.

Problems with this topology :

It is challenging to design the architecture of the Hybrid Network.

Hubs used in this topology are very expensive.

The infrastructure cost is very high as a hybrid network requires a lot of cabling and network devices.

LAN access topologies

There are three common topologies found in LANs: star, ring, and bus. In a star topology, all devices on the LAN are interconnected through some central device. In a ring topology, the nodes are connected by a set of point-to-point links that are organized in a circle.

This chapter provides an overview of local area network (LAN) concepts, topologies, technologies, and design. Historically, LANs have been broadcast networks, meaning that every LAN station hears every transmission on the medium. LAN topologies have to support the broadcast nature of the network and provide full connectivity between all stations. The physical topology describes how the LAN stations are physically connected so that they can communicate with each other. The logical topology describes how the broadcast nature of the LAN is actually affected, and, therefore, how stations participate in the process of obtaining permission to transmit on the medium. There are three common topologies found in LANs: star, ring, and bus. In a star topology, all devices on the LAN are interconnected through some central device. In a ring topology, the nodes are connected by a set of point-to-point links that are organized in a circle. In a bus topology, all devices are connected to a single electrically continuous medium. The chapter also discusses concepts related to media access control, interconnection devices, and network operating systems.

 

CSMA/CD- based LAN

Carrier-sense multiple access with collision detection (CSMA/CD) is a medium access control (MAC) method used most notably in early Ethernet technology for local area networking. It uses carrier-sensing to defer transmissions until no other stations are transmitting. This is used in combination with collision detection in which a transmitting station detects collisions by sensing transmissions from other stations while it is transmitting a frame. When this collision condition is detected, the station stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to resend the frame.

CSMA/CD is a modification of pure carrier-sense multiple access (CSMA). CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, thus shortening the time required before a retry can be attempted.

With the growing popularity of Ethernet switches in the 1990s, IEEE 802.3 deprecated Ethernet repeaters in 2011, making CSMA/CD and half-duplex operation less common and less important.

CSMA/CD (Carrier Sense Multiple Access/ Collision Detection) is a media access control method that was widely used in Early Ethernet technology/LANs When there used to be shared 
Bus Topology and each node ( Computers) were connected By Coaxial Cables. Now a Days Ethernet is Full Duplex and CSMA/CD is not used as Topology is either Star (connected via Switch or Router) 
or Point to Point ( Direct Connection) but they are still supported though. 

Consider a scenario where there are ‘n’ stations on a link and all are waiting to transfer data through that channel. In this case, all ‘n’ stations would want to access the link/channel to transfer their own data. Problem arises when more than one station transmits the data at the moment. In this case, there will be collisions in the data from different stations. 

CSMA/CD is one such technique where different stations that follow this protocol agree on some terms and collision detection measures for effective transmission. This protocol decides which station will transmit when so that data reaches the destination without corruption. 

How CSMA/CD works? 

Step 1: Check if the sender is ready for transmitting data packets.

Step 2: Check if the transmission link is idle? 
Sender has to keep on checking if the transmission link/medium is idle. For this, it continuously senses transmissions from other nodes. Sender sends dummy data on the link. If it does not receive any collision signal, this means the link is idle at the moment. If it senses that the carrier is free and there are no collisions, it sends the data. Otherwise, it refrains from sending data.

Step 3: Transmit the data & check for collisions. 
Sender transmits its data on the link. CSMA/CD does not use an ‘acknowledgment’ system. It checks for successful and unsuccessful transmissions through collision signals. During transmission, if a collision signal is received by the node, transmission is stopped. The station then transmits a jam signal onto the link and waits for random time intervals before it resends the frame. After some random time, it again attempts to transfer the data and repeats the above process.

Step 4: If no collision was detected in propagation, the sender completes its frame transmission and resets the counters.

How does a station know if its data collide? 

Consider the above situation. Two stations, A & B. 
Propagation Time: Tp = 1 hr ( Signal takes 1 hr to go from A to B) 

At time t=0, A transmits its data.

        t= 30 mins : Collision occurs.

After the collision occurs, a collision signal is generated and sent to both A & B to inform the stations about a collision. Since the collision happened midway, the collision signal also takes 30 minutes to reach A & B. 

Therefore, t=1 hr: A & B receive collision signals.

This collision signal is received by all the stations on that link. Then, 

A token-ring network is a local area network (LAN) topology that sends data in one direction throughout a specified number of locations by using a token. The token is the symbol of authority for control of the transmission line.

What is a token ring?

A token ring is a data link for a local area network (LAN) in which all devices are connected in a ring or star topology and pass one or more tokens from host to host. A token is a frame of data transmitted between network points. Only a host that holds a token can send data, and tokens are released when receipt of the data is confirmed. IBM developed token ring technology in the 1980s as an alternative to Ethernet.

What is a token ring network?

Also known as IEEE (Institute of Electrical and Electronics Engineers) 802.5, a token ring network connects all devices, including computers, in a circular or closed-loop manner. In this scenario, the word token describes a segment of data sent through the network.

Token ring networks prevent data packets from colliding on a network segment because only a token holder can send data, and the number of tokens available is also controlled. When a device on the network successfully decodes that token, it receives the encoded data.

Token ring history

Attached Resource Computer Network, Fiber Distributed Data Interface (FDDI) and the token bus used the token ring. But the most broadly deployed token ring protocols were those of IBM, released in the mid-1980s, and the standardized version of it known as IEEE 802.5, which appeared in the late 1980s.

The use of token rings and 802.5 started declining in the 1990s. Today, they are considered inactive and obsolete. Enterprise organizations gradually phased out the token ring and adopted Ethernet technology, which dominates LAN designs today. The IEEE 802.5 working group is now listed as disbanded.

Token rings were popular because they worked well with large amounts of traffic, but they were not well suited to large networks, particularly if those networks were spread widely or had physically remote nodes. To overcome some of these limitations, multistation access units (MSAUs), which are like hubs on Ethernet, were added. MSAUs are centralized wiring hubs and are also known as concentrators.

What is token ring star topology?

In a star topology, token ring access could connect up to 225 nodes at 4 million, 16 million or 100 million bits per second, conforming to the IEEE 802.5 standard. An MSAU connects all stations using a twisted pair cable. For example, users could connect six nodes to an MSAU in one office and connect that MSAU to an MSAU in another office that served eight other nodes. In turn, that MSAU could connect to another MSAU that connected to the first MSAU.

Such a physical configuration is called a star topology. However, the actual configuration is a ring topology because every message passes through every computer, one at a time, until it forms a ring.

An advantage of an MSAU is that, if one computer fails in the ring, the MSAU can bypass it, and the ring will remain intact. Typically, each node connection cannot exceed 382 feet, depending on the cable type. However, you can increase this distance by up to a mile and a half using token ring repeaters.

What are Type 1 and Type 3 token ring networks?

Token ring networks are generally considered either Type 1 or Type 3 configurations. Type 1 networks can support up to 255 stations per network ring and use shielded twisted pair wires with IBM-style Type 1 connectors.

Type 3 networks can support up to 72 stations per network and use unshielded twisted pair wires with Cat3, Cat4 or Cat5 with RJ-45 connectors. Like Ethernet, the token ring functions at Layers 1 and 2 of the Open Systems Interconnection (OSI) model.

What is a full-duplex token ring?

In a dedicated token ring, also called full-duplex token ring, switching hubs enable stations to send and receive data simultaneously on the network. In this case, a token ring switching hub divides the network into smaller segments. When a data packet is transmitted, the token ring switch reads the packet's destination address and forwards the information directly to the receiving station.

The switch establishes a dedicated connection between the two stations. This enables data to be transmitted and received simultaneously. But, in a full-duplex token ring, the token-passing protocol is suspended, making the network a "tokenless" token ring. Full-duplex token rings are designed to improve network performance by increasing the sending and receiving bandwidth for connected stations.