Network Topology & Protocols

Standards and Protocols:

Standards and protocols are required to govern the physical and logical connections between terminals, Computers and other equipment. They are vital for data communications and computer networking.

Typically, standard fall into two groups official standards (from national standards bodies) and defacto standards established by common usage.

An early official standard was the EIA RS232 for data transfer over wires (Electronic Industries Association Recommended Standard).

Network Topology:

The topology defines how the devices (Computers, printers… etc) are connected and how the data flows from one device to another. There are two conventions while representing the topologies. The physical topology defines how the devices are physically wired. The logical topology defines how the data flows from one device to another.

Broadly categorized into i) Bus ii) Ring iii) Star iv) Mesh

Bus Topology:

Logical Topology illustration of bus topology

In a bus topology all devices are connected to the transmission medium as backbone. There must be a terminator at each end of the bus to avoid signal reflections, which may distort the original signal. Signal is sent in both directions, but some buses are unidirectional. Good for small networks. Can be used for 10BASE5 (thick net), 10BASE2 (thin net) or 10BROAD36 (broad band) co-axial bus standards.

Physical Topology of Bus topology

The main problem with the bus topology is failure of the medium will seriously affect the whole network. Any small break in the media the signal will reflect back and cause errors.

The whole network must be shut down and repaired. In such situations it is difficult to troubleshoot and locate where the break in the cable is or which machine is causing the fault; when one device fails the rest of the LAN fails.

Logical Topology illustration of bus topology

Ring Topology

Ring topology was in the beginning of LAN area. In a ring topology, each system is connected to the next as shown in the following picture.

Each device has a transceiver which behaves like a repeater which moves the signal around the ring; ideal for token passing access methods. In this topology signal degeneration is low; only the device that holds the token can transmit which reduces collisions. If you see its negative aspects it is difficult to locate a problem cable segment; expensive hardware.

Star Topology:

In a star topology each station is connected to a central node. The central node can be either a hub or a switch. The star topology does not have the problem as seen in bus topology. The failure of a media does not affect the entire network. Other stations can continue to operate until the damaged segment is repaired.

Commonly used for 10BASE5, 10BASE-T or 100BASE-TX types.

The advantages are cabling is inexpensive, easy to write, more reliable and easier to manage because of the use of hubs which allow defective cable segments to be routed around; locating and repairing bad cables is easier because of the concentrators; network growth is easier.

The Disadvantages are all nodes receive the same signal therefore dividing bandwidth; Maximum computers are 1,024 on a LAN. Maximum UTP (Un Shielded Twisted Pair) length is 100 meters; distance between computers is 2.5 meters.

Mesh Topology:

A mesh physical topology is when every device on the network is connected to every device on the network; most commonly used in WAN configurations helps find the quickest route on the network; provides redundancy. Very expensive and not easy to set up.

Hybrid Topology:

A hybrid topology is a combination of any two or more network topologies in such a way that the resulting network does not have one of the standard forms. For examples, a tree network connected to a tree network is still a tree network, but two-star networks connected together exhibit hybrid network topologies. A hybrid topology is always produced when two different basic network topologies are connected.

The OSI Model

It has seven layers. They are separate but related. Each layer has well defined tasks and provides services to the corresponding lower layer while transmission. In receiving mode, the lower layer provides the necessary services to the upper layer. Any changes in one layer should not require changes in other layers. This kind of standardization allows communication across all types of computers.

The Seven layers of OSI and their conceptual services –

·         Application – (Layer – 7): allows application to use the network. The user may want to access the network for various purposes. Like for sending e-mail, transferring a file, surfing the web, dedicated service.

Services – e-mail, news groups, web applications, file transfer, remote host, directory services, network management, file services

Easy to remember these layers!.....   Please Do Not Touch Sai Pet Animal

·         Presentation – (Layer – 6): Translates data into a form usable by the application layer. The redirector operates here. Responsible for protocol conversion, translating and encrypting data, and managing data compression. Messages are sent between layers

Services – Pop, SMTP (email, post office protocol, Simple Mail Transfer protocol), Usenet (for news groups), HTTP (hyper text transfer protocol for web applications), FTP, TFTP (File Transfer Protocol, Trivial FTP for file transfer), Telnet (Terminal Network, A general purpose program enabling remote login into some other computer and function as if it is directly connected to that remote computer), Domain name Server (finding IP addresses for domain names), SNMP (simple network management protocol).

·         Session – (Layer 5): Allows applications on connecting systems to standard ports & establish a session. Provides synchronization between communicating computers. Messages are sent between layers

Services – Various port numbers are POP (25), USENET (532), HTTP (80), FTP (20/21), Telnet (23), DNS (53), SNMP (161/162) etc…

·         Transport – (Layer 4): Responding for packet handling. Ensures error – free delivery. Repackages messages (while receiving), divides messages into smaller packets (while transmitting), and handles error handling. Segments of message fragments are sent between layers.

Services – TCP – Connection – oriented communication for application to ensure error free delivery; UDP – connectionless communications and does not guarantee packet delivery between transfer points.

·         Network – (Layer 3): Translate system names into addresses. Responsible for addressing determining routes for sending, managing network traffic problems, packet switching, routing, data congestion, and reassembling data. Datagrams are sent between layers.

Services- Software & hardware addresses and packet routing between hosts and networks (IP). Two versions IP4 (32bits) & IP6(128bits)

·         Data Link – (Layer 2): Sends data from network layer to physical layer. Manages physical layer communications between connecting systems. Data frames are sent between layers

Services – SLIP/PPP, 802.2 SNAP, Ethernet

·         Physical – (Layer 1): Transmit data over a physical medium. Defines cables, cards, and physical aspects. Data bits are sent.

Services: ISDN, ADSL, ATM, FDDI, CAT 1-5, Coaxial cable

The Internet Model

There are four layers in this model. They are 1) Application Layer 2) Transport Layer 3) Network Layer 4) Data Link & Physical Layer.

·         Application Layer: Most of the responsibilities of the three top most layers of OSI model are in application layer of Internet model. The services are as depicted in the fig.

·         Transport Layer: It has two protocols. TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). TCP is are liable protocol that allows two application layers to converse with each other. While transmitting it divides the stream of characters into manageable segments. While receiving it create stream of characters for application layer from received segments from network layer. Its function is much more than as depicted in OSI model. Some of the responsibilities of OSI’s session layer are dissolved into internet models transport layer defined in OSI model. It is used when fast delivery of packets is needed without worrying much about error control.

·         Network Layer: The main protocol is IP (Internet Protocol) is responsible for creating network layer packets called IP datagrams. The datagrams travel network to network or LAN to WAN and the packets may reach out sequence. It is the responsibility of upper layers to put them into proper order.

·         Datalink & Physical Layer: The internet model does not discuss much about these layers making this protocol machine independent to a large extent. It is left to the user to choose the proper standard or protocol according to what they desire.

FTP Servers:

One of the oldest of the internet services, File Transfer Protocol makes it possible to move one or more files securely between computers while providing file security and organisation as well as transfer control.

Mail Servers:

Almost as ubiquitous and crucial as web servers, mail servers move and store mail over corporate networks (via LANs and WANs) and across the internet.

Proxy Servers:

Proxy servers sit between a client program (typically a web browser) and a external server (typically another server on the web) to filter requests, improve performance, and share connections.

Telnet Servers:

A Telnet server enables users to log on to a host computer and perform tasks as if they’re working on the remote computer itself.

Web Servers:

At its core, a web server serves static content to a web browser by loading a file from a disk and serving it across the network to a user’s web browser and server talking to each other using HTTP his entire exchange.

Gateways:

The internet is the collection of heterogeneous computers with different hardware and software platforms. Without gateways computers will never be able to understand and communicate with each other. Essentially, gateways perform protocol translation between networks. Gateways are generally designed and used for LAN-WAN connections and not for inter LAN communications. Gateways function is to do any necessary conversion of protocols between networks. Gateways are customized and designed to perform a specific function and are used on a case-by-case basis. Gateways may do anything from converting protocols to converting application data. Gateways make a connection between two totally different networks.

·         Transform the packet format

·         Transform the address format

·         Transform the protocol

Network interface cards (NIC), also called network adapters include a cable socket allowing computers to be connected to the network. All NICs have a unique address (sometimes called a MAC address), placed in them by their manufacturer. Before sending data onto the network, the network card also organizes data into frames and then sends them out on the network. Notebook computers often use NICs that are plugged into the PCMCIA port.

Wireless LAN adapters are needed for WLANs.

NIC card functions:

LAN adapters have their own onboard architectures and they carry NIC functions out several important functions out several important functions including

·         Monitoring activity on the communication medium

·         Providing each workstation/server with a unique identification address (MAC)

·         Recognizing and receiving data transmitted to the computer

·         Creating (building) the frames needed to transmit data on the communication medium

·         Controlling LAN transmission speed

·         Transmission error detection and recovery

Hubs/Switches:

Hubs act as junction boxes, linking cables from several computers on a network. Hubs are usually sold with 4,8,16 or 24 ports. Some hubs allow connection of more than one kind of cabling, such as UTP and coax. Hubs also repeat (reconstruct and strengthen) incoming signals. This is important since all signals become weaker with distance. The maximum LAN segment distance for a cable can therefore be extended using hubs.

In general use Hub can be referred to any connecting device and can be considered as multipoint repeater. Hubs can be used to create tree structure like topology (Technically it is bus topology). Network point of view it is a logical device and does not have an address. The added benefit of using Hub is it removes the length restriction 100 metres in 10BaseT.

A switch is more sophisticated than hub and can remember and check node addresses. In fact, this phenomenon can affect logical topology of the network! They physically resemble hubs and like hubs, they vary in number of ports, stand-alone vs. stackable, and managed vs. unmanaged. While a hub broadcasts data frames to all ports, the switch reads the destination address of the data frame and only sends it to the corresponding port. The effect is to turn the network into a group of point-to-point circuits and thus changes the logical topology of the network from a bus to a star.

Switched Ethernet still uses CSMA/CD media access control, but collisions are less likely as each network segment operates independently.

For example, computer A can send a message to computer B at the same time that computer C sends one to computer B. if two computers send frames to the same destination at the same time, the switch stores the second data frame in memory until it has finished sending the first, then forwards the second.

Switched Ethernet can dramatically improve network performance. Shared Ethernet 10BaseT networks are only capable of using about 50% of capacity before collision are problem. Switched Ethernet, however, runs at upto 95% on 10BaseT. Using a 10/100 switch that uses a 100BaseT connection for the server(s) and/or routers, i.e., the network segments experiencing the highest volume of LAN traffic, can make another performance improvement.

File Transfer Protocol (FTP)

This allows you to transfer files between your host and another host machine. A shell is provided which cables you to

·         Change directories on the host or local machine

·         Display directories and files

·         Change the file transfer mode (binary or Text)

·         Execute commands

·         Delete or rename files

·         Open and close connections to a host

Terminal Emulation (telnet)

This allows you to logon to a host using terminal emulation. You can execute commands on the host and run application software.

Domain Name Servers

This is a hierarchical naming system for identifying host computers on networks or the internet. each host name is comprised of domain labels separated by periods. If your machine is connected to the internet, you assign local domain name host names only, and your higher-level domain name is assigned to you.

For example, the domain name assigned to Osmania university is Osmania.ac.in has the higher-level domain names of ac (academic) and in (India). Examples of the host computers at Osmania university are called staff, results, and www. Their host names in the domain are staff.osmania.ac.in & results.osmania.ac.in

Simple Network Management Protocol (SNMP)

This provides a means for managing a network environment. Each host, router ot gateway running SNMP can be interrogated for information related to the network.

Examples of information are,

·         Names

·         Packets transmitted and received

·         Errors

·         Routing information

Web Browsers:

Simply putting, a browser is a program that reads HTML (Hyper Text Markup Language, the programming language of the web) and displays it to the viewer.

While there is currently one recognised standard language used to write web pages (HTML4), there are a myriad of browsers available to view it with. Choosing the right one for your individual needs can help maximize your web experience, which will also cut down on installations and uninstallations.

Currently the two most popular browsers are Microsoft Internet Explorer, and Netscape Navigator, Microsoft Internet Explorer is by far more wide spread in use and is one of the most popular browsers to gain wide use and supports most HTML functions but not the newest attributes like frames or animated images.

Repeaters:

Since a signal loses strength as it passes along a cable, it is often necessary to boost the signal with a device called a repeater. The repeater electrically amplifies the signal it receives and rebroadcasts it. Repeaters can be separate devices or they can be incorporated into a concentrator.

They are used when the total length of your network cable exceeds the standards set for the type of cable being used.

A good example of the use of repeaters would be in a local area network using a star topology with unshielded twisted-pair cabling. The length limit for unshielded twisted-pair cable is 100 meters. The most common configuration is for each workstation to be connected by twisted-pair cable to a multi-port active concentrator. The concentrator amplifies all the signals that pass through it allowing for the total length of cable on the network to exceed the 100meter limit.

Bridges:

A bridge is a device that allows you to segment a large network into two smaller, more efficient networks. If you are adding to an older wiring scheme and want the new network to be up-to-date, a bridge can connect the two.

A bridge monitors the information traffic on both sides of the network so that it can pass packets of information to the correct location. Most bridges can “listen” to the network and automatically figure out the address of each computer on both sides of the bridge. The bridge can inspect each message and, if necessary, broadcast it on the other side of the network. The bridge manages the traffic to maintain optimum performance on both sides of the network. You might say that the bridge is like a traffic cop at a busy intersection during rush hour. It keeps information flowing on both sides of the network, but it does not allow unnecessary traffic through. Bridges can be used to connect different types of cabling, or physical topologies. They must, however, be used between networks with the same protocol.

Routers:

A router translates information from one network to another; it is similar to a super intelligent bridge. Routers select the best path to route a message, based on the destination address and when to direct traffic to prevent head-on collisions, and is smart enough to know when to direct traffic along back roads and shortcuts.

While bridges know the addresses of all computers on each side of the network, routers know the addresses of computers, bridges, and other routers on the network. Routers can even “listen” to the entire network to determine which sections are busiest – they can then redirect data around those sections until they clear up.

If you have a school LAN that you want to connect to the internet, you will need to purchase a router. In this case, the router serves as the translator between the information on your LAN and the internet. It also determines the best route to send the data over the internet.

Routers can:

·         Direct signal traffic efficiently

·         Route messages between any two protocols

·         Route messages between linear bus, star, and star wired ring topologies

·         Route messages across fibre optic, coaxial, and twisted-pair cabling

Switch:

A concentrator is a device that provides a central connection point for cables from work station, servers and peripherals. In a star topology, twisted pair wire is run from each work station to a central switch/hub. Most Switches are active.