COMPUTER NETWORKS NOTES

COMPUTER NETWORKS
IT-305 E
COURSE MATERIAL
UNIT 1:OSI REFRENCE MODEL & NETWORK ARCHITECTURE
LECTURE NO.1
INTRODUCTION TO COMPUTER NETWORKS:
“Computer communications” refers to the electrical transmission of data from one system to another,it describes the manner in which computers exchange information with each other.
“Networking” refers to the concept of connecting a group of systems for the express purpose of sharing information.
A computer network is a group of interconnected computers. In the world of Computer Networks:
· The connected entities of a network are called computers or other devices.
· The link through which communications takes place is called a Network medium.
· The rules that govern the manner in which data are exchanged between devices are achieved through a common network protocol.
“A Computer Network is a collection of computers & other devices that use a common network protocol to share resources with each other over a network medium”
A computer network is any set of computers or devices connected to each other with the ability to exchange data.[2] Examples of different networks are:
Local area network (LAN), which is usually a small network constrained to a small geographic area.
Wide area network (WAN) that is usually a larger network that covers a large geographic area.
Wireless LANs and WANs (WLAN & WWAN) are the wireless equivalent of the LAN and WAN.
All networks are interconnected to allow communication with a variety of different kinds of media, including twisted-pair copper wire cable, coaxial cable, optical fiber, and various wireless technologies. The devices can be separated by a few meters (e.g. via Bluetooth) or nearly unlimited distances (e.g. via the interconnections of the Internet).

ARPANET: In the mid 1960’s,the mainframe computers in research organisations were standalone devices.Computers from different manufactuers were unable to communicate with each other.The Advanced research Projects agency in the Department of defense was intrested in finding a way to connect computers so that the researchers they funded could share their findings.
In 1967, at an association for computing machinery meeting,ARPA presents its ideas for ARPANET,a small network of connected computers.The idea was that each host computer would be attached to a specialized computer called an Interface Message Processor(IMP).The IMP’s would be in turn be connected to one another.Each IMP had to be able to communicate with other IMP’s as well as with its own attached host.
In 1969, work began on the ARPAnet, grandfather to the Internet. Designed as a computer version of the nuclear bomb shelter, ARPAnet protected the flow of information between military installations by creating a network of geographically separated computers that could exchange information via a newly developed protocol (rule for how computers interact) called NCP (Network Control Protocol). One opposing view to ARPAnet's origins comes from Charles M. Herzfeld, the former director of ARPA. He claimed that ARPAnet was not created as a result of a military need, stating "it came out of our frustration that there were only a limited number of large, powerful research computers in the country and that many research investigators who should have access were geographically separated from them." ARPA stands for the Advanced Research Projects Agency, a branch of the military that developed top secret systems and weapons during the Cold War. The first data exchange over this new network occurred between computers at UCLA and Stanford Research Institute. On their first attempt to log into Stanford's computer by typing "log win", UCLA researchers crashed their computer when they typed the letter 'g'.
Ø Four computers were the first connected in the original ARPAnet. They were located in the respective computer research labs of UCLA (Honeywell DDP 516 computer), Stanford Research Institute (SDS-940 computer), UC Santa Barbara (IBM 360/75), and the University of Utah (DEC PDP-10). As the network expanded, different models of computers were connected, creating compatibility problems. The solution rested in a better set of protocols called TCP/IP (Transmission Control Protocol/Internet Protocol) designed in 1982.






LECTURE NO 2

INTERNET:
The Internet is a global system of interconnected computer networks that interchange data by packet switching using the standardized Internet Protocol Suite (TCP/IP). It is a "network of networks" that consists of millions of private and public, academic, business, and government networks of local to global scope that are linked by copper wires, fiber-optic cables, wireless connections, and other technologies.
The Internet carries various information resources and services, such as electronic mail, online chat, file transfer and file sharing, online gaming, and the inter-linked hypertext documents and other resources of the World Wide Web (WWW)
The Internet is a specific internetwork. It consists of a worldwide interconnection of governmental, academic, public, and private networks based upon the networking technologies of the Internet Protocol Suite. It is the successor of the Advanced Research Projects Agency Network (ARPANET) developed by DARPA of the U.S. Department of Defense. The Internet is also the communications backbone underlying the World Wide Web (WWW). The 'Internet' is most commonly spelled with a capital 'I' as a proper noun, for historical reasons and to distinguish it from other generic internetworks.
Participants in the Internet use a diverse array of methods of several hundred documented, and often standardized, protocols compatible with the Internet Protocol Suite and an addressing system (IP Addresses) administered by the Internet Assigned Numbers Authority and address registries. Service providers and large enterprises exchange information about the reachability of their address spaces through the Border Gateway Protocol (BGP), forming a redundant worldwide mesh of transmission paths.
Private network
In Internet terminology, a private network is typically a network that uses private IP address space, following the standards set by RFC 1918 and RFC 4193. These addresses are common in home and office local area networks (LANs), as using globally routable addresses is seen as impractical or unnecessary. Private IP addresses were originally created due to the shortage of publicly registered IP addresses created by the IPv4 standard, but are also a feature of the next generation Internet Protocol, IPv6.
These addresses are private because they are not globally assigned, meaning they aren't allocated to a specific organisation--instead, any organisation needing private address space can use these addresses without needing approval from a regional Internet registry (RIR). Consequently, they are not routable on the public Internet, meaning that if such a private network wishes to connect to the Internet, it must use either a Network Address Translation (NAT) gateway, or a proxy server.
The most common use of these addresses is in home networks, since most Internet Service Providers (ISPs) only allocate a single IP address to each customer, but many homes have more than one networking device (for example, several computers, or a printer). In this situation, a NAT gateway is almost always used to provide Internet connectivity. They are also commonly used in corporate networks, which for security reasons, are not connected directly to the internet, meaning globally routable addresses are unnecessary. Often a proxy, SOCKS gateway, or similar is used to provide restricted internet access to internal users. In both cases, private addresses are seen as adding security to the internal network, since it's impossible for an Internet host to connect directly to an internal system.
Because many internal networks use the same private IP addresses, a common problem when trying to merge two such networks (e.g. during a company merger or takeover) is that both organisations have allocated the same IPs in their networks. In this case, either one network must renumber, often a difficult and time-consuming task, or a NAT router must be placed between the networks to translate one network's addresses before they can reach the other side.
It is not uncommon for private address space to "leak" onto the Internet in various ways. Poorly configured private networks often attempt reverse DNS lookups for these addresses, putting extra load on the Internet's root nameservers. The AS112 project mitigates this load by providing special "blackhole" anycast nameservers for private addresses which only return "not found" answers for these queries. Organisational edge routers are usually configured to drop ingress IP traffic for these networks, which can occur either by accident, or from malicious traffic using a spoofed source address. Less commonly, ISP edge routers will drop such ingress traffic from customers, which reduces the impact to the Internet of such misconfigured or malicious hosts on the customer's network.
A common misconception is that these addresses are not routable. However, while not routable on the public Internet, they are routable within an organisation or site.
The Internet Engineering Task Force (IETF) has directed IANA to reserve the following IPv4 address ranges for private networks, as published in RFC 1918:
RFC1918 name
IP address range
number of addresses
classful description
largest CIDR block (subnet mask)
host id size
24-bit block
10.0.0.0 – 10.255.255.255
16,777,216
single class A
10.0.0.0/8 (255.0.0.0)
24 bits
20-bit block
172.16.0.0 – 172.31.255.255
1,048,576
16 contiguous class Bs
172.16.0.0/12 (255.240.0.0)
20 bits
16-bit block
192.168.0.0 – 192.168.255.255
65,536
256 contiguous class Cs
192.168.0.0/16 (255.255.0.0)
16 bits
Note that classful addressing is obsolete and no longer used on the Internet. For example, while 10.0.0.0/8 would be a single class A network, it is not uncommon for organisations to divide it into smaller /16 or /24 networks.

Network Classification
The following list presents categories used for classifying networks.
Network topology
Computer networks may be classified according to the network topology upon which the network is based, such as Bus network, Star network, Ring network, Mesh network, Star-bus network, Tree or Hierarchical topology network. Network Topology signifies the way in which devices in the network see their logical relations to one another. The use of the term "logical" here is significant. That is, network topology is independent of the "physical" layout of the network. Even if networked computers are physically placed in a linear arrangement, if they are connected via a hub, the network has a Star topology, rather than a Bus Topology. In this regard the visual and operational characteristics of a network are distinct; the logical network topology is not necessarily the same as the physical layout. Networks may be classified based on the method of data used to convey the data, these include digital and analog networks
1. Bus network
A bus network topology is a network architecture in which a set of clients are connected via a shared communications line, called a bus. There are several common instances of the bus architecture, including one in the motherboard of most computers, and those in some versions of Ethernet networks.Bus networks are the simplest way to connect multiple clients, but may have problems when two clients want to transmit at the same time on the same bus. Thus systems which use bus network architectures normally have some scheme of collision handling or collision avoidance for communication on the bus, quite often using Carrier Sense Multiple Access(Carrier Sense Multiple Access (CSMA) is a probabilistic Media Access Control (MAC) protocol in which a node verifies the absence of other traffic before transmitting on a shared transmission medium, such as an electrical bus, or a band of the electromagnetic spectrum."Carrier Sense" describes the fact that a transmitter listens for a carrier wave before trying to send. That is, it tries to detect the presence of an encoded signal from another station before attempting to transmit. If a carrier is sensed, the station waits for the transmission in progress to finish before initiating its own transmission."Multiple Access" describes the fact that multiple stations send and receive on the medium. Transmissions by one node are generally received by all other stations using the medium.) or the presence of a bus master which controls access to the shared bus resource.
The bus topology makes the addition of new devices straightforward. The term used to describe clients is station or workstation in this type of network. Bus network topology uses a broadcast channel which means that all attached stations can hear every transmission and all stations have equal priority in using the network to transmit[1] data.
Advantages and disadvantages of a bus network
Advantages
Easy to implement and extend
Well suited for temporary or small networks not requiring high speeds (quick setup)
Cheaper than other topologies.
Cost effective as only a single cable is used
Cable faults are easily identified
Disadvantages
Limited cable length and number of stations.
If there is a problem with the cable, the entire network goes down.
Maintenance costs may be higher in the long run.
Performance degrades as additional computers are added or on heavy traffic.
Proper termination is required (loop must be in closed path).
Significant Capacitive Load (each bus transaction must be able to stretch to most distant link).
It works best with limited number of nodes.
It is slower than the other topologies.



2. Star networks are one of the most common computer network topologies. In its simplest form, a star network consists of one central switch, hub or computer, which acts as a conduit to transmit messages. Thus, the hub and leaf nodes, and the transmission lines between them, form a graph with the topology of a star. If the central node is passive, the originating node must be able to tolerate the reception of an echo of its own transmission, delayed by the two-way transmission time (i.e. to and from the central node) plus any delay generated in the central node. An active star network has an active central node that usually has the means to prevent echo-related problems.
The star topology reduces the chance of network failure by connecting all of the systems to a central node. When applied to a bus-based network, this central hub rebroadcasts all transmissions received from any peripheral node to all peripheral nodes on the network, sometimes including the originating node. All peripheral nodes may thus communicate with all others by transmitting to, and receiving from, the central node only. The failure of a transmission line linking any peripheral node to the central node will result in the isolation of that peripheral node from all others, but the rest of the systems will be unaffected


Advantages
Better performance: Passing of Data Packet through unnecessary nodes is prevented by this topology. At most 3 devices and 2 links are involved in any communication between any two devices which are part of this topology. This topology induces a huge overhead on the central hub, however if the central hub has adequate capacity, then very high network utilization by one device in the network does not affect the other devices in the network.
Isolation of devices: Each device is inherently isolated by the link that connects it to the hub. This makes the isolation of the individual devices fairly straightforward, and amounts to disconnecting the device from the hub. This isolated nature also prevents any non-centralized failure to affect the network.
Benefits from centralization: As the central hub is the bottleneck, increasing capacity of the central hub or adding additional devices to the star, can help scale the network very easily. The central nature also allows the inspection traffic through the network. This can help analyze all the traffic in the network and determine suspicious behavior.
Simplicity: The topology is easy to understand, establish, and navigate. The simple topology obviates the need for complex routing or message passing protocols. As noted earlier, the isolation and centralization simplifies fault detection, as each link or device can be probed individually.
Disadvantages
The primary disadvantage of a star topology is the high dependence of the system on the functioning of the central hub. While the failure of an individual link only results in the isolation of a single node, the failure of the central hub renders the network inoperable, immediately isolating all nodes. The performance and scalability of the network also depend on the capabilities of the hub. Network size is limited by the number of connections that can be made to the hub, and performance for the entire network is capped by its throughput. While in theory traffic between the hub and a node is isolated from other nodes on the network, other nodes may see a performance drop if traffic to another node occupies a significant portion of the central node's processing capability or throughput. Furthermore, wiring up of the system can be very complex.
3. A ring network is a network topology in which each node connects to exactly two other nodes, forming a single continuous pathway for signals through each node - a ring. Data travels from node to node, with each node along the way handling every packet.
Because a ring topology provides only one pathway between any two nodes, ring networks may be disrupted by the failure of a single link. A node failure or cable break might isolate every node attached to the ring. FDDI networks overcome this vulnerability by sending data on a clockwise and a counterclockwise ring: in the event of a break data is wrapped back onto the complementary ring before it reaches the end of the cable, maintaining a path to every node along the resulting "C-Ring". 802.5 networks -- also known as IBM Token Ring networks -- avoid the weakness of a ring topology altogether: they actually use a star topology at the physical layer and a Multistation Access Unit to imitate a ring at the datalink layer.
Advantages
Very orderly network where every device has access to the token and the opportunity to transmit
Performs better than a star topology under heavy network load
Can create much larger network using Token Ring
Does not require network server to manage the connectivity between the computers
Disadvantages
One malfunctioning workstation or bad port in the MAU can create problems for the entire network
Moves, adds and changes of devices can affect the network
Network adapter cards and MAU's are much more expensive than Ethernet cards and hubs
Much slower than an Ethernet network under normal load



LECTURE NO 3

4. Mesh networking is a way to route data, voice and instructions between nodes. It allows for continuous connections and reconfiguration around broken or blocked paths by “hopping” from node to node until the destination is reached. A mesh network whose nodes are all connected to each other is a fully connected network. Mesh networks differ from other networks in that the component parts can all connect to each other via multiple hops, and they generally are not mobile. Mesh networks can be seen as one type of ad hoc network. Mobile ad-hoc networks (MANETs) and mesh networks are therefore closely related, but MANETs also have to deal with the problems introduced by the mobility of the nodes.Mesh networks are self-healing: the network can still operate even when a node breaks down or a connection goes bad. As a result, a very reliable network is formed. This concept is applicable to wireless networks, wired networks, and software interaction. Wireless mesh networks is the most topical application of mesh architectures. Wireless mesh was originally developed for military applications but have undergone significant evolution in the past decade. As the cost of radios plummeted, single radio products evolved to support more radios per mesh node with the additional radios providing specific functions- such as client access, backhaul service or scanning radios for high speed handover in mobility applications. The mesh node design also became more modular - one box could support multiple radio cards - each operating at a different frequency
5. Tree Topology
Tree topologies integrate multiple star topologies together onto a bus. In its simplest form, only hub devices connect directly to the tree bus, and each hub functions as the "root" of a tree of devices. This bus/star hybrid approach supports future expandability of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the number of hub connection points) alone
6.Hybrid topology: Hybrid network topologies
The hybrid topology is a type of network topology that is composed of one or more interconnections of two or more networks that are based upon the same physical topology, but where the physical topology of the network resulting from such an interconnection does not meet the definition of the original physical topology of the interconnected networks (e.g., the physical topology of a network that would result from an interconnection of two or more networks that are based upon the physical star topology might create a hybrid topology which resembles a mixture of the physical star and physical bus topologies or a mixture of the physical star and the physical tree topologies, depending upon how the individual networks are interconnected, while the physical topology of a network that would result from an interconnection of two or more networks that are based upon the physical distributed bus network retains the topology of a physical distributed bus network).
Ø Star-bus
A type of network topology in which the central nodes of one or more individual networks that are based upon the physical star topology are connected together using a common 'bus' network whose physical topology is based upon the physical linear bus topology, the endpoints of the common 'bus' being terminated with the characteristic impedance of the transmission medium where required – e.g., two or more hubs connected to a common backbone with drop cables through the port on the hub that is provided for that purpose (e.g., a properly configured 'uplink' port) would comprise the physical bus portion of the physical star-bus topology, while each of the individual hubs, combined with the individual nodes which are connected to them, would comprise the physical star portion of the physical star-bus topology.

Ø Hybrid mesh
A type of hybrid physical network topology that is a combination of the physical partially connected topology and one or more other physical topologies the mesh portion of the topology consisting of redundant or alternate connections between some of the nodes in the network – the physical hybrid mesh topology is commonly used in networks which require a high degree of availability.



















LECTURE NO. 4
Types of networks
1.Personal Area Network (PAN)
A Personal Area Network (PAN) is a computer network used for communication among computer devices close to one person. Some examples of devices that are used in a PAN are printers, fax machines, telephones, PDAs and scanners. The reach of a PAN is typically about 20-30 feet (approximately 6-9 meters), but this is expected to increase with technology improvements.
2. Local Area Network (LAN)
A Local Area Network (LAN) is a computer network covering a small physical area, like a home, office, or small group of buildings, such as a school, or an airport. This is a network covering a small geographic area, like a home, office, or building. Current LANs are most likely to be based on Ethernet technology. For example, a library may have a wired or wireless LAN for users to interconnect local devices (e.g., printers and servers) and to connect to the internet. On a wired LAN, PCs in the library are typically connected by category 5 (Cat5) cable, running the IEEE 802.3 protocol through a system of interconnected devices and eventually connect to the Internet..
3.Metropolitan Area Network (MAN)
A Metropolitan Area Network (MAN) is a network that connects two or more Local Area Networks or Campus Area Networks together but does not extend beyond the boundaries of the immediate town/city. Routers, switches and hubs are connected to create a Metropolitan Area Network.
4.Wide Area Network (WAN)
A Wide Area Network (WAN) is a computer network that covers a broad area (i.e., any network whose communications links cross metropolitan, regional, or national boundaries [1]). Less formally, a WAN is a network that uses routers and public communications links [1]. Contrast with personal area networks (PANs), local area networks (LANs), campus area networks (CANs), or metropolitan area networks (MANs) which are usually limited to a room, building, campus or specific metropolitan area (e.g., a city) respectively. The largest and most well-known example of a WAN is the Internet. A WAN is a data communications network that covers a relatively broad geographic area (i.e. one city to another and one country to another country) and that often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer.
5. Internetwork
A Internetworking involves connecting two or more distinct computer networks or network segments via a common routing technology. The result is called an internetwork (often shortened to internet). Two or more networks or network segments connected using devices that operate at layer 3 (the 'network' layer) of the OSI Basic Reference Model, such as a router. Any interconnection among or between public, private, commercial, industrial, or governmental networks may also be defined as an internetwork.
In modern practice, the interconnected networks use the Internet Protocol. There are at least three variants of internetwork, depending on who administers and who participates in them:
Intranet
Extranet
Internet
Intranets and extranets may or may not have connections to the Internet. If connected to the Internet, the intranet or extranet is normally protected from being accessed from the Internet without proper authorization. The Internet is not considered to be a part of the intranet or extranet, although it may serve as a portal for access to portions of an extranet.
Intranet:An intranet is a set of networks, using the Internet Protocol and IP-based tools such as web browsers and file transfer applications, that is under the control of a single administrative entity. That administrative entity closes the intranet to all but specific, authorized users. Most commonly, an intranet is the internal network of an organization. A large intranet will typically have at least one web server to provide users with organizational information.
Extranet:An extranet is a network or internetwork that is limited in scope to a single organization or entity but which also has limited connections to the networks of one or more other usually, but not necessarily, trusted organizations or entities (e.g. a company's customers may be given access to some part of its intranet creating in this way an extranet, while at the same time the customers may not be considered 'trusted' from a security standpoint). Technically, an extranet may also be categorized as a CAN, MAN, WAN, or other type of network, although, by definition, an extranet cannot consist of a single LAN; it must have at least one connection with an external network.
Internet: The Internet is a specific internetwork. It consists of a worldwide interconnection of governmental, academic, public, and private networks based upon the networking technologies of the Internet Protocol Suite. It is the successor of the Advanced Research Projects Agency Network (ARPANET) developed by DARPA of the U.S. Department of Defense. The Internet is also the communications backbone underlying the World Wide Web (WWW). The 'Internet' is most commonly spelled with a capital 'I' as a proper noun, for historical reasons and to distinguish it from other generic internetworks.
Participants in the Internet use a diverse array of methods of several hundred documented, and often standardized, protocols compatible with the Internet Protocol Suite and an addressing system (IP Addresses) administered by the Internet Assigned Numbers Authority and address registries. Service providers and large enterprises exchange information about the reachability of their address spaces through the Border Gateway Protocol (BGP), forming a redundant worldwide mesh of transmission paths.





















LECTURE NO. 5
OSI MODEL:
The Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer network protocol design. It was developed as part of the Open Systems Interconnection (OSI) initiative.In its most basic form, it divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model.
v A layer is a collection of conceptually similar functions that provide services to the layer above it and receives service from the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path.
Description of OSI layers
OSI Model

Data unit
Layer
Function
Hostlayers
Data
7. Application
Network process to application
6. Presentation
Data representation and encryption
5. Session
Interhost communication
Segment
4. Transport
End-to-end connections and reliability
Medialayers
Packet
3. Network
Path determination and logical addressing
Frame
2. Data Link
Physical addressing (MAC & LLC)
Bit
1. Physical
Media, signal and binary transmission
Layer 7: Application Layer
The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).
Layer 6: Presentation Layer
The Presentation Layer establishes a context between Application Layer entities, in which the higher-layer entities can use different syntax and semantics, as long as the Presentation Service understands both and the mapping between them. The presentation service data units are then encapsulated into Session Protocol Data Units, and moved down the stack.
This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.
Layer 5: Session Layer
The Session Layer controls the dialogues/connections (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for "graceful close" of sessions, which is a property of TCP, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The Session Layer is commonly implemented explicitly in application environments that use remote procedure calls (RPCs).it offers various services, including
1. dialog control:The session layer allows two systems to enter into a dialog.It allows the communication between two process to take place either in half duplex or full duplex mode.
2. synchronization:It allows a process to add checkpoints or synchronization points to a stream of data.for eg.,if a system is sending a file of 2000 pages,it is advisable to insert checkpoints after every 100 pages to ensure that eacg 100 page unit is received & acknowleged indepedently..















LECTURE NO. 6
Layer 4: Transport Layer
The Transport Layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. It is responsible for process to process delivery of entire message. It ensures that whole message arrives intact & in order,overseeing both error & flow control at the source to destination level.The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the Transport Layer can keep track of the segments and retransmit those that fail.
1.Service Point Addressing: Computers often run several programs at the same time. For this reason, source to destination delivery means delivery not only from one computer to the next but also from a specific process on one computer to a specific process on other computer .The transport layer header must therefore include a type of address called port address .The transport layer gets the entire message to the correct process or computer.
2. Segmentation& reassemble: A message is divided into transmittable segments with each segment containing a sequence no. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination & to identify &replace packets thatt were lost in transmission.
3.Connection Control :It can be either connectionless or connection-oriented.A connection less Transport layer treat each segment as an independent packet & delivers it to transport layer at destination machine. A connection oriented Transport layer makes a connection with the transport layer at the destination machine before delievering the packets.After all data is transferred & connection is terminated.
4. Flow control: It is performed end to end rather than single link.
5. Error control: t is performed process tp process rather than single link.The sending transport layer makes sure that the entire message arrives at the receiving tranport layer without error(damage,loss or duplication).it is achieved through retransmission.

Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the Transport Layer, the best known examples of a Layer 4 protocol are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
Layer 3: Network Layer
The Network Layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks, while maintaining the quality of service requested by the Transport Layer. The Network Layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer—sending data throughout the extended network and making the Internet possible.The best-known example of a Layer 3 protocol is the Internet Protocol (IP). It manages the connectionless transfer of data one hop at a time, from end system to ingress router, router to router, and from egress router to destination end system. It is not responsible for reliable delivery to a next hop, but only for the detection of errored packets so they may be discarded. When the medium of the next hop cannot accept a packet in its current length, IP is responsible for fragmenting into sufficiently small packets that the medium can accept it.
1.Logical addressing: The physical addressing implemented by data link layer handles the addressing problem locally.the network layer adds a header to the packet coming from the upper layer includes logical address of sender & reciever.
2.Routing: When independent networks are connected to create internetworks,the router route the packet to final destination.
Layer 2: Data Link Layer
The Data Link Layer is the second layer in the OSI model, above the Physical Layer, which ensures that the error free data is transferred between the adjacent nodes in the network.
1. Framing :It breaks the datagrams passed down by above layers and convert them into frames ready for transfer. This is called Framing. It provides two main functionalities
Reliable data transfer service between two peer network layers
Flow Control mechanism which regulates the flow of frames such that data congestion is not there at slow receivers due to fast senders.
2. Error Control
The bit stream transmitted by the physical layer is not guaranteed to be error free. The data link layer is responsible for error detection and correction. The most common error control method is to compute and append some form of a checksum to each outgoing frame at the sender's data link layer and to recompute the checksum and verify it with the received checksum at the receiver's side. If both of them match, then the frame is correctly received; else it is erroneous. The checksums may be of two types: # Error detecting : Receiver can only detect the error in the frame and inform the sender about it. # Error detecting and correcting : The receiver can not only detect the error but also correct it.
3. Flow Control
Consider a situation in which the sender transmits frames faster than the receiver can accept them. If the sender keeps pumping out frames at high rate, at some point the receiver will be completely swamped and will start losing some frames. This problem may be solved by introducing flow control. Most flow control protocols contain a feedback mechanism to inform the sender when it should transmit the next frame. 4. Access Control: When two or more devices are connected to the same link,data link layer protocols are necessary to determine which device has control over the link at any given time.

5. Physical addressing: If frames are to be distributed to different systems on the network,the data link layer adds the header to the frame to define the sender or receiver of the frame.
Layer 1: Physical Layer
The Physical Layer is the first level in the seven-layer OSI model of computer networking. It translates communications requests from the Data Link Layer into hardware-specific operations to effect transmission or reception of electronic signals.
The Physical Layer is a fundamental layer upon which all higher level functions in a network are based. However, due to the plethora of available hardware technologies with widely varying characteristics, this is perhaps the most complex layer in the OSI architecture. The implementation of this layer is often termed PHY.
The Physical Layer defines the means of transmitting raw bits rather than logical data packets over a physical link connecting network nodes. The bit stream may be grouped into code words or symbols and converted to a physical signal that is transmitted over a hardware transmission medium. The Physical Layer provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes of the electrical connectors, which frequencies to broadcast on, which modulation scheme to use and similar low-level parameters are specified here.
List of Physical Layer services
The major functions and services performed by the Physical Layer are:
Bit-by-bit delivery
Providing a standardized interface to physical transmission media, including
Mechanical specification of electrical connectors and cables, for example maximum cable length
Electrical specification of transmission line signal level and impedance
Radio interface, including electromagnetic spectrum frequency allocation and specification of signal strength, analog bandwidth, etc.
Specifications for IR over optical fiber or a wireless IR communication link
Modulation
Line coding
Bit synchronization in synchronous serial communication
Start-stop signalling and flow control in asynchronous serial communication
Circuit mode multiplexing,[citation needed] as opposed to statistical multiplexing performed at the higher level
Establishment and termination of circuit switched connections
Carrier sense and collision detection utilized by some level 2 multiple access protocols
Equalization filtering, training sequences, pulse shaping and other signal processing of physical signals
Forward error correction,[citation needed] for example bitwise convolutional coding
Bit-interleaving and other channel coding
The Physical Layer is also concerned with
Point-to-point, multipoint or point-to-multipoint line configuration
Physical network topology, for example bus, ring, mesh or star network
Serial or parallel communication
Simplex, half duplex or full duplex transmission mode
Autonegotiation
Physical Layer examples
V.92 telephone network modems
IRDA Physical Layer
USB Physical Layer
Firewire
EIA RS-232, EIA-422, EIA-423, RS-449, RS-485
ITU Recommendations: see ITU-T
DSL
ISDN
T1 and other T-carrier links, and E1 and other E-carrier links
10BASE-T, 10BASE2, 10BASE5, 100BASE-TX, 100BASE-FX, 100BASE-T, 1000BASE-T, 1000BASE-SX and other varieties of the Ethernet physical layer
Varieties of 802.11
SONET/SDH