OSI Model Concepts PDF

1.2.1 Open Systems Interconnection Model The International Organization for Standardization (ISO) developed the Open Systems Interconnection (OSI) reference model () to promote understanding of how components in a network system work. It does this b separating the functions of hardware and software...

1.2.1 Open Systems Interconnection Model The International Organization for Standardization (ISO) developed the Open Systems Interconnection (OSI) reference model () to promote understanding of how components in a network system work. It does this b separating the functions of hardware and software components into seven d layers. Each layer performs a different group of tasks required for network communication. Description The seven layers from bottom to top are as follows: 1. Physical. 2. Data link.  3. Network. 4. Transport. 5. Session. 6. Presentation. 7. Application. The OSI model. Although not all network systems implement layers using this precise struct all implement each task in some way. The OSI model is not a standard or a specification; it serves as a functional guideline for designing network proto software, and appliances and for troubleshooting networks. Copyright © The Computing Technology Industry Association, Inc. All rights reserved. 1.2.2 Data Encapsulation and Decapsulation A network protocol is a set of rules for exchanging data in a structured form network protocol has two principal functions: Addressing—Describing where data messages should go. At each OS layer, there are different mechanisms for identifying nodes and rules they can send and receive messages. Encapsulation—Describing how data messages should be packaged transmission. Encapsulation is like an envelope for a letter, with the d that each layer requires its own envelope. At each layer, the protocol a fields in a header to whatever payload data it receives from an applica other protocol. A network will involve the use of many different protocols operating at diffe layers of the OSI model. At each layer, for two nodes to communicate they m running the same protocol. The protocol running at each layer communicat its peer layer on the other node. This communication between nodes at the s layer is described as a same layer interaction. To transmit or receive a communication, on each node, each layer provides services for the layer abo uses the services of the layer below. This is referred to as adjacent layer inte  Description Two users: A and B. The data, 0 1 1 0 1 0 1 0 0 1 0 1 from user A is encry and sent to user B, where the data is decrypted. The headers of user A a user B for each layer are as follows. Physical: 0 1 1 0 1 0 1 0 0 1 0 1. Data D L, N, T, S, P, and A. Network: N, T, S, P, and A. Transport: T, S, P, and A. Session: S, P, and A. Presentation: P and A. Application: A. Each layer has data along with the headers. Encapsulation and decapsulation. (Images © 123RF.com.) When a message is sent from one node to another, it travels down the stack on the sending node, reaches the receiving node using the transmission me then passes up the stack on that node. At each level (except the Physical laye sending node adds a header to the data payload, forming a “chunk” of data protocol data unit (PDU). This is the process of encapsulation. For example, on the sending node, data is generated by an application, such HyperText Transfer Protocol (HTTP), which will include its own application he the Transport layer, a Transmission Control Protocol (TCP) header is added t  application data. At the Network layer, the TCP segment is wrapped in an Int Protocol (IP) header. The IP packet is encapsulated in an Ethernet frame at t Link layer, then the stream of bits making up the frame is transmitted over t network at the Physical layer as a modulated electrical signal. The receiving node performs the reverse process, referred to as decapsulati receives the stream of bits arriving at the Physical layer and decodes an Ethe frame. It extracts the IP packet from this frame and resolves the information header, then does the same for the TCP and Application headers, eventually extracting the HTTP application data for processing by a software program, web browser or web server. You might notice that this example seems to omit some OSI layers. Th because "real-world" protocols do not conform exactly to the OSI mod Copyright © The Computing Technology Industry Association, Inc. All rights reserved. 1.2.3 Layer 1 - Physical The Physical layer (PHY) of the OSI model is defined as layer 1. The Physica responsible for the transmission and receipt of the signals that represent bit data. Transmission media can be classified as cabled or wireless: Cabled—A physical signal conductor is provided between two nodes. Examples include copper or fiber optic cable types. Cabled media can described as bounded media. Wireless—Uses free space between nodes, such as microwave radio. media can also be described as unbounded media. The Physical layer specifies the following: Physical topology—The layout of nodes and links as established by t transmission media. An area of a larger network is called a segment. A network is typically divided into segments to cope with the physical restrictions of the network media used, to improve performance, or to improve security. At the Physical layer, a segment is where all the nod access to the same media. Physical interface—Mechanical specifications for the network mediu cabled media, this means the construction of the cable, the interface/connector form factor, and the number and functions of the connector. For wireless media, it means radio transceiver and antenna specifications. Signaling—The process of transmitting and receiving encoded data o network medium. A modulation scheme describes how electrical, ligh radio signals represent bits. Timing and synchronization schemes ens senders and receivers can identify groups of signals as a chunk or fra data. Devices that operate at the Physical layer include the following: Transceiver—The part of a network interface that sends and receives over the network media.  Repeater—A device that amplifies an electronic signal to extend the m allowable distance for a media type. Hub—A multiport repeater, deployed as the central point of connectio nodes. Media converter—A device that converts one media signaling type to another. Copyright © The Computing Technology Industry Association, Inc. All rights reserved. 1.2.4 Layer 2 - Data Link Layer 2 is referred to as the Data Link layer. It is responsible for transferrin between nodes on the same logical segment. At the Data Link layer, a segm one where all nodes can send traffic to one another using hardware address regardless of whether they share access to the same media. A layer 2 segme include multiple physical segments. This is referred to as a logical topology. Local networks do not typically connect hosts directly with point to point or links. To reduce cabling and interface costs, each host is connected to a cent such as a switch or a wireless access point. The central node provides a forw function, receiving the communication from one node and sending it to ano do this, each node interface must have a Data Link layer address. The addre interfaces within the same layer 2 segment are described as local addresses hardware addresses. The Data Link layer also performs an encapsulation function. It organizes th of bits arriving from the Physical layer into structured units called frames. E frame contains a Network layer packet as its payload. The Data Link layer ad control information to the payload in the form of header fields. These fields source and destination hardware addresses, plus a basic error check to test frame was received intact.  Description The steps are as follows. 1. At the data link layer, each network interface is identified by a hardwa address, represented as A A on this host. 2. When A A sends a frame to A C, the frame travels along the cable to a switch. 3. The switch keeps track of which local addresses are connected to its interfaces and so forwards the frame out of port G 2. 4. The receiving host recognizes a frame addressed to A C and so proces it. A dashed arrow from host A A points to G 0 of the switch. A dashed a from G 2 of switch points to host A C. Communications at layer 2 of the OSI model. (Images © 123RF.com.)  Devices that operate at the Data Link layer include the following: Network adapter or network interface card (NIC)—A NIC joins an e system host to network media (cabling or wireless) and enables it to communicate over the network by assembling and disassembling fram Bridge—A bridge is a type of intermediate system that joins physical segments while minimizing the performance reduction of having mor on the same network. A bridge has multiple ports, each of which func network interface. Switch—An advanced type of bridge with many ports. A switch create between large numbers of nodes more efficiently. Wireless access point (AP)—An AP allows nodes with wireless netwo to communicate and creates a bridge between wireless networks and ones. Copyright © The Computing Technology Industry Association, Inc. All rights reserved. 1.2.5 Layer 3 - Network Layer 3 is the Network layer. This layer is responsible for moving data arou network of networks, known as an internetwork. While the Data Link layer is of forwarding data by using hardware addresses within a single segment, th Network layer moves information around an internetwork by using logical n and host IDs. The networks are often heterogeneous; that is, they use a vari Physical layer media and Data Link protocols. The main appliance working a is the router. Description The steps are as follows. 1. At the network layer, each interface is identified by an address with a network part (1. for this router interface) and a host part (254). 2. When host 1.2 wants to send to host 2.3, the packet must be delivere the remote network via routers.  3. Networks 1 and 2 are connected via an intermediate network k (9). 4. Router B recognizes that network 2 is directly connected and uses dat link protocols to send the packet to host 2.3. Communications at layer 3 of the OSI model. (Images © 123RF.com.) At layer 3, each packet is given a destination network address. Routers are configured with information about how to reach these different logical netw packet is forwarded, router by router (or hop by hop), through the internetw the target network. Once it has reached the destination network, the hardw address can be used to deliver the packet to the target node. The general convention is to describe PDUs packaged at the Network as packets or datagrams and messages packaged at the Data Link lay frames. Packet is often used to describe PDUs at any layer, however. It is usually important for traffic passing between networks to be filtered. A firewall operates at layer 3 to enforce an access control list (ACL). A netwo a list of the addresses and types of traffic that are permitted or blocked. Copyright © The Computing Technology Industry Association, Inc. All rights reserved. 1.2.6 Layer 4 - Transport The first three layers of the OSI model are primarily concerned with moving and datagrams between nodes and networks. At the Transport layer—also as the end-to-end or host-to-host layer—the content of the packets become significant. Any given host on a network will be communicating with many o hosts using many different types of networking data. One of the functions o Transport layer is to identify each type of network application by assigning it number. For example, data requested from an HTTP web application can be identified as port 80, while data sent to an email server can be identified as At the Transport layer, on the sending host, data from the upper layers is pa as a series of layer 4 PDUs, referred to as segments. Each segment is tagged application's port number. The segment is then passed to the Network layer delivery. Many different hosts could be transmitting multiple HTTP and ema at the same time. These are multiplexed using the port numbers along with source and destination network addresses onto the same link. Description The steps are as follows. 1. Hosts 2.2 and 2.3 send different types of traffic to host 2.1.  2. The traffic is sent over the network as layer 3 packets encapsulated in layer 2 frames. 3. Host 2.1 identifies each type of data from its port number and passes the relevant application for processing. Communications at layer 4 (Transport layer) of the OSI model. (Images © 123RF.com) At the Network and Data Link layers, the port number is ignored—it become the data payload and is invisible to the routers and switches that implement addressing and forwarding functions of these layers. At the receiving host, e segment is decapsulated, identified by its port number, and passed to the re handler at the Application layer. Put another way, the traffic stream is de- multiplexed. The Transport layer can also implement reliable data delivery mechanisms, s the application require it. Reliable delivery means that any lost or damaged are resent. Devices working at the Transport layer include multilayer switches—usually as load balancers—and many types of security appliances, such as more adv firewalls and intrusion detection systems (IDSs). Copyright © The Computing Technology Industry Association, Inc. All rights reserved. 1.2.7 Upper Layers The upper layers of the OSI model are less clearly associated with distinct re protocols. These layers collect various functions that provide useful interface between software applications and the Transport layer. Layer 5—Session Most application protocols require the exchange of multiple messages betw client and server. This exchange of such a sequence of messages is called a or dialog. The Session layer (layer 5) represents functions that administer t process of establishing a dialog, managing data transfer, and then ending (o down) the session. Layer 6—Presentation The Presentation layer (layer 6) transforms data between the format requ the network and the format required for the application. For example, the Presentation layer is used for character set conversion, such as between Am Standard Code for Information Interchange (ASCII) and Unicode. The Presentation layer can also be conceived as supporting data compression and encryption. However, in practical terms, encryption implemented by devices and protocols running at lower layers of the or simply within a homogenous Application layer. Layer 7—Application The Application layer (layer 7) is at the top of the OSI stack. An Application protocol doesn't encapsulate any other protocols or provide services to any Application layer protocols provide an interface for software programs on ne  hosts that have established a communications channel through the lower-le protocols to exchange data. More widely, upper-layer protocols provide most of the services that make a useful, rather than just functional, including web browsing, email and communications, directory lookup, remote printing, and database services. Copyright © The Computing Technology Industry Association, Inc. All rights reserved. 1.2.8 OSI Model Summary The following image summarizes the OSI model, listing the PDUs at each lay with the types of devices that work at each layer. Description The devices and concepts for each layer are as follows. 1. Physical: transceiver, cable, media converter, and hub. 2. Data link: frame, M A C address or E U I, bridge, and switch. Network adapter connects frame a transceiver. Access point lies between physical and data link layers. 3. Network: datagram, I P address, basic firewall, and router. 4. Transport: segment. 5, 6, and 7. Session, presentation, and application: application protocols (web, email, file transfer), stateful or application layer security appliance, and multilayer switch.  Devices and concepts represented at the relevant OSI model layer. Copyright © The Computing Technology Industry Association, Inc. All rights reserved.