Chapter 2: The OSI Model
Chapter 2 Outline
- On the Test
2.1. Given an example, identify a MAC address.
2.2. Identify the seven layers of the OSI Model and their functions
2.4. Identify the OSI layers at which the following network components operate: hubs; switches; bridges; routers; network interface cards.
- The OSI Model
- The OSI Model was designed by the International Standards Organization (ISO) as a structural framework for the rules of data communication.
- This model allows any vendor who adheres to the framework to design a component for data communications with the assurance that it can be used with products from any other vendor and provide seamless interoperability.
- The Seven Layers
- Each layer of the OSI Model adds information to the data packet so that it can be passed to the next layer in the model
- Every layer is responsible for a part of the communications process.
- To remember the seven layers, there are two mnemonics:
- “All People Seem To Need Data Processing”
- “Please Do Not Throw Sausage Pizza Away”
- The Physical Layer
- The Physical layer defines the way data is transmitted over the physical connections of the network.
- Other component characteristics of this layer include physical topology, connection types, signal type, baseband and broadband, and bit synchronization.
- Physical Topology
- Bus: The bus topology is the easiest to install because all computers connect to the same piece of cable.
- Star
- The most common physical setup
- Requires a hub or switch and is easy to troubleshoot and repair
- Ring
- Connects nodes in a circle where the nodes can only “talk” to two other nodes – the one immediately preceding on the ring and the one immediately succeeding on the ring.
- Data moves in a single direction.
- A node can only “talk” if it has a token in its possession.
- Mesh
- All nodes are physically connected to all other nodes on the network.
- This is 100% fault tolerant.
- Wireless
- Wireless is rapidly emerging in the network communications market.
- Wireless systems rely on either radio frequency (RF) or infrared (IR) frequencies to connect directly to each other or to access points (AP).
- Wireless devices connect using either ad hoc mode or infrastructure (multipoint networking).
- Ad Hoc Mode
- In ad hoc mode, devices send out radio waves that can be intercepted by other wireless devices whenever they move within a certain distance range of each other.
- The network is created as needed. Network association can be changed by moving the location of the device closer to another wireless device.
- Infrastructure (Multipoint networking)
- Devices connect to a fixed network through a wireless bridge functioning as an AP.
- Usually the AP is wired to the network backbone and serves essentially the same purpose as a switch or hub in a wired network.
- Connection Types
- Point-to-point connections occur when two computers are connected to each other.
- Multipoint connections occur when many devices are connected to the network or transmission media.
- Signal Types
- Analog signaling uses a continuously varying voltage or signal. Signaling types include Amplitude Shift Keying, Frequency Shift Keying, and Phase Shift Keying.
- Digital signaling uses an electrical pulse to transmit data. Signaling types include current-state encoding and state-transition encoding.
- Baseband versus Broadband
- Baseband and broadband refer to the way the signals are passed across the media.
- Baseband transmission allows a single transmission to use the entire bandwidth, while broadband transmission requires that many signals share the same bandwidth.
- Bit Synchronization
- Bit synchronization is the timing function that allows devices to understand the beginning and end of a transmission.
- Asynchronous transmissions require that a start bit begin the transmission and a stop bit close the transmission.
- Synchronous transmissions use a clocking signal to identify the beginning and ending of a transmission.
- The Data Link Layer
- The Data Link layer is subdivided into two sublayers known as the logical link control layer (LLC) and the media access control layer (MAC).
- The LLC is responsible for the standards that govern how network communication will take place.
- The MAC layer maintains the physical addressing scheme used by network-connected nodes, allowing each node to be uniquely identified as a participant on the network.
- The Network Layer
- The Network layer is responsible for two important functions: logical addressing of nodes on the network or internetwork and routing of packets from source to destination.
- The logical addressing scheme used most frequently is IP addressing, a binary method for building large numbers of unique addresses.
- Routing is a store and forward action that allows the best path to be chosen when sending a packet from source node to a destination node.
- The Transport Layer
- The Transport layer makes sure that the data is transmitted reliably between nodes.
- This layer also segments large packets based on the type of network.
- Each segment is given a sequence number so that the receiving node can recreate the message correctly.
- Transport layer function also includes flow control, or the management of the rate of data transmission.
- To maintain reliable delivery of segments, this layer will issue either an ACK (acknowledgement) or a request for retransmission if errors are detected.
- The Session Layer: Session layer functions include the management of sessions or conversations between nodes.
- The Presentation Layer
- The Presentation layer makes sure that both nodes understand in what format the data will arrive.
- Its function is that of translator when two different data formats are present.
c.Encryption/decryption are managed at this layer.
d.Compression/decompression of data is managed at this layer.
15.The Application Layer
a.The Application layer provides services to software applications used by a user.
b.Some of the services include file access services, printing services, e-mail services, file transfer services, and file management services.