SYSTEM ADMINISTRATION Chapter 2 The OSI Model

The OSI Model was designed by the International Standards Organization (ISO) as a structural framework for the rules of data communications. 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 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 The physical topology of the network defines the way the network is laid out, including: –Cabling (star, bus, mesh, ring, and wireless) –Interconnection of segments –Devices

Star The star topology is the most commonly used physical layout for networks today. When building a star topology, a hub or switch will be the central device connecting all nodes on the segment. When a node failure occurs, there are three places where the failure can take place: the port on the hub or switch, the cable connecting the node and the hub or switch, or the network interface on the node itself. A star topology is relatively easy to set up, and can be expanded so long as the physical limitations are not exceeded.

Bus The bus topology is the easiest to install and the most difficult to troubleshoot. Cables must be terminated. The bus topology has strict physical limitations of cable length and distance from other nodes.

Mesh Mesh networks provide redundant links between all nodes on the network, thus creating a fault-tolerant data communications topology. The mesh network is time-consuming and somewhat expensive to install because of the multiple connections between nodes. Troubleshooting mesh networks is relatively easy because the failure points are easily identified.

Ring The ring topology connects nodes in a circle, allowing only nodes directly connected to talk to each other. Data is transmitted in a single direction only. A node can accept and respond to packets addressed to it, then pass the packets on to the next node on the ring.

Wireless A wireless topology relies on either radio frequency (RF) or infrared (IR) frequencies, or channels, to connect directly to each other or to access points (APs). Wireless communication devices connect, or network, using either ad hoc or infrastructure networking.

Connection Types Point-to-point connections occur when two computers are connected and exchange information. An example is modem connections. Multipoint connections use many devices connected to transmission media, sharing the available bandwidth. The corporate network is an example.

Signal Types There are two broad categories of signal types: digital and analog. Analog signals use constantly varying voltages or waves. Digital signals use electrical pulses. All signals are subject to deterioration of signal over distance or attenuation. Each category of signal has a set of specific encoding schemes that allow for the efficient transmission of the data.

Baseband vs. 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. Broadband transmission requires that many signals share the same bandwidth. Broadband traffic is more efficient for data transmission.

Bit Synchronization 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. Broadband transmission requires that many signals share the same bandwidth. Broadband traffic is more efficient for data transmission.

The Datalink Layer The Datalink layer is subdivided into two sub-layers 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. Encryption/decryption are managed at this layer. Compression/decompression of data is managed at this layer.

The Application Layer The Application layer provides services to software applications used by a user. Some of the services include file access services, printing services, services, file transfer services, and file management services.