|  | 8.1 | Basic Networking Knowledge
| 8.1.8 | The OSI model and the protocols, services, functions that pertain to each layer
Application Layer The Application Layer (the seventh layer) of the OSI Model provides network services, which are closest to the user. Such programs as Internet Explorer, Netscape Communicator, Eudora Pro, and other end-user application software. This layer establishes communication with intended partners, synchronizes agreement on procedures for error recovery and control of data integrity. The protocols, which function on this layer, are Server Message Block (SMB) and Network Control Program (NCP). Services, which provide network access, include: Telnet and File Transfer Protocol (FTP) Trivial File Transfer Protocol (TFTP) Network File System (NFS) Simple Network Management Protocol (SNMP) Simple Mail Transfer Protocol (SMTP) Hyper Text Transfer Protocol (HTTP) Devices function up to this layer include hosts and gateways. Presentation Layer The Presentation Layer ensures that the information that the application layer of one system sends out is readable by the application layer of another system. If necessary, the presentation layer translates between multiple data formats by using a common format. The Presentation Layer also provides data encryption to ensure protection as data journeys through the network. When the encrypted data is received, it decrypts and formats the message before passing along to the Application Layer. Protocols include NCP. Data formats include ASCII, EBCDIC, encrypted, jpeg, gif, mpeg, quicktime, flash, wav, avi, and mp3. Devices functioning up to this layer include hosts and gateways. Session Layer The session layer establishes, manages, terminates sessions between two communicating hosts, and provides its services to the presentation layer. It also synchronizes dialogue between the two hosts and manages their data exchange. The Session Layer also offers provisions for efficient data transfer, class of service, security authorization, and exception reporting of session layer, presentation layer, and application layer problems. Three types of dialogs used in the Session Layer are simplex, half-duplex, and full-duplex. A simplex dialog allows information to flow from one device to another without requiring a reply transmission. Half-duplex, which is also known as a two-way alternate (TWA) transmission, allows data to flow in two directions from one device to another; however, each device cannot send a transmission until the previous signal has been completely received. When one device sends a transmission and requires the destination device to respond, the destination device must wait until the initial transmission is complete before it can send its response. Full-duplex, which is also known as a two-way simultaneous (TWS) transmission, allows devices to send data to another device without having to wait until the wire is clear. When a device transmits a signal, the destination device does have not to wait until the signal is complete to send a reply to the source device. Full-duplex enables two-way traffic to occur simultaneously during one communication session. A telephone is an example of full-duplex. Protocols include Network File System (NFS), Structured Query Language (SQL), Remote Procedure Call (RPC), X-Window System, AppleTalk Session Protocol (ASP), and Digital Network Architecture Session Control Protocol (DNA SCP). Devices functioning up to this layer include hosts and gateways. Transport Layer The Transport Layer segments data from the sending host's system and reassembles the data into a data stream on the receiving host's system. The boundary between the session layer and the transport layer can be thought of as the boundary between media-layer protocols and host-layer protocols. Whereas the application, presentation, and session layers are concerned with application issues, the lower three layers are concerned with data transport issues. The transport layer attempts to provide a data transport service that shields the upper layers from transport implementation details. Specifically, the main concern of the transport layer includes issues such as how reliable transport between two hosts is accomplished. In providing communication service, the transport layer establishes, maintains, and properly terminates virtual circuits. In providing reliable service, transport error detection-and-recovery and information flow controls are used. When the Transport Layer receives data from the upper layers, it breaks up the information into segments (smaller pieces) to be sent through the lower levels of the OSI Model and then to the destination device. Protocols used in this layer are: Sequenced Package Exchange (SPX) Transmission Control Protocol (TCP) User Datagram Protocol (UDP) NetBIOS Extended User Interface (NetBEUI) Services used at this layer use TCP to provide connection-oriented communication with error free delivery and UDP to provide connectionless communications without guaranteed packet delivery (unreliable delivery). Devices functioning up to this layer include hosts and gateways. Network Layer The network layer is a complex layer that provides connectivity and path selection between two host systems that may be geographically separated. Layer 3 can be remembered as addressing, path selection, routing, and switching. Protocols functioning on this layer include: Routed Protocols IPX IP Layer 3 Protocols Internet Control Message Protocol (ICMP) Address Resolution Protocol (ARP) Reverse Address Resolution Protocol (RARP) Routing Protocols Routing Information Protocol (RIP) Internet Gateway Routing Protocol (IGRP) Enhanced IGRP (EIGRP) Open Shortest Path First (OSPF) Exterior Gateway Protocol (EGP) Internet Management Group Protocol (IGMP) Group with Routed Protocols but label as a Non-routable Protocol NetBEUI Group with Routed Protocols DecNET Services include software and hardware addressing, packet routing between hosts and networks, resolution of hardware and software addresses, and reports of packet delivery. Devices functioning up to this layer include routers and brouters. Data Link Layer The Data Link Layer provides reliable transit of data across a physical link. In so doing, the data link layer is concerned with physical (as opposed to logical) addressing, network topology, network access, error notification, ordered delivery of frames, and flow control. Layer 2 can be remembered by frames and media access control. Ethernet CSMA/CD also operates on this layer to determine which devices should transmit at a given time in order to avoid collisions. The NIC is also responsible for CSMA/CD on Ethernet. In the case where two or more devices attempt to transmit signals at the same time, a collision will occur. CSMA/CD instructs the device to wait a given amount of time before transmitting another signal to avoid another collision. The Data Link Layer is broken down into two sublayers by the 802 standards: Logical Link Control (LLC) and Media Access Control (MAC). The LLC sublayer (IEEE 802.2) establishes and maintains communication with other devices and provides connectivity with servers when data is being transferred. LLC manages link control and defines service access points (SAPs). The MAC sublayer maintains a table of physical addresses of devices. Each device is assigned and must have a unique MAC address if the device is to participate on the network. For example, the MAC address is similar to the individual's physical residence address, which the post office uses to deliver snail mail. Protocols used at this layer include High Level Data Link Control (HDLC) for WAN connections, including synchronous and asynchronous transmissions. The LLC protocol (IEEE 802.2) provides flow control at this layer. Technologies which operate on this layer include more than 18 varieties of Ethernet (specified in the IEEE 802.3 and other standards), Token Ring (IEEE 802.5), and other LAN technologies which rely on frames. Communications with the NIC are also provided. Devices functioning up to this layer include NICs, bridges, and switches. While routers and brouters are classified as layer 3 devices, in order to perform their functions, they must operate on layer 1 and 2 as well. Physical Layer The Physical Layer (Layer 1) defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other, similar, attributes are defined by physical layer specifications. The Physical Layer is responsible for moving bits of data through physical media. Data, in the form of ones and zeros, are turned into electrical signals, pulses of light, or wireless signals. These signals are placed on the copper cables, optical fibers, or emitted as wireless, using a NIC. When receiving data from the network, the NIC turns the electrical signals, pulses of light or wireless signals back into ones and zeros to be sent up the hierarchy of the OSI Model. Protocols are the cabling, signaling, and connection standards. Services include Ethernet, Token Ring, FDDI, and other LAN technologies. Devices, which function at this layer are repeaters, multiport repeaters (also called hubs), media access units (MAUs), and transceivers (transmitter/receivers, for converting one signal type into another).
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