The OSI Reference Model
November 19th, 2009 by BobThe OSI Reference Model
To pass the ICND1 exam, you must be conversant in a protocol specification with which you are very unlikely to ever have any hands-on experience—the OSI reference model. The difficulty these days when discussing the OSI protocol specifications is that you have no point of reference, because most people cannot simply walk down the hall and use a computer whose main, or even optional, networking protocols conform to the entire OSI model.
OSI is the Open System Interconnection reference model for communications.
OSI Layers and Their Functions
Cisco requires that CCNAs demonstrate a basic understanding of the functions defined by each OSI layer, as well as remembering the names of the layers. It is also important that, for each device or protocol referenced throughout the book, you understand which layers of the OSI model most closely match the functions defined by that device or protocol. The
upper layers of the OSI reference model (application, presentation, and session—Layers 7, 6, and 5) define functions focused on the application. The lower four layers (transport, network, data link, and physical—Layers 4, 3, 2, and 1) define functions focused on endto-end delivery of the data. The CCNA exams focus on issues in the lower layers—in particular, with Layer 2, upon which LAN switching is based, and Layer 3, upon which routing is based. Table 2-4 defines the functions of the seven layers.
| Layer | Functional Description |
|---|---|
| 7 | Layer 7 provides an interface between the communications software and any applications that need to communicate outside the computer on which the application resides. It also defines processes for user authentication. |
| 6 | This layer’s main purpose is to define and negotiate data formats, such as ASCII text, EBCDIC text, binary, BCD, and JPEG. Encryption also is defined by OSI as a presentation layer service. |
| 5 | The session layer defines how to start, control, and end conversations (called sessions). This includes the control and management of multiple bidirectional messages so that the application can be notified if only some of a series of messages are completed. This allows the presentation layer to have a seamless view of an incoming stream of data. |
| 4 | Layer 4 protocols provide a large number of services, as described in Chapter 6 of this book. Although OSI Layers 5 through 7 focus on issues related to the application, Layer 4 focuses on issues related to data delivery to another computer—for instance, error recovery and flow control. |
| 3 | The network layer defines three main features: logical addressing, routing (forwarding), and path determination. The routing concepts define how devices (typically routers) forward packets to their final destination. Logical addressing defines how each device can have an address that can be used by the routing process. Path determination refers to the work done by routing protocols by which all possible routes are learned, but the best route is chosen for use. |
| 2 | The data link layer defines the rules (protocols) that determine when a device can send data over a particular medium. Data link protocols also define the format of a header and trailer that allows devices attached to the medium to send and receive data successfully. The data link trailer, which follows the encapsulated data, typically defines a Frame Check Sequence (FCS) field, which allows the receiving device to detect transmission errors. |
| 1 | This layer typically refers to standards from other organizations. These standards deal with the physical characteristics of the transmission medium, including connectors, pins, use of pins, electrical currents, encoding, light modulation, and the rules for how to activate and deactivate the use of the physical medium. |
Note: You need to memorize the table above. Here are a few mnemonic phrases to help you:
* All People Seem To Need Data Processing (Layers 7 to 1)
* Please Do Not Take Sausage Pizzas Away (Layers 1 to 7)
* Pew! Dead Ninja Turtles Smell Particularly Awful (Layers 1 to 7)
| Layer Name | Protocols and Specifications | Devices |
|---|---|---|
| Application, presentation, session (Layers 5–7) |
Telnet, HTTP, FTP, SMTP, POP3, VoIP, SNMP |
Firewall, intrusion detection system |
| Transport (Layer 4) | TCP, UDP | |
| Network (Layer 3) | IP | Router |
| Data link (Layer 2) | Ethernet (IEEE 802.3), HDLC, Frame Relay, PPP |
LAN switch, wireless access point, cable modem, DSL modem |
| Physical (Layer 1) | RJ-45, EIA/TIA-232, V.35, Ethernet (IEEE 802.3) |
LAN hub, repeater |
Next up: Chapter 3 Fundamentals of LANs