Open Systems Interconnection Models are conceptual models for communication systems
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| Questions | Answers |
|---|---|
| Layer 1 (OSI model) | Physical |
| Layer 2 (OSI model) | Data Link |
| Layer 3 (OSI model) | Network |
| Layer 4 (OSI model) | Transport |
| Layer 5 (OSI model) | Session |
| Layer 6 (OSI model) | Presentation |
| Layer 7 (OSI model) | Application |
| Physical Layer | Layer 1 The Physical layer of the OSI model sets standards for sending and receiving electrical signals between devices. |
| Protocols at the Physical layer identify: | •How digital data (bits) are converted to electric pulses, radio waves, or pulses of lights. •Specifications for cables and connectors. •The physical topology. Data at the Physical layer is referred to as bits. |
| Data Link Layer | Layer 2 The Data Link layer defines the rules and procedures for hosts as they access the Physical layer. |
| These rules and procedures specify or define: | •How hosts on the network are identified (physical or MAC address). •How and when devices can transmit on the network medium (media access control and logical topology). •How to verify that the data received from the Physical layer is error free (parity and CRC). •How devices control the rate of data transmissions between hosts (flow control). Data at the Data Link layer is referred to as a frame. |
| Network Layer | Layer 3 The Network layer describes how data is routed across networks and on to the destination. |
| Network layer functions include: | •Identifying hosts and networks using logical addresses. •Maintaining a list of known networks and neighboring routers. •Determining the next network point to which data should be sent. Routers use a routing protocol to take into account various factors such as the number of hops in the path, link speed, and link reliability to select the optimal path for data. Data at the Network layer is referred to as a packet. |
| Transport Layer | Layer 4 The Transport layer provides a transition between the upper and lower layers of the OSI model, making the upper and lower layers transparent from each other. |
| Transport layer functions include: | •End-to-end flow control. •Port and socket numbers. •Segmentation, sequencing, and combination. •Connection services, either reliable (connection-oriented) or unreliable (connectionless) delivery of data. Data at the Transport layer is referred to as a segment. |
| Session Layer | Layer 5 The Session layer's primary function is managing the sessions in which data is transferred. |
| Functions at this layer include: | •Management of multiple sessions (each client connection is called a session). A server can concurrently maintain thousands of sessions. •Assignment of the session ID number to each session to keep data streams separate. •Set up, maintain, and tear down communication sessions. |
| Presentation Layer | Layer 6 The Presentation layer formats or "presents" data into a compatible form for receipt by the Application layer or the destination system. |
| Specifically, the Presentation layer ensures: | •Formatting and translation of data between systems. •Negotiation of data transfer syntax between systems, through converting character sets to the correct format. •Encapsulation of data into message envelopes by encryption and compression. •Restoration of data by decryption and decompression |
| Application Layer | Layer 7 The Application layer integrates network functionality into the host operating system, and enables network services. The Application layer does not include specific applications that provide services, but rather provides the capability for services to operate on the network. Most Application layer protocols operate at multiple layers down to the Session and even Transport layers. However, they are classified as Application layer protocols because they start at the Application layer (the Application layer is the highest layer where they operate). |
| Services typically associated with the Application layer include: | •HTTP •Telnet •FTP •TFTP •SNMP |
