CCNA Section 2 OSI Reference Model
What are the seven layers of the OSI model?
7 Application All 6 Presentation People 5 Session Seem 4 Transport To 3 Network Need 2 Data-link Data 1 Physical Processing
As data is passed from the user application down the virtual layers of the OSI model, each layer adds a __________ (and sometimes a trailer) containing protocol information specific to that layer. These headers are called ________________, and the process of adding these headers is called _________________.
As data is passed from the user application down the virtual layers of the OSI model, each layer adds a header (and sometimes a trailer) containing protocol information specific to that layer. These headers are called Protocol Data Units (PDUs), and the process of adding these headers is calledencapsulation.
What is the Protocol Data Unit( PDU) of the Physical Layer?
BIts
The Physical layer provides specifications for a variety of hardware:
Cabling Connectors and transceivers Network interface cards (NICs) Wireless radios Hubs
Transport layer communication falls under what two categories?
Connection-oriented - requires that a connection with specific agreed-upon parameters be established before data is sent. Connectionless - requires no connection before data is sent.
During encapsulation on the sending host:
Data from the user application is handed off to the Transport layer. The Transport layer adds a header containing protocol-specific information, and then hands the segment to the Network layer. The Network layer adds a header containing source and destination logical addressing, and then hands the packet to the Data-Link layer. The Data-Link layer adds a header containing source and destination physical addressing and other hardware-specific information. The Data-Link frame is then handed off to the Physical layer to be transmitted on the network medium as bits.
The Internet Protocol (IP) was originally developed by
Department of Defense (DoD), and was a cornerstone for a group of protocols that became known as the TCP/IP protocol suite
Why do networks have layers?
Each layer represents a specific networking function.These functions are controlled by protocols, which are rules that govern end-to-end communication between devices.
The encapsulation type will vary depending on the underlying technology. Common Data-link layer technologies include following:
Ethernet - the most common LAN data-link technology Token Ring - almost entirely deprecated FDDI (Fiber Distributed Data Interface) 802.11 Wireless •Frame-Relay ATM (Asynchronous Transfer Mode
What is the Protocol Data Unit( PDU) of the Data-Link Layer?
Frames
Sessions communication falls under one of three categories:
Full-Duplex - simultaneous two-way communication Half-Duplex - two-way communication, but not simultaneous Simplex - one-way communication
Who was the OSI model developed by and why?
International Organization for Standardization (ISO), and formalized in 1984. It provided the first framework governing how information should be sent across a network.
switches are generally identified as Layer-
Layer-2 devices, as switches process information stored in the Data-Link header of a frame, such as Ethernet MAC addresses.
routers are identified as Layer-
Layer-3 devices, as routers process logical addressing information in the Network header of a packet, such as IP addresses.
The Data-Link layer consists of two sublayers:
Logical Link Control (LLC) sublayer Media Access Control (MAC) sublayer
What are the two key responsibilities of The Network layer (Layer-3)
Logical addressing - provides a unique address that identifies both the host, and the network that host exists on. Routing - determines the best path to a particular destination network, and then routes data accordingly.
internetwork
Multiple networks connected together
What is the Protocol Data Unit( PDU) of the Network Layer?
Packets
Protocols that operate at the upper layers are for?
Protocols that operate at these layers manage application-level functions, and are generally implemented in software.
What important services do connection orientated protocols provide?
Segmentation and sequencing - data is segmented into smaller pieces for transport. Each segment is assigned a sequence number, so that the receiving device can reassemble the data on arrival Connection establishment - connections are established, maintained, and ultimately terminated between devices. Acknowledgments - receipt of data is confirmed through the use of acknowledgments. Otherwise, data is retransmitted, guaranteeing delivery. Flow control (or windowing) - data transfer rate is negotiated to prevent congestion.
What is the Protocol Data Unit( PDU) of the Transport Layer?
Segments
What is the most prevalent protocol suite?
TCP/IP suite is the most prevalent protocol suite, and is the foundation of the Internet.
During decapsulation on the receiving host, the reverse occurs:
The frame is received from the physical medium. The Data-Link layer processes its header, strips it off, and then hands it off to the Network layer. The Network layer processes its header, strips it off, and then hands it off to the Transport layer. The Transport layer processes its header, strips it off, and then hands the data to the user application.
What Layers of the OSI model are often refered to as the upper layers?
The top three layers of the OSI model are often referred to as the upper layers: •Layer-7 - Application layer •Layer-6 - Presentation layer •Layer-5 - Session layer
The TCP/IP protocol suite incorporates what two Transport layer protocols
Transmission Control Protocol (TCP) - connection-oriented •User Datagram Protocol (UDP) - connectionless
Hardware addresses uniquely identify
a host within a network, and are often hardcoded onto physical network interfaces.
computer network
connects two or more devices together to share information and services
The Transport layer (Layer-4) :
does not actually senddata, despite its name. Instead, this layer is responsible for the reliable transfer of data, by ensuring that data arrives at its destination error-free and in order.
The Session layer (Layer-5) is responsible for
establishing, maintaining, and ultimately terminating sessions between devices. If a session is broken, this layer can attempt to recover the session.
The Data-link layer packages the higher-layer data into frames, so that the data can be put onto the physical wire. This packaging process is referred to as
framing or encapsulation.
The Network layer (Layer-3) controls
internetwork communication,
If two devices do not support the same format or syntax, the Presentation layer can
provide conversion or translation services to facilitate communication
What is the purpose of The Application layer(Layer-7)
provides the interface between the user application and the network. A web browser and an email client are examples of user applications.
Network reference model
serves as a blueprint, detailing how communication between network devices should occur.
Protocols that operate at the bottom 4 layers controll what?
the end-to-end transport of data between devices, and are implemented in both software and hardware.
The Presentation layer (Layer-6) controls :
the formatting and syntax of user data for the application layer. This ensures that data from the sending application can be understood by the receiving application.
The Logical Link control(LLC) sublayer serves as:
the intermediary between the physical link and all higher layer protocols. It ensures that protocols like IP can function regardless of what type of physical technology is being used.
Note that in the TCP/IP protocol suite only the:
the lower layers perform encapsulation
The MAC sublayer controls access to
the physical medium, serving as mediator if multiple devices are competing for the same physical link. Data-link layer technologies have various methods of accomplishing this - Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD), and Token Ring utilizes a token.
The Physical layer (Layer-1) controls
the signaling and transferring of raw bits onto the physical medium.
While the Network layer is concerned with transporting data between networks, the Data-Link layer (Layer-2) is responsible for
transporting data within a network
What are examples of Application layer(Layer-7) protocols?
•FTP, via an FTP client •HTTP, via a web browser •POP3 and SMTP, via an email client •Telnet
What are the functions of the Application layer(Layer-7) ?
•Identifies communication partners •Determines resource availability •Synchronizes communication
The most common Network layer (Layer-3) protocol is
•Internet Protocol (IP)
What are examples of Presentation layer (Layer-6)
•Text - RTF, ASCII, EBCDIC •Images - GIF, JPG, TIF •Audio - MIDI, MP3, WAV •Movies - MPEG, AVI, MOV
The two most recognized network reference models are:
•The Open Systems Interconnection (OSI) model •The Department of Defense (DoD) model
