Cisco Semester 1 Chapter 2 The

Cisco – Semester 1 – Chapter 2 The OSI Model

International Standards Organization Interoperability The ISO recognized that there was a need to create a network model that would help network builders implement networks that could communicate and work together Released model in 1984 OSI reference model is networking scheme that supports networking standards

The 7 Layers • • 7 6 5 4 3 2 1 Application Presentation Session Transport Network Data Layer Physical

Why Layers? • • • Reduces complexity Standardizes interface Facilitates modular engineering Ensures interoperability Accelerates evolution Easier to teach and learn (or so they say)

Physical Layer – Layer 1 • Defines the electrical and functional specifications for the link between end systems • Defines voltage levels, physical data rates, maximum transmission distance, physical connections • THINK MEDIA AND SIGNALS

Layer 2 – Data Link • Provides reliable transit of data across a physical link • Concerned with physical addressing (MAC) address), network topology, and media access, error notification, delivery of frames, and flow control • THINK FRAMES AND MEDIA ACCESS CONTROL

Layer 3 – Network Layer • Provides connectivity and path selection between two host systems • THINK PATH SELECTION, ROUTING, AND ADDRESSING (logical or IP)

Layer 4 - Transport • Provides a data transport service that shields the upper layers from transport implementation details • Concerned with reliable transport between two hosts • Establishes, maintains, and properly terminates virtual circuits • Uses error detection-and-recovery and information flow control • THINK QUALITY OF SERVICE AND RELIABILITY

Layer 5 - Session • Establishes, manages, and terminates sessions between two communicating hosts. • Provides its services to the presentation layer • Synchronizes dialogue between the two hosts' presentation layers and manages their data exchange • Provides data expedition, class of service, and exception reporting • THINK DIALOG AND CONVERSATIONS

Layer 6 - Presentation • Ensures the information sent is readable by receiving host • Concerned with data structures & syntax • Translates between multiple formats by using a common format • Encodes, encrypts, and compresses • THINK COMMON DATA FORMAT

Layer 7 - Application • Provides network services to user applications • DOES NOT provide services to any other OSI layer • Provides services to applications outside the OSI model • THINK BROWSERS

Layer Information • In general each layer communicates with three other layers – The layer above – The layer below – The same layer in the peer host • Know specifics of each layer – E. g. Net. Beui operates at layer 5

Encapsulation • Wraps data with the necessary protocol information before sending to next layer and for network transit. • Data receives headers (control information), trailers, and other information • Data – Segments – Packets – Frames – Bits – THIS IS IMPORTANT (defines datagram at each layer)

Headers • Control information placed before the data • Adds address information, et. al. • Exact format depends on layer

Layers and Encapsulation • • • Presentation – Data Transport – Segments Network – Packets Data Link – Frames Physical - Bits

Protocols • Set of rules that determines format and transmission of data; aka standards • Different protocols for different layers • PDU – Protocol Data Unit – exchange between peer layers

TCP/IP • Transmission Control Protocol/Internet Protocol • Makes data communication possible between any two computers, anywhere in the world • Speed approaches speed of light • WWW and Internet protocol – defacto standard

TCP/IP Layers • Application Layer – Combines Application, Presentation, & Session • Transport Layer – Same as OSI Transport Layer • Internet Layer – Same as OSI Network Layer • Network Access Layer – Includes Physical & Data Link; Includes LAN and WAN details

TCP/IP Protocols • Application Layer – FTP, TFTP, HTTP, SMPT, DNS, TFTP, Telnet, Redirector • Transport Layer – UDP and TCP • Internet Layer – IP • Network Access Layer – LAN and WAN technology used

Model Similarities • Network professionals need to know both • Both use layers • Both are packet switched (not circuit switched) • Both have similar transport and network layers

Model Differences • TCP/IP application layer includes OSI presentation and session layers • TCP/IP network access layer includes OSI data link and physical layers • TCP/IP protocols are standards used to build Internet • No networks are built around OSI protocols • TCP/IP APPEARS simpler

Cisco Uses OSI Because • Generic and protocol independent • More details which makes it useful for teaching and learning • More details which makes it useful for troubleshooting • USE OSI MODEL but use TCP/IP PROTOCOLS