CCNA Data CenterHost to Host

Communications

Understanding Host to Host Comm.» Initial host-to-host communications models were proprietary per vendor» Business and technology innovations drove multivendor compatibility

OSI Reference Model» Purpose of OSI Reference Model • Reduce complexity • Standardized interfaces • Modular Engineering • Interoperability • Promotes innovation • Simplifies teaching and learning

OSI Model Layers and Functions» Network Process to Applications» Data Representation» Interhost Communications» End-to-End Connections» Data Delivery» Access to Media» Binary Transmission

Layer 1» Physical Layer – Binary Transmission

Defines the electrical, mechanical procedures, and functional specifications for activating, maintaining, and deactivating the physical layer

Physical cabling and hardware devices such as

hubs, network interface cards (NICs), and host bus adapters (HBAs) exist at this layer

Layer 2» Data Link Layer – Access to Media

Defines how data is formatted for transmission and how the network is controlled along with error detection

Example hardware - Bridges and switches

Layer 3» Network Layer – Data Delivery

• Routes data packets • Selects best path to deliver data • Provides logical addressing and path

selection • Example hardware - Routers

Layer 4» Transport Layer – End-to-End Connections

• Handles transport issues between hosts• Ensures data transport reliability• Establishes, maintains, and terminates virtual

circuits• TCP/UDP Protocols and ports exist at this

layer

Layer 5» Session Layer – Interhost Communication

• Establishes, manages, and terminates sessions between applications

• Synchronizes dialog between hosts

Layer 6» Presentation Layer – Data Representation

• Ensures data is readable by receiving system

• Formats and structures data • Negotiates data transfer syntax for

application • Provides encryption

Layer 7» Application Layer – Network Processes to

Applications

• Provides network services to application processes

• Provides user authentication • Examples include (Email, file transfer,

terminal emulation)

Why use Layers?» Devices only need to be aware of their own

layer • Web server doesn’t care if Ethernet or Wireless

was used to reach it • Ethernet Switch doesn’t care if it’s transmitting

IPv4 or IPv6 packets» Allows interoperability between devices and

vendors • Cisco Ethernet Switch can connect to Netgear

Ethernet Switch • Both agree on Ethernet standards

Encapsulation and Decapsulation» Data is added or remove when adjacent layers

talk • Resulting data that a layer creates is called a

Protocol Data Unit (PDU)» Encapsulation • Process of adding formatting data on the sending

host to create a PDU» Decapsulation• Process of removing data formatting on the

receiving host to expose a PDU

Encapsulation» Encapsulation occurs when data moves down

the OSI stack • 7 > 6 > 5 > 4 > 3 > 2 > 1» Each layers adds its formatting in front or

behind the data • Encapsulation Header • New data added to the front • Encapsulation Trailer • New data added to the end» Data is then passed to the layer below» Process repeats until physical layer is reached

Decapsulation» Decapsulation occurs when data moves up the OSI stack • 1 > 2 > 3 > 4 > 5 > 6 > 7» Each layer removes its own header/trailer» Data is then passed up to the layer above» Process repeats until the application is reached

Protocol Data Units» Transport Layer (Layer 4) • Segments» Internet Layer (Layer 3) • Packets» Data Link Layer (Layer 2) • Frames» Physical Layer (Layer 1) • Bits

Peer-to-Peer Communications

TCP/IP Model» Transmission Control Protocol (TCP) /

Internet Protocol (IP) Model» Similar to OSI model but simplified into four

layers • Application • Transport • Internet • Network Access» Also known as “TCP/IP Stack” or simply “IP

Stack”

TCP/IP Model Layers» Application • Equivalent of OSI layers 5 – 7» Transport • Equivalent of OSI layer 4» Internet • Equivalent of OSI layer 3 (Network)» Network Access • Equivalent of OSI layers 1 – 2

OSI vs. TCP/IP Model