Institute of Technology Sligo - Dept of Computing
Paul Flynn

Institute of Technology Sligo - Dept of Computing
• The OSI layer was introduced by the International Organization for
Standardization (ISO) in 1984 in order to provide a reference model to make sure products of different vendors would interoperate in networks.
• OSI is short for Open Systems Interconnection.
• The OSI layer shows WHAT needs to be done to send data from an application on one computer, trough a network, to an application on another computer, not HOW it should be done.
• A layer in the OSI model communicates with three other layers: the layer above it, the layer below it, and the same layer at its communication partner.
• Data transmitted between software programs passes all 7 OSI layers.
• The Application, Presentation and Session layers are also known as the
Upper Layers.
• The Data Link and Physical layers are often implemented together to define LAN and WAN specifications.

Institute of Technology Sligo - Dept of Computing
• A Protocol is a set of rules that make communication on a network more efficient.
• E.G. - When answering the telephone, someone says, "Hello," then the person calling says, "Hello. This is.... "; and so it goes back and forth
• One technical definition of a data communications protocol is: a set of rules, or an agreement, that determines the format and transmission of data. Layer n on one computer communicates with Layer n on another computer.

Institute of Technology Sligo - Dept of Computing

Institute of Technology Sligo - Dept of Computing

Institute of Technology Sligo - Dept of Computing
• All
• People
• Seem
• To
• Need
• Domino’s
• Pizza
7 - Application
6 - Presentation
5 - Session
4 - Transport
3 - Network
2 - Data Link
1 - Physical

Institute of Technology Sligo - Dept of Computing
• Away
• Pizza
• Sausage
• Throw
• Not
• Do
• Please
7 - Application
6 - Presentation
5 - Session
4 - Transport
3 - Network
2 - Data Link
1 - Physical

Institute of Technology Sligo - Dept of Computing
• The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems.
• Physical layer specifications define characteristics such as:
• voltage levels
• timing of voltage changes
• physical data rates
• maximum transmission distances
• physical connectors
• Physical layer implementations can be categorized as either LAN or WAN specifications.
• more on next slide…

Institute of Technology Sligo - Dept of Computing
• Examples of LAN specifications are:
• Ethernet
• FastEthernet
• Token Ring
• FDDI
• Examples of WAN specifications are:
• HSSI
• V.24
• V.35
• BRI
• RS-232
• Transmits bits. (bitstream)
• Repeaters operate at this layer.

Institute of Technology Sligo - Dept of Computing
10 Base 2 – Co Ax
10 Base 5 – Thicknet
Fibre Optic Connectors UTP

Institute of Technology Sligo - Dept of Computing
• Defines psychical addressing, network topology, and is also concerned with error notification, sequencing of frames and flow control.
• Examples of network topologies are:
• Star
• Bus
• Ring
• Physical addresses are also known as hardware and BIA's (Burned In Addressess) but most commonly as MAC addresses.
• Examples of Data Link LAN specifications are:
• Ethernet
• FastEthernet
• Token Ring
• FDDI
• More on next slide…

Institute of Technology Sligo - Dept of Computing
• Examples of Data Link WAN specifications are:
• Frame Relay (operates also on the Physical layer)
• PPP (operates also on the Physical layer)
• X.25 (operates also on the Physical and Network layer)
• Transmits Frames.
• Bridges and Switches operate at this layer.
• The Data Link layer consists of two sublayers :
• LCC (Logical Link Control) sublayer
• Manages communication between devices over a single link of a network.
• Enables multiple higher-layer protocols to share a single physical data link.
• MAC sublayer
• Manages protocol access to the physical network medium.
• Determines hardware addresses.

Institute of Technology Sligo - Dept of Computing
Network Interface
Card (NIC)
Workgroup Switch

Institute of Technology Sligo - Dept of Computing
• Defines logical addressing for nodes and networks/segments.
• Enables internetworking, passing data from one network to another.
• Defines the logical network layout so routers can determine how to forward packets trough an internetwork.
• Routing occurs at this layer, hence Routed and Routing protocols reside on this layer.
• Routed protocols are used to encapsulate data into packets. The header added by the
Network layer contains a network address so it can be routed trough an internetwork.
• Examples of Network layer Routed protocols are:
• IP, IPX, AppleTalk
• Routing protocols are used to create routing tables; routing tables are used to determine the best path / route. Routing protocols provide periodic communication between routers in an internetwork to maintain information on network links in a routing table.
• Examples of Network layer Routing protocols are:
• OSPF, IGRP/EIGRP, RIP, BGP.
• Transmits Packets.
• Routers operate at this layer.

Institute of Technology Sligo - Dept of Computing
• The main purpose of this layers is making sure that the data is delivered error-free and in the correct sequence.
• Establishes, maintains and terminates virtual circuits.
• Provides error detection and recovery.
• Is concerned with reliable and unreliable transport. When using a connectionoriented, reliable transport protocol, such as TCP, acknowledgments are send back to the sender to confirm that the data has been received.
• Provides Flow Control and Windowing.
• Provides multiplexing; the support of different flows of data to different applications on the same host.
• Examples of Transport layer protocols are:
• TCP (connection-oriented, reliable, provides guaranteed delivery.)
• UDP (connectionless, unreliable, less overhead, reliability can be provided by the Application layer)
• SPX
•Transmits Segments.

Institute of Technology Sligo - Dept of Computing
• The session layer establishes, manages, maintains and terminates communication channels between software programs on network nodes.
• Provides error reporting for the Application and Presentation layer.
• Examples of Session layer protocols are:
• NFS
• SQL
• RPC
•Transmits Data.

Institute of Technology Sligo - Dept of Computing
• Defines coding and conversion functions.
• Ensures that information sent from the application layer of one system is readable by the application layer of another system.
• Includes common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes, common examples of these formats and schemes are:
• MPEG, QuickTime
• ASCII, EBCDIC
• GIF, TIFF, JPEG
• Transmits Data.

Institute of Technology Sligo - Dept of Computing
• Provides network services directly to applications. Software programs itself are not part of the OSI model.
• Determines the identity and availability of communication partners, and determines if sufficient resources are available to start program-to-program communication.
• This layer is closest to the user.
• Examples of Application layer protocols are:
• Telnet
• SMTP
• FTP
• SNMP
• Transmits Data.

Institute of Technology Sligo - Dept of Computing
• Data Encapsulation is the process of adding a header to wrap the data that flows down the OSI model.
• Each OSI layer may add it's own header to the data received from above. (from the layer above or from the software program 'above' the Application layer.)
• The 5 Steps of Data Encapsulation are:
1. The Application, Presentation and Session layers create DATA from users‘ input.
2. The Transport layer converts the DATA to SEGMENTS
3. The Network layer converts the SEGMENTS to PACKETS (or datagrams)
4. The Data Link layer converts the PACKETS to FRAMES
5. The Physical layer converts the FRAMES to BITS.
• At the sending computer the information goes from top to bottom while each layers divides the information received from upper layers in to smaller pieces and adds a header. At the receiving computer the information flows up the model discarding the corresponding header at each layer and putting the pieces back together.

Institute of Technology Sligo - Dept of Computing
Data Encapsulation Diagram

Institute of Technology Sligo - Dept of Computing
Each layer contains a Protocol Data Unit (PDU). PDU ’s are used for peer-to-peer conversations.

Institute of Technology Sligo - Dept of Computing
• The TCP/IP Model was develop by the Department of
Defense in the late 60s’ to ensure data communications would not be interrupted even under the worse circumstances.
• Since then, TCP/IP has become the de facto method we use for data communications on the Internet.

Institute of Technology Sligo - Dept of Computing
Application
Transport
Internet
Network Access
Includes all the functions of the OSI’s
Application,
Presentation, & Session layers including:
• Data representation
• Data encryption and
• Dialog control

Institute of Technology Sligo - Dept of Computing
Application
Transport
Internet
Network Access
• FTP-File Transfer
Protocol
• HTTP-Hypertext
Transfer Protocol
• SMTP-Simple Mail
Transfer Protocol
• DNS-Domain Name
Service
• TFTP-Trivial File
Transfer Protocol

Institute of Technology Sligo - Dept of Computing
Application
Transport
Internet
Network Access
Uses the TCP protocol and is responsible for quality of service issues including:
• Reliability
• Flow Control and
• Error Correction

Institute of Technology Sligo - Dept of Computing
Application
• TCP-Transmission
Control Protocol
• UDP-User Datagram
Protocol
Transport
Internet
Network Access

Institute of Technology Sligo - Dept of Computing
Application
Uses the IP protocol and is responsible for:
• Path determination and
• Packet switching.
Transport
Internet
Network Access

Institute of Technology Sligo - Dept of Computing
• IP – Internet Protocol
Application
Transport
Internet
Network Access

Institute of Technology Sligo - Dept of Computing
Application
Transport
Internet
Network Access
Includes all the functions of the OSI’s
Data Link & Physical layers including:
• Processes required by
IP to ensure a packet reaches its destination.
• All the various LAN &
WAN Technologies such as 100BaseTX & Frame
Relay.

Institute of Technology Sligo - Dept of Computing
Application
• LAN and WAN
Technologies
Transport
Internet
Network Access

Institute of Technology Sligo - Dept of Computing
TCP/IP
Application
OSI
Application
Presentation
Session
Transport
Internet
Network Access
Transport
Network
Data Link
Physical

Institute of Technology Sligo - Dept of Computing
• TCP/IP is the most popular “protocol specific” model used on the Internet.
• However, TCP/IP does not cover all the protocols and standards we will be exploring.
• The OSI Model is “protocol independent.” Therefore, all topics covered in the curriculum can fit into its framework.

Institute of Technology Sligo - Dept of Computing
• The OSI Model’s layers & their key characteristics and functions.
• The process of data encapsulation and the various
PDUs.
• The TCP/IP layers and its corresponding protocols.
• Comparison of the two model’s layers.

Institute of Technology Sligo - Dept of Computing
In the TCP/IP model which layer would deal with reliability, flow control, and error correction?
A – Transport Layer
B – Internet Layer
C – Network Layer
D – Application Layer

Institute of Technology Sligo - Dept of Computing
In the TCP/IP model which layer would deal with reliability, flow control, and error correction?
A – Transport Layer
B – Internet Layer
C – Network Layer
D – Application Layer

Institute of Technology Sligo - Dept of Computing
What is the control information called that is placed before data when encapsulating that data for network transmission?
A – Frame
B – Header
C – Capsule
D – Routing Information

Institute of Technology Sligo - Dept of Computing
What is the control information called that is placed before data when encapsulating that data for network transmission?
A – Frame
B – Header
C – Capsule
D – Routing Information

Institute of Technology Sligo - Dept of Computing