1. Networks 2 Network Layer Models Dr John Cowell
phones off (please)
CSCI1412 Lecture 16
Networks 2 Network Layer Models
Dr John Cowell

2. Overview Protocol layer models The OSI seven layer model
what is a layer model and why use it?
standards and standards agencies
The OSI seven layer model
overview of the layer model
the seven layers explained
The real world
how the theoretical model applies to the Internet

3. Layer Models

4. Protocol Layer Models Head of State
Formulate message in country’s interest
Receive message and decide upon response
High-level communication no common language
Ambassador/Diplomats
Rephrase in diplomatic language
Rephrase using appropriate language
Communicate ideas no generation or change of meaning
Translator
Communicate, but no concept of meaning
Translate to intermediate language
Translate into national language

5. Protocol Standards
A protocol is a set of rules governing communication issues - a standard
diplomats have protocols
so do networks
so do computer networks!
Two types of standards
de-facto - devolved standards
e.g. many IBM products, Microsoft Office, TCP/IP
designed - written standards
submitted to a standards agency for approval
agreed by a group of companies or organisations

6. Agencies Protocols often written/regulated by agencies, e.g. ANSI
American National Standards Institute
FDDI, ASCII
CCITT
Comité Consultatif Internationale de Télégraphic et Téléphonique
V series for telephone communications (modems)
X series for network interfaces & public Networks
IEEE
Institute of Electrical & Electronic Engineers
Project LAN standards
IAB
Internet Activities Board
set and document internet policies and standards, e.g. TCP/IP

7. Who is ISO? International Organisation for Standardisation ISO’s name
a worldwide federation of national standards bodies from 130 countries, one from each country
a non-governmental organization established in 1947
to promote the development of standardization
ISO’s work results in international agreements which are published as International Standards
ISO’s name
shouldn’t the acronym be “IOS”?
yes, if it were an acronym; but it is not
“ISO” is a word, from the Greek isos, meaning “equal”

8. ISO’s OSI Model Open Systems Interconnection The seven layer model
a standard description or “reference model” for how messages should be transmitted between any two points in a telecommunication network
any two computers can communicate when connected
regardless of underlying architecture
The seven layer model
the process of communication between two users in a comms. network can be divided into layers, with each layer adding its own set of special, related functions
written standard
not implemented

9. The OSI 7 Layer Model

10. Origins of the Model Late 1970’s 2 separate projects by ISO CCITT
To standardise a framework for emerging network technologies.
1984 published as ISO 7498 and CCITT as X.200
The Internet and growth of TCP/IP (Transmission Control Protocol/ Internet Protocol) became the de-facto standard.

11. The Seven Layers Application layer
User services, application, activities
Application layer
Presentation layer
Session layer
Transport layer
Actual data transmission communications subnet
Network layer
Data Link layer
Physical layer

12. What They Do The layers are in two groups
the upper four layers are used whenever a message passes from or to a user
within the same computer or to another computer
lower three layers used when any message passes through the host computer to another
messages destined for some other host are not passed up to the upper layers but are forwarded to another host
Purpose is to separate specific functions
makes implementation transparent to other components
allows independent testing & maintenance of each layer

13. Information Processing
Process sending information
Logical communication between layers
Process receiving information
Application layer
Application layer
Presentation layer
Presentation layer
Session layer
Session layer
same computer
Transport layer
Transport layer
Network layer
Network layer
Data Link layer
Data Link layer
Physical layer
Physical layer
other computers

14. Data Unit at Different Layers
Each layer deals with different grouping of data
Data - Application, Presentation, Session
Segment – Transport
Packet – Network
Frame – Data Link
Bit - Physical

15. Application (Layer 7) The application layer Provides services such as
interacts with applications that offer some form of communication component
not an application program
Provides services such as
identifying communicating partners
determining the identity and availability of partners
determining resource availability
decide whether sufficient networking resources are available
synchronising communication
all communication between applications requires cooperation

16. Presentation (Layer 6) The presentation layer
presents data in user understandable format
allows user to see information, not data
translates between formats
Provides a variety of coding and conversion functions that are applied to application layer data
data representation
e.g. JPEG, GIF, TIFF formats for graphics
character representation
e.g. ASCII, EBCDIC encodings for characters
data compression schemes
data encryption schemes

17. Session (Layer 5) The session layer The layer
responsible for managing connections between two machines during the course of communication
determines whether all pertinent information for the session has been transmitted and/or received
The layer
establishes logical connection
controls synchronisation between two ends
manages acknowledgements and retransmissions
reconnects on premature termination (at lower level)
releases logical connection

18. Transport (Layer 4) The transport layer
end-to-end communications
decides upon network to use
according to data type, e.g. packets or continuous bit stream
Transport layer functions typically include
flow control
ensuring there is enough ‘space’ to send/receive messages
multiplexing
combining more than one message on the same connection
packeting
breaking larger units down into small pieces to be transferred
error checking and recovery

19. Network (Layer 3) The network layer It provides functions such as
provides routing and addressing functions that enable multiple data links to be combined into a network
sending it in the right direction to the right destination on outgoing transmissions
receiving incoming transmissions at the packet level
It provides functions such as
routing strategies
deciding which route to use and why
forwarding
sending on packets destined for other hosts
controlling communications subnet

20. Data Link (Layer 2) The data link layer
provides the first level of organization of the data bits into a rudimentary structure called a datalink frame
this is organized into fields of information that convey
the beginning and ending of the frame
the address of the sender
the address of the receiver
a method to ensure that the frame does not contain errors
an area to provide some basic administrative function
It is sometimes broken down in two sub layers
logical link control layer (LLC)
media access control layer (MAC)

21. Physical (Layer 1) The physical layer This layer describes
defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems
transmits/receives bits as electrical impulses
This layer describes
the network topology (shape, structure)
the transmission media (cables, broadcasts)
the transmission devices (network cards, modems)
the electrical or optical signalling characteristics

22. The Real World

23. The Real World! Loosely, a five layer model OSI 7 6 5 4 3 2 1
application layer
presentation layer
session layer
transport layer
network layer
data link layer
physical layer
OSI 7 6 5 4 3 2 1
world wide web (HTML/HTTP)
(POP / SMTP)
file transfers (FTP)
remote sessions (TELNET)
transmission control protocol (TCP)
internet protocol (IP)
(physical level)
ethernet
The Real World 5 4 3 2 1
PPP
modem

24. Application Layer Top layer (5) comprises other protocols such as
HTML / HTTP (the World Wide Web)
POP / SMTP ( )
FTP (file transfer)
TELNET (remote sessions)
DNS (Domain Name Server)
maps domain names to IP addresses 
This application layer roughly corresponds to layers 5, 6, and 7 of the OSI model

25. TCP/IP
TCP/IP is the basic communication protocol (language) of the Internet
Transmission Control Protocol
manages assembling a message or file into smaller parts (packets) that will be transmitted over a network
reassembles packets when received
TCP  OSI layer 4 (transport)
Internet Protocol
handles the address part of each packet to ensure that it gets to the correct destination
IP  OSI layer 3 (network)

26. Lower Levels
Three of the most common methods of connecting to the Internet are
Ethernet
LAN protocol developed by Xerox, DEC & Intel in 1976
specifies the way data is transmitted as well as the physical connection options
incorporates layers 1 and 2 of the OSI model
PPP over a modem link
point-to-point protocol controls how the data is sent (level 2)
now included in Windows and other operating systems
modem handles transmission of data over telephone line (level 1)
DSL (Broadband)
utilises unused bandwidth on standard telephone wires
need frequency divider (DSL ‘modem’) to separate voice call from data signal

27. Summary Protocol layer models The OSI seven layer model The real world
what is a layer model and why use it?
standards and standards agencies
The OSI seven layer model
overview of the layer model
the seven layers explained
The real world
how the theoretical model applies to the Internet