9. Network Software
Protocol Hierarchies
Design Issues for the Layers
Connection-Oriented and Connectionless Services
Service Primitives
The Relationship of Services to Protocols
16. Services to Protocols Relationship
The relationship between a service and a protocol.
17. Reference Models
The OSI Reference Model
The TCP/IP Reference Model
A Comparison of OSI and TCP/IP
A Critique of the OSI Model and Protocols
A Critique of the TCP/IP Reference Model
18. OSI Reference model :
The Open System Interconnection (OSI) Reference Model is a description for layered
communications and computer network protocol design. It was developed as part of the
Open Systems Interconnection (OSI) initiative. It divides network architecture into seven
layers which are, from top to bottom, the Application, Presentation, Session, Transport,
Network, Data-Link, and Physical Layers. It is therefore often referred to as the OSI
Seven Layer Model.
A layer is a collection of similar functions that provide services to the layer above it and
receives service from the layer below it.
On each layer an instance provides services to the instances at the layer above and
requests service from the layer below.
19. 19
Why do we need the OSI Model?
To address the problem of networks increasing in size
and in number, the International Organization for
Standardization (ISO) researched many network
schemes and recognized that there was a need to
create a network model that would help network
builders implement networks that could communicate
and work together and therefore, released the OSI
reference model in 1984.
20. OSI Reference Model
Data unit Layer Function
Data
7. Application Network process to application
6. Presentation Data representation and encryption
5. Session Host-to-host communication
Segment 4. Transport End-to-end connections and reliability
Packet 3. Network Path determination and logical addressing
Frame 2. Data Link Physical addressing
Bit 1. Physical Media, signal and binary transmission
22. Layer 7: Application Layer
the application layer provides services for an application program to ensure that effective
communication with another application program in a network is possible. It is a service
layer that provides these services:
•Allows applications to use the network.
•Message authenticates either the message sender or receiver or both
•Makes sure that necessary communication resources exist
•Determines protocol and data syntax rules at the application level
•Interface between the user & the computer (applications & Gateways). Provides
services that directly support user applications, such as the USER INTERFACE, E-
MAIL, FILE TRANSFER, TERMINAL EMULATION, DATABASE ACCESS.
•Handles Network access, flow control & error recovery.
•Messages are sent between layers.
23. Layer 7 - The Application Layer
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
This layer deal with
networking applications.
Examples:
Email
Web browsers
PDU - User Data
24. Layer 6: Presentation Layer
The Presentation Layer enables translation between Application Layer entities, in which
the higher-layer entities can use different syntax and semantics, as long as the
Presentation Service understands both and the mapping between them.
•This layer formats and encrypts data to be sent across a network.
•The presentation service data units are then encapsulated into Session Protocol
Data Units, and moved to the lower layers.
•Translation of data into understandable format for transmission (into a form usable
by the application layer i.e. translates data between the formats the network
requires and the computer expects).
•Handles character encoding, bit order and byte order issues. Encodes and
decodes data.
•Data compression and encryption takes place at this layer.
•Generally determines the structure of data
•The redirector works at this layer.
•Responsible for protocol conversion
25. Layer 6 - The Presentation Layer
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
This layer is responsible
for presenting the data in
the required format which
may include:
Encryption
Compression
PDU - Formatted Data
26. Layer 5: Session Layer
•The Session layer controls the dialogues (connections) between computers.
It establishes, manages and terminates the connections between the local
and remote application. It provides for full-duplex, half-duplex, or simplex
operation.
•Provides synchronization between communicating computers (nodes),
messages are sent between layers (i.e. Manages upper layer errors).
•Places checkpoints in the data flow, so that if transmission fails, only the data
after the last checkpoint needs to be retransmitted.
27. Layer 5 - The Session Layer
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
This layer establishes,
manages, and terminates
sessions between two
communicating hosts.
Example:
Client Software
( Used for logging in)
PDU - Formatted Data
28. Layer 4: Transport Layer
•The Transport Layer provides transparent transfer of data between end users, providing
reliable data transfer services to the upper layers.
•Responsible for PACKET HANDLING. Ensures error free delivery. Repackages
messages, divides messages into smaller packets (Fragments and reassembles data),
and handles error handling
•Ensures proper sequencing and without loss and duplication.
•Takes action to correct faulty transmissions
•Controls flow of data
•Acknowledges successful receipt of data
•Sliding window is at this Layer -segments of message fragments are sent between
layers
•TCP - connection oriented communication for applications to ensure error free delivery.
•UDP- connectionless communications and does not guarantee packet delivery between
transfer points
29. Layer 4 - The Transport Layer
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
This layer breaks up the
data from the sending host
and then reassembles it in
the receiver.
It also is used to insure
reliable data transport
across the network.
PDU - Segments
30. Layer 3: Network Layer
The Network Layer provides the functional and procedural means of transferring variable
length data sequences from a source to a destination via one or more networks. The
Network Layer performs network routing functions, and might also perform
fragmentation and reassembly, and report delivery errors.
A well known example of the network layer protocol is the Internet Protocol (IP).
Logical addressing - software addresses to hardware addresses are resolved
(ARP/RARP).
Determining the best route (Makes routing decisions & forwards packets.
Layer 2: Data Link Layer
The Data Link Layer provides the functional and procedural means to transfer data
between network entities and to detect and correct errors that may occur in the Physical
Layer
Data link layer arrange bits, from the Physical Layer, into logical sequences called
frames.
31. Layer 3 - The Network Layer
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
Sometimes referred to as the
“Cisco Layer”.
Makes “Best Path
Determination” decisions
based on logical addresses
(usually IP addresses).
PDU - Packets
32. Layer 2 - The Data Link Layer
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
This layer provides reliable
transit of data across a
physical link.
Makes decisions based on
physical addresses (usually
MAC addresses).
PDU - Frames
33. Layer 1: Physical Layer
The Physical Layer defines the electrical and physical specifications for devices. This
includes the layout of pins, voltages, cable specifications, Hubs, repeaters, network
adapters, and more.
The Physical Layer will tell one device how to transmit to the communication medium, and
another device how to receive from it
The major functions and services of the Physical Layer are:
•Establishment and termination of a connection to a communications medium.
•Flow control.
•Modulation (conversion between the representation of digital data) .These are
signals operating over the physical cabling (such as copper and optical fiber) or over
a radio link
34. Layer 1 - The Physical Layer
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
This is the physical media
through which the data,
represented as electronic
signals, is sent from the
source host to the
destination host.
Examples:
CAT5 (what we have)
Coaxial (like cable TV)
Fiber optic
PDU - Bits
35. 35
Host Layers
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
These layers
only exist in the
source and
destination host
computers.
36. 36
Media Layers
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
These layers manage
the information out in
the LAN or WAN
between the source
and destination hosts.
