1. Network Models
Network uses a combination of hardware and software to send data from one location to another
Performing a task is performed on different layers [Higher ---> Lower]
LAYERED TASKS
Concept of layers in our daily life
Two friends communicate through postal mail

2. Layered Tasks

3. Layered Tasks Hierarchy Services
Task must be done in the order given in the hierarchy
Sender site from up to down
Receiver site from down to up
Services
Sender site each layer uses the services of the layer immediately below it
Higher  Middle  Lower  Carrier

4. The OSI Model
1947 established ISO : International Standards Organization
ISO model covers network communications is OSI : Open System Interconnection
Introduced in the late 1970s.
Open system: a set of protocols that allows any two different systems to communicate regardless of their underlying architecture.
Purpose of the OSI : to show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software.
OSI model not a protocol, it is a model for understanding and designing a network architecture that is flexible, robust, and interoperable.

5. ISO is the organization. OSI is the model

6. THE MODEL
Is a layered framework, for design of computer systems that allows communication across all types of computer systems
Consists of seven layers each defines a part of the process of moving information across network
Understanding of the fundamentals of the OSI model provides a solid basis for exploring data communications.

7. Seven layers of the OSI model

8. Layered Architecture When device A sent message to device B
Message may pass intermediate nodes
Only the first three layers of the model involve

9. Layered Architecture At single machine Between Machines
Each layer defines functions distinct from other layers
Each calls upon the services of the layer below it
Layer 3 uses services from layer 2
Layer 3 provides services for layer 4
Between Machines
Layer x on one machine communicates with layer x on another machine by protocols.
Communication is governed by an agreed-upon series of rules and conventions called protocols
Process that communicate a given layer on one machine are called Peer-to-Peer process
Communication between machines is therefore peer-to-peer process using the protocols appropriate to a given layer

10. Peer-to-Peer process On the sending machine On receiving machine
Each layer in sending device
adds its own information to the message it receives from the layer just above it
pass the whole package to the layer below it
At layer 1 the message converted to a form that can be transferred to the receiving machine
On receiving machine
Message is unwrapped layer by layer
layer 2 removes the data meant for it, then passes the rest to layer 3 and so on.

11. Interfaces Between Layers
Between any pair of adjacent layers sending and receiving done by interface
Each one defines what information and services must provide for layer above it.
Well-defined interfaces and layer functions provide modularity to a network ( could modified or replace without affect surrounding )

12. Organization of the Layers
Seven layers could be belonged to three subgroups
Network support layers: layers 1,2 and 3
deal with physical aspects of moving data from one device to another
Such as electrical specifications, physical connections, physical addressing, and transport timing and reliability
User support layer: layer 5, 6, 7
It allows interoperability among unrelated software systems
Layer 4, transport layer
It links the two subgroups
Ensures that what the lower layers transmitted is in a form that the upper layers can use
The upper OSI layers are implemented in software
Lower layers implemented in hardware and software except for the physical layer which is mostly hardware.

13. Organization of the Layers

14. Organization of the Layers
Each layer adds a header to data except layer 2 adds also trailer
When data passes physical layer (layer 1) changed into an electromagnetic signal and transported through physical link
When reaching destination, the signal passes into layer 1 and it transformed back into digital form.
When data reaches the next higher layer
header and trailers corresponding sending layer are removed
Action appropriate to that layer are taken
In layer 7, message will be in appropriate format ( application )

15. Note The physical layer is responsible for movements of
individual bits from one hop (node) to the next.

16. Physical Layer
The physical layer is responsible for transmitting individual bits from one node to the next

17. Physical Layer
The major duties of this layer are:
Physical characteristics of interfaces and medium :
Defines :
characteristic of the interface between the devices and the transmission medium
Type of transmission medium
Representation of bits : Defines the type of encoding (how 0s and 1s are changed to signals: electrical or optical)
Data rate ( transmission rate ) : number of bits sent each second (duration of a bit)
Synchronization of bits : synchronized at bit level of sender and receiver

18. Physical Layer
Line configuration: is concerned with the connection of the devices to the media
Point-to-point configuration
Multipoint configuration
Physical topology: defines how devices are connected to make a network.
Mesh, star, ring, bus, or a hybrid.
Hybrid is a combination of two or more topologies
Transmission mode: defines the direction of transmission between two devices: simplex, half-duplex, or full-duplex.

19. Note
The data link layer is responsible for moving frames from one hop (node) to the next.

20. Data Link Layer

21. Data Link Layer
The data link layer is responsible for transmitting frames from one node to the next
Framing : divided bits into manageable data units called frames
Physical addressing : If frames distributed
On same the network, header (physical address of sender and receiver) added to frame
On different networks, receiver address is the address of device that connects networks
Flow control : controls transmission rate between sender and receiver
Error control : Link layer adds reliability to the physical layer by adding trailer to frame :
Mechanisms to detect and retransmit damaged or lost frames
Mechanisms to prevent duplication of frames
Access control : which device has control over at any given time

22. Hop-to-hop (node-to-node) delivery

23. Example
A node with physical address 10 sends a frame to a node with physical address 87
Frame contains physical address in the header
Rest of header contains other information needed at this level
The trailer contains extra bits needed for error detection

24. the source host to the destination host.
Note
The network layer is responsible for the delivery of individual packets from
the source host to the destination host.

25. Network Layer
Responsible for the source-to-destination delivery across multiple networks
Ensures that each packet gets from its point of origin to its final destination
No need for network layer if systems on the same networks

26. Network Layer Duties of the network layer:
Logical addressing : physical address locally but for universal
A header added to the packet include logical address of sender and receiver
Routing : internetwork connected by devices ( routers or gateways ) which route or switch the packets to their final destination, network layer provide this mechanism

27. Source-to-destination delivery

28. Example
Send from A to P ( network address ) or from 10 to 95 ( physical address )
Different networks
Two addresses required
Network address is universal address
Network address = logical address
Logical address remain the same from source to destination ( A and P )
Physical address changes

29. Note
The transport layer is responsible for the delivery of a message from one process to another.

30. Transport Layer
Responsible for process-to-process delivery of the entire message
Compare with Network layer
Delivery of individual packets
Does not recognize any relationship between those packets
Ensures that whole message
Intact: not changed or broken
In order ( manage error control + flow control)

31. Transport Layer

32. Transport Layer
Service-point addressing: transport layer gets the entire message to the correct process on that computer (from specific process [ running program ] on one computer to a specific process [ running program ] on the other.
Transport layer adds header called a service—point address (port address)
Network layer gets each packet to the correct computer
Transport layer gets the entire message to the correct process on that computer
Segmentation and reassembling: the message is reassembled
Correctly upon sequence number
Identify and replace packets that lost in the transmission
Connection control : either
connectionless : each packets treat independent and delivers it to destination
connection-oriented : make connection first then delivers packets then terminated
Flow control: not on single link (data link) But end to end.
Error control: not on single link? But process-to-process
Sending transport layer ensures message arrives at transport layer in receiving without error (damage, loss, or duplication)
error correction is achieved by retransmission.

33. Reliable process-to-process delivery of a message

34. The session layer is responsible for dialog control and synchronization.

35. Session Layer The session layer is the network dialog controller.
IT establishes, maintains, and synchronizes the interaction among communicating systems.
Responsibilities:
Dialog control:
Allows two system to enter into a dialog
Allows the communication between two processes to take place in:
Half-duplex or
Full-duplex
Synchronization:
Allows process to add checkpoints (synchronization points) to a stream of data
Example : add checkpoint every 100 pages for sending 2000 pages
If crash happens during the transmission of page 523 retransmission began at page 501.

36. Session Layer

37. The presentation layer is responsible for translation, compression, and encryption.

38. Presentation Layer
Concern with the syntax and semantic of the information exchanged between two systems

39. Presentation Layer Responsibilities
Translation : Information should change to bits stream before transmit
Working with different encoding methods ( different computers)
Presentation layer is responsible for interoperability between these different encoding methods.
At sender
Change information ( sender-dependent format to common format)
At receiving
Change information (common format to receiver-dependent format)
Encryption
Encryption at sending and Decryption at receiving
Compression :
Data compression reduces the number of bits to be transmitted
Compression needs in transmitted multimedia

40. Note
The application layer is responsible for providing services to the user.

41. Application layer
Enables the user (human or software) accessing the network
It provides user interfaces and services ( electronic mail, remote file access and transfer, World Wide Web)

42. Application layer
Mail services : the basis for forwarding and storage
File transfer and access: lets user to access file in a remote host
Change or read
Retrieve files
Manage or control files
Remote log-in: log into a remote computer and access the resources
Accessing the WWW

43. Summary of Layers

44. TCP/IP PROTOCOL SUITE Invented before OSI
The layers in the TCP/IP protocol suite do not exactly match those in the OSI model.
The original TCP/IP protocol suite was defined as having four layers:
host-to-network equivalent to physical + data link
internet, equivalent to network
transport, equivalent to part of duties of session
application equivalent to session + presentation + application
However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers:
physical,
data link,
network,
transport,
application.

45. TCP/IP PROTOCOL SUITE
The first four layers provide functions that correspond to the first four layers of the OSI model:
Physical standers
Network interfaces
Internetworking
Transport functions
The three topmost layers functions in the OSI model are represented in TCP/IP by application layer

46. TCP/IP and OSI model

47. TCP/IP PROTOCOL SUITE
TCP/IP is a hierarchical protocol made up of interactive modules
Each layer in TCP/IP provides specific function
The modules are not necessarily interdependent, but in OSI module specifies which functions belong to each of its layers.
Each layer has independent protocols
TCP/IP is a hierarchical protocol : each upper-level supported by one or more lower-level protocols depending on the needs of the system.
AT transport layer, TCP/IP defines three protocols
Transmission Control Protocol (TCP)
User Datagram Protocol (UDP)
Stream Control Transmission Protocol (SCTP)
At network layer, the main protocol defined by TCP/IP is the Internetworking Protocol (IP)

48. TCP/IP PROTOCOL SUITE Physical and Data Link Layers Network Layer
TCP/IP does not define any specific protocol but it supports all the standard protocols.
A network in TCP/IP internetwork can be local or a wide-area network
Network Layer
At Internetwork layer, TCP/IP supports the Internetworking Protocol. IP uses four supporting protocols:
ARP
RARP
ICMP
IGMP

49. TCP/IP PROTOCOL SUITE Internetworking Protocol (IP)
It is the transmission mechanism used by the TCP/IP protocols.
It is unreliable and connectionless protocol (a best-effort delivery service).
A best effort: IP provides no error checking or tracking.
IP assumes the unreliability of the underlying layers and does its best to get a transmission through to its destination, but with no guarantees.
IP transport data in packets separately called datagrams.
Datatgrams can travel along different routes and can arrive out of sequence or be duplicated.
IP does not keep track of the routes and has no facility for reordering datagrams when they arrive their destination.
IP provides bare-bones transmission service that free the user to add only those facilities necessary for a given application and thereby allows for maximum efficiency. (covers the weakness in its functionality)

50. TCP/IP PROTOCOL SUITE Address Resolution Protocol
Associated a logical address with a physical address.
On a typical physical network, LAN, each device is identified by a physical or station address, usually imprinted on the network interface card (NIC).
ARP is used to find the physical address of the node when its Internet address is known.
Reverse Address Resolution Protocol
RARP allows a host to discover its Internet address when it knows only its physical address.
RARP is used when a computer is connected to a network for the first time or when a diskless computer is booted.

51. TCP/IP PROTOCOL SUITE Internet Control Message protocol
ICMP is a mechanism used by a host and gateways to send notification of datagram problems back to the sender.
ICMP sends query and error reporting messages.
Internet Group Message Protocol
IGMP is used to facilitate the simultaneous transmission of a message to a group of recipients.
Transport Layer
Traditionally the transport layer represented in TCP/IP by TCP and UDP .
IP is host-to-host protocol, deliver a packet from one physical device to another.
UDP and TCP are transport level protocols responsible for delivery of a message from a process to another process.
SCTP, has been devised to meet the needs of some newer applications.

52. TCP/IP PROTOCOL SUITE User Datagram Protocol
UDP is the simple of the two standard TCP/IP transport protocol.
UDP is process-to-process adds:
port addresses
Checksum error control
Length information to the data from the upper layer
Transmission Control Protocol
TCP provides full transport-layer services to applications.
TCP is a reliable stream (connection-oriented) transport protocol.
At sending end of each transmission:
TCP divides a stream of data into smaller units called segments
Each segment includes a sequence number for recording after receipt
An acknowledgment number for the segments received
Segments carried across the internet inside of IP datagrams.
At receiving end
TCP collects each datagram as it comes in and reorders the transmission based on sequence numbers

53. TCP/IP PROTOCOL SUITE Stream Control Transmission Protocol
SCTP provides support for newer applications as voice over the Internet.
SCTP is a transport layer protocol that combines the best features of UDP and TCP.
Application Layer
Application layer in TCP/IP is equivalent to the combined session, presentation, and applications layers in the OSI model.
Many protocols are defined at this layer.

54. ADDRESSING
Four levels of addresses are used in an internet employing the TCP/IP protocols:

55. ADDRESSING
Each address is related to a specific layer in the TCP/IP architecture.

56. ADDRESSING Physical Addresses It is known as the link address.
It is the address of a node as defined by its LAN and WAN.
It is included in the frame used by the data link layer.
It is the lowest-level address.
It has authority over the network (LAN or WAN).
The size and format of these addresses vary depending on the network.
The Ethernet uses a 6-byte (48 bit) physical address that is imprinted on NIC.
LocalTalk (Apple) has 1-byte dynamic address.

57. ADDRESSING
In Figure 2.19 a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link (bus topology LAN). As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver.

58. A 6-byte (12 hexadecimal digits) physical address.
ADDRESSING
As we will see in Chapter 13, most local-area networks use a 48-bit (6-byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below:
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address.

59. ADDRESSING Logical Addresses
It is necessary for universal communications.
Physical addresses are not adequate in an internetwork environment where different networks can have different address format.
A universal addressing system is needed in which each host can be identified uniquely, regardless of the underlying physical newtwork.
A logical address in the Internet is currently a 32-bit address.
No two publicly addressed and visible hosts on the Internet can have the same IP address.

60. IP Addresses
Figure 2.20 shows a part of an internet with two routers connecting three LANs. Each device (computer or router) has a pair of addresses (logical and physical) for each connection. In this case, each computer is connected to only one link and therefore has only one pair of addresses. Each router, however, is connected to three networks (only two are shown in the figure). So each router has three pairs of addresses, one for each connection.

61. Port Addresses
The end objective of Internet communication is a process communicating with another process.
In TCP/IP architecture, the label assigned to a process is called a port address.
A port address in TCP/IP is 16 bits in length.

62. Port Addresses
Figure 2.21 shows two computers communicating via the Internet. The sending computer is running three processes at this time with port addresses a, b, and c. The receiving computer is running two processes at this time with port addresses j and k. Process a in the sending computer needs to communicate with process j in the receiving computer. Note that although physical addresses change from hop to hop, logical and port addresses remain the same from the source to destination

63. Addresses The physical addresses will change from hop to hop,
but the logical addresses usually remain the same.

64. A 16-bit port address represented as one single number.
Port Addresses
As we will see in Chapter 23, a port address is a 16-bit address represented by one decimal number as shown.
753
A 16-bit port address represented as one single number.

65. Specific Addresses
address
Universal Resource Locator (URL)