1. Data and Communication Systems
Lecture 2
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Engr. Ahmad Bilal
Assistant Professor
University of Lahore

2. Protocol
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3. Protocol diagrams A human protocol and a computer network protocol: Hi
TCP connection
request Hi
TCP connection
response
Got the
time?
Get
2:00
<file>
time

4. 2-1 LAYERED TASKS
We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office.

5. Figure 2.1 Tasks involved in sending a letter

6. 2-2 THE OSI MODEL
Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late 1970s.

7. ISO is the organization. OSI is the model.
Note
ISO is the organization. OSI is the model.

8. The Function of a Layer Each layer deals with one aspect of networking
Layer 1 deals with the communication media
Each layer communicates with the adjacent layers
In both directions
Ex: Network layer communicates with:
Transport layer
Data Link layer
Each layer formats the data packet
Ex: Adds or deletes addresses

9. Layering Simplifies complex systems
Each layer relies on services from layer below and exports services to layer above
Hides implementation, eases maintenance and updating of system
Layer implementations can change without disturbing other layers (black box)
Introduction
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10. Host-to-host connectivity
Layering
Examples:
Topology and physical configuration hidden by network-layer routing
Applications require no knowledge of this
New applications deployed without coordination with network operators or operating system vendors
Application
Host-to-host connectivity
Link hardware
Introduction
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11. Seven layers of the OSI model

12. Figure 2.3 The interaction between layers in the OSI model

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14. Role of Layers Node A 7. Application 6. Presentation Data In To/from
1. Physical
To/from
Node B
Data Out

15. The Role of Layers in Point-to-point Communication
Node b
Node a
7. Application
7. Application
1.Physical
1. Physical

16. Virtual Communication Between Layers
7. Application
7. Application
3. Network
3. Network

17. Example information flow supporting virtual communication in layer.

18. Encapsulation source destination application transport network link
message M
application
transport
network
link
physical
segment Ht M
datagram Ht Hn M
frame Ht Hn Hl M
link
physical Ht Hn Hl M Ht Hn Hl M
switch
destination
network
link
physical Ht Hn M Ht Hn M
application
transport
network
link
physical Ht Hn Hl M M Ht Hn Hl M Ht M Ht Hn M
router Ht Hn Hl M
Introduction
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19. Communication Between Layers
7. Application
Data
Encapsulation
6. Presentation
Data
Stripping
5. Session

20. The OSI Environment

21. Figure 2.4 An exchange using the OSI model

22. Key Elements of a Protocol
syntax - data format
semantics - control info & error handling
timing - speed matching & sequencing
Communication is achieved by having the corresponding, or peer, layers in two systems communicate. The peer layers communicate by means of formatted blocks of data that obey a set of rules or conventions known as a protocol. The key features of a protocol are:
• Syntax: Concerns the format of the data blocks
• Semantics: Includes control information for coordination and error handling
• Timing: Includes speed matching and sequencing

23. Service Primitives
Five service primitives for implementing a simple connection-oriented service.

24. Service Primitives (2)
Packets sent in a simple client-server interaction on a connection-oriented network.

25. 2-3 LAYERS IN THE OSI MODEL
In this section we briefly describe the functions of each layer in the OSI model.

26. 1. Physical Layer Purpose Example
Deals with the transmission of 0s and 1s over the physical media
Translation of bits into signals
Example
Pulse duration determination
Transmission synchronization
etc.

27. Physical Layer Function
Encode bits into signals
Carry data from the h higher layers
Define the interface to the card
Electrical
Mechanical
Functional
Example: Pin count on the connector

28. Lower Layers Application Areas
Special significance to network card design
Applies to general LAN hardware design
Exceptions
Routers etc.
802. standards
Centered around the lower layers
Applies to networks

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

30. Figure 2.5 Physical layer

31. Media Access Control Application
Network Interface Card driver
NETWORK
CARD
NETWORK
SOFTWARE
NIC Driver
facilitates data
transfer

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

33. Figure 2.6 Data link layer

34. Network Layer Function
Address messages
Address translation from logical to physical
Ex: >
Routing of data
Based on priority
Best path at the time of transmission
Congestion control

35. Figure 2.8 Network layer

36. 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.

37. 4. Transport Layer Purpose Example
Repackage proper and efficient delivery of packages
Error free
In sequence
Without duplication
Example

38. Transport Layer Function
For sending data
Repackage the message to fit into packets
Split long messages
Assemble small messages
On receiving data
Perform the reverse
Send an acknowledgment to the sender
Solve packet problems
During transmission and reception

39. Figure 2.10 Transport layer

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

41. Figure 2.11 Reliable process-to-process delivery of a message

42. Session Layer Function
Performs name recognition and related security
Synchronization between sender and receiver
Assignment of time for transmission
Start time
End time etc.

43. Figure Session layer

44. Note
The session layer is responsible for dialog control and synchronization.

45. Presentation Layer Function
Protocol conversion
Data translation
Encryption
Character set conversion

46. Figure 2.13 Presentation layer

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

48. 7. Application Layer Purpose Examples
User application to network service interface
Examples
File request from server
services
etc.

49. Application Layer Function
General network access
Flow control
Error recovery

50. Figure 2.14 Application layer

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

52. Figure 2.15 Summary of layers

53. 2-4 TCP/IP PROTOCOL SUITE
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, internet, transport, and 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, and application.

54. Figure 2.16 TCP/IP and OSI model

55. 2-5 ADDRESSING
Four levels of addresses are used in an internet employing the TCP/IP protocols: physical, logical, port, and specific.

56. Figure 2.17 Addresses in TCP/IP

57. Figure 2.18 Relationship of layers and addresses in TCP/IP

58. Example 2.1
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.

59. Figure 2.19 Physical addresses

60. A 6-byte (12 hexadecimal digits) physical address.
Example 2.2
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.

61. Figure IP addresses

62. Figure Port addresses

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

64. Traditional vs Multimedia Applications
traditionally Internet dominated by info retrieval applications
typically using text and image transfer
eg. , file transfer, web
see increasing growth in multimedia applications
involving massive amounts of data
such as streaming audio and video
The Internet, until recently, has been dominated by information retrieval applications, , and file transfer, plus Web interfaces that emphasized text and images. Increasingly, the Internet is being used for multimedia applications that involve massive amounts of data for visualization and support of real-time interactivity. Streaming audio and video are perhaps the best known of such applications.
Although traditionally the term multimedia has connoted the simultaneous use of multiple media types (e.g., video annotation of a text document), the term has also come to refer to applications that require real-time processing or communication of video or audio alone. Thus, voice over IP (VoIP), streaming audio, and streaming video are considered multimedia applications even though each involves a single media type.

65. Elastic and Inelastic Traffic
can adjust to delay & throughput changes over a wide range
eg. traditional “data” style TCP/IP traffic
some applications more sensitive though
inelastic traffic
does not adapt to such changes
eg. “real-time” voice & video traffic
need minimum requirements on net arch
Traffic on a network or internet can be divided into two broad categories: elastic and inelastic. Elastic traffic can adjust, over wide ranges, to changes in delay and throughput across an internet and still meet the needs of its applications. This is the traditional type of traffic supported on TCP/IP-based internets and is the type of traffic for which internets were designed. Elastic applications include common Internet-based applications, such as file transfer, electronic mail, remote logon, network management, and Web access. But there are differences among the requirements of these applications.
Inelastic traffic does not easily adapt, if at all, to changes in delay and throughput across an internet. The prime example is real-time traffic, such as voice and video. The requirements for inelastic traffic may include the following: minimum throughput may be required, may be delay-sensitive, may require a reasonable upper bound on delay variation, may vary in the amount of packet loss, if any, that they can sustain.
These requirements are difficult to meet in an environment with variable queuing delays and congestion losses. Accordingly, inelastic traffic introduces two new requirements into the internet architecture.

66. OSI Layers

67. OSI v TCP/IP
There are a number of reasons why the TCP/IP architecture has come to dominate. Perhaps the most important is that the key TCP/IP protocols were mature and well tested at a time when similar OSI protocols were in the development stage. When businesses began to recognize the need for interoperability across networks, only TCP/IP was available and ready to go. Another reason is that the OSI model is unnecessarily complex, with seven layers to accomplish what TCP/IP does with fewer layers. Stallings DCC8e Figure 2.7 illustrates the layers of the TCP/IP and OSI architectures, showing roughly the correspondence in functionality between the two.

68. Physical Layer
concerned with physical interface between computer and network
concerned with issues like:
characteristics of transmission medium
signal levels
data rates
other related matters
The physical layer covers the physical interface between a data transmission device (e.g., workstation, computer) and a transmission medium or network. This layer is concerned with specifying the characteristics of the transmission medium, the nature of the signals, the data rate, and related matters.

69. Network Access Layer
exchange of data between an end system and attached network
concerned with issues like :
destination address provision
invoking specific services like priority
access to & routing data across a network link between two attached systems
allows layers above to ignore link specifics
The network access layer is concerned with the exchange of data between an end system (server, workstation, etc.) and the network to which it is attached. The sending computer must provide the network with the address of the destination computer, so that the network may route the data to the appropriate destination. The sending computer may wish to invoke certain services, such as priority, that might be provided by the network. The specific software used at this layer depends on the type of network to be used; different standards have been developed for circuit switching, packet switching (e.g., frame relay), LANs (e.g., Ethernet), and others. Thus it makes sense to separate those functions having to do with network access into a separate layer.

70. Internet Layer (IP) routing functions across multiple networks
for systems attached to different networks
using IP protocol
implemented in end systems and routers
routers connect two networks and relays data between them
The internet layer provides procedures used to allow data to traverse multiple interconnected networks, to provide communications between devices are attached to different networks. The Internet Protocol (IP) is used at this layer to provide the routing function across multiple networks. This protocol is implemented not only in the end systems but also in routers. A router is a processor that connects two networks and whose primary function is to relay data from one network to the other on its route from the source to the destination end system.

71. Transport Layer (TCP) common layer shared by all applications
provides reliable delivery of data
in same order as sent
commonly uses TCP
The host-to-host layer, or transport layer, collects mechanisms in a common layer shared by all applications to provide reliable delivery of data. Regardless of the nature of the applications, there is usually a requirement that data be exchanged reliably, ensuring that all of the data arrives at the destination application and that the data arrives in the same order in which they were sent. These mechanisms for providing reliability are essentially independent of the nature of the applications. The Transmission Control Protocol (TCP) is the most commonly used protocol to provide this functionality.

72. Application Layer provide support for user applications
need a separate module for each type of application
Finally, the application layer contains the logic needed to support the various user applications. For each different type of application, such as file transfer, a separate module is needed that is peculiar to that application.

73. Addressing Requirements
two levels of addressing required
each host on a subnet needs a unique global network address
its IP address
each application on a (multi-tasking) host needs a unique address within the host
known as a port
For successful communication, every entity in the overall system must have a unique address. Actually, two levels of addressing are needed. Each host on a subnetwork must have a unique global internet address; this allows the data to be delivered to the proper host. Each process with a host must have an address that is unique within the host; this allows the host-to-host protocol (TCP) to deliver data to the proper process. These latter addresses are known as ports.

74. Transmission Control Protocol (TCP)
usual transport layer is (TCP)
provides a reliable connection for transfer of data between applications
a TCP segment is the basic protocol unit
TCP tracks segments between entities for duration of each connection
For most applications running as part of the TCP/IP protocol architecture, the transport layer protocol is TCP. TCP provides a reliable connection for the transfer of data between applications. A connection is simply a temporary logical association between two entities in different systems. A logical connection refers to a given pair of port values. For the duration of the connection each entity keeps track of TCP segments coming and going to the other entity, in order to regulate the flow of segments and to recover from lost or damaged segments.

75. TCP Header
TCP segments include a header. Stallings DCC8e Figure 2.3a shows the header format for TCP, which is a minimum of 20 octets, or 160 bits. The Source Port and Destination Port fields identify the applications at the source and destination systems that are using this connection. The Sequence Number, Acknowledgment Number, and Window fields provide flow control and error control. The checksum is a 16-bit frame check sequence used to detect errors in the TCP segment. Chapter 20 provides more details.

76. User Datagram Protocol (UDP)
an alternative to TCP
no guaranteed delivery
no preservation of sequence
no protection against duplication
minimum overhead
adds port addressing to IP
In addition to TCP, there is one other transport-level protocol that is in common use as part of the TCP/IP protocol suite: the User Datagram Protocol (UDP). UDP does not guarantee delivery, preservation of sequence, or protection against duplication. UDP enables a procedure to send messages to other procedures with a minimum of protocol mechanism. Some transaction-oriented applications make use of UDP; eg SNMP (Simple Network Management Protocol). Because it is connectionless, UDP has very little to do. Essentially, it adds a port addressing capability to IP.

77. UDP Header
Because it is connectionless, UDP has very little to do. just adding a port addressing capability to IP. This is best seen by examining the UDP header, shown in Stallings DCC8e Figure 2.3b. The UDP header also includes a checksum to verify that no error occurs in the data; the use of the checksum is optional.

78. IP Header
For decades, the keystone of the TCP/IP protocol architecture has been IP. Stallings DCC8e Figure 2.4a shows the IP header format, which is a minimum of 20 octets, or 160 bits. The header, together with the segment from the transport layer, forms an IP-level PDU referred to as an IP datagram or an IP packet. The header includes 32-bit source and destination addresses. The Header Checksum field is used to detect errors in the header to avoid misdelivery. The Protocol field indicates which higher-layer protocol is using IP. The ID, Flags, and Fragment Offset fields are used in the fragmentation and reassembly process. Chapter 18 provides more detail.

79. IPv6 Header
In 1995, the Internet Engineering Task Force (IETF), which develops protocol standards for the Internet, issued a specification for a next-generation IP, known then as IPng. This specification was turned into a standard in 1996 known as IPv6. IPv6 provides a number of functional enhancements over the existing IP, designed to accommodate the higher speeds of today's networks and the mix of data streams, including graphic and video, that are becoming more prevalent. But the driving force behind the development of the new protocol was the need for more addresses. The current IP uses a 32-bit address to specify a source or destination. With the explosive growth of the Internet and of private networks attached to the Internet, this address length became insufficient to accommodate all systems needing addresses. As Stallings DCC8e Figure 2.4b shows, IPv6 includes 128-bit source and destination address fields. Ultimately, all installations using TCP/IP are expected to migrate from the current IP to IPv6, but this process will take many years, if not decades.

80. TCP/IP Applications
have a number of standard TCP/IP applications such as
Simple Mail Transfer Protocol (SMTP)
File Transfer Protocol (FTP)
Telnet
A number of applications have been standardized to operate on top of TCP. We mention three of the most common here.
The Simple Mail Transfer Protocol (SMTP) provides a basic electronic mail transport facility for transferring messages among separate hosts. The SMTP protocol does not specify the way in which messages are to be created; some local editing or native electronic mail facility is required. The target SMTP module will store the incoming message in a user's mailbox.
The File Transfer Protocol (FTP) is used to send files from one system to another under user command. Both text and binary files are accommodated. FTP sets up a TCP connection to the target system for the exchange of control messages. Once a file transfer is approved, a second TCP data connection is set up for the data transfer, without the overhead of any headers or control information at the application level. When the transfer is complete, the control connection is used to signal the completion and to accept new file transfer commands.
TELNET provides a remote logon capability, which enables a user at a terminal or personal computer to logon to a remote computer and function as if directly connected to that computer. The protocol was designed to work with simple scroll-mode terminals. Terminal traffic between User and Server TELNET is carried on a TCP connection.

81. Internet protocol stack
application: supporting network applications
FTP, SMTP, HTTP
transport: process-process data transfer
TCP, UDP
network: routing of datagrams from source to destination
IP, routing protocols
link: data transfer between neighboring network elements
PPP, Ethernet
physical: bits “on the wire”
application
transport
network
link
physical
Introduction
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82. OSI Layers
Stallings DCC8e Figure 2.6 illustrates the OSI model and provides a brief definition of the functions performed at each layer. The intent of the OSI model is that protocols be developed to perform the functions of each layer.

83. Layering in Networks: OSI Model
Physical
how to transmit bits
Data link
how to transmit frames
Network
how to route packets host-to-host
Transport
how to send packets end2end
Session
how to tie flows together
Presentation
byte ordering, formatting
Application: everything else
Host
Application
Transport
Network
Data Link
Presentation
Session
Physical
Introduction
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