Data Communications and Networking

 

Network Models

RS Maniaol

Network

combination of hardware and software that sends data from one location to another

Hardware consists of physical equipment that carries signals

Software consists of instruction sets that make possible the services that we expect from a network

OSI Model

International Standards Organization (ISO) introduced the Open Systems Interconnection model in the late 1970s

OSI model ISO/IEC 7498-1

Open system – a set of protocols that allows any two different systems to communicate regardless of the underlying architecture

OSI is not a protocol; it is a model for understanding and designing a network architecture that is flexible, robust, and interoperable

OSI Layers

Physical

Data Link

Network

Transport

Session

Presentation

Application

1

2

3

4

5

6

7

Interaction between Layers

Application

Application

Presentation

Presentation

Session

Session

Transport

Transport

Network

Network

Data Link

2 - 1 Interface

Physical

Physical

Data Link

3 - 2 Interface

4 - 3 Interface

5 - 4 Interface

6 - 5 Interface

7 - 6 Interface

Physical

Physical

Data Link

Data Link

Network

Network

7 - 6 Interface

6 - 5 Interface

5 - 4 Interface

4 - 3 Interface

3 - 2 Interface

3 - 2 Interface

3 - 2 Interface

2 - 1 Interface

2 - 1 Interface

2 - 1 Interface

Encapsulation

D7

H7

D6

H6

D5

D7

H7

H5

H4

H3

H2

H1

H6

H5

H4

H3

H2

H1

D6

D5

D4

D4

D3

D3

D2

D1

D2

D1

Physical Layer

Defines the connector and interface specifications as well as the medium (cable) requirements

Electrical, mechanical, functional and procedural specifications are provided for sending a bit stream on a computer network

Physical Layer

Physical characteristics of interfaces and medium

Representation of bits - encoding

Data rate - number of bits sent each second

Synchronization of bits - timing

Line configuration – point-to-point, multipoint

Physical topology – bus, star, ring, mesh, hybrid

Transmission mode – simplex, half-duplex, duplex

Components of a Physical Layer

  • Cabling system components
  • Adapters that connect media to physical interfaces
  • Connector design and pin assignments
  • Hub, repeater and patch panel specifications
  • Wireless system components
  • Parallel SCSI
  • Network Interface Card (NIC)

Data Link Layer

Transforms the physical layer to a reliable link

Makes the physical layer appear error-free to the upper layer

Data Link Layer

Framing – defining frames

Physical addressing – defines sender/receiver

Flow control – prevents overwhelming the receiver

Error control – detect and retransmit damaged or lost frames, use of trailers

Access control – defines which device has control over the link when it is shared by devices

Components that function at Layer 2

  • NIC drivers
  • Ethernet and Token Ring Switches
  • Bridges

Hop to Hop Delivery

Network Layer

Source-to-destination delivery of packets, possibly across multiple networks

No need for network layer if two systems are connected to the same link

Network Layer

Logical addressing – distinguish source and destination systems if packets pass network boundary

Routing – makes possible the delivery of packets in internetworks, connecting devices are called routers and switches

Source to Destination Delivery

Transport Layer

Responsible for process-to-process delivery

Ensures that the whole message arrives intact and in order

Transport Layer

Service-point addressing – several programs can use the network, uses ports

Segmentation and reassembly – message divided into transmittable segments (uses sequence numbers)

Connection control – maybe connectionless or connection-oriented

Flow control – end-to-end flow control rather than a single link

Error control – process-to-process flow control rather than single link

Process to Process Delivery

Session Layer

Network dialog controller

Establishes, maintains, and synchronizes interactions among communicating systems

Dialog control – dialog between two systems

Synchronization – add checkpoints

Presentation Layer

Concerned with the syntax and semantics of the information exchanged between two systems

Translation – apply encoding schemes

Encryption – provides privacy

Compression – reduce the number of bits contained in the information

Application Layer

Enables the user (human or software) to access the network

Provides user interfaces and support services: email, remote file access and transfer, shared DBM, distributed information services

Application Layer

Network virtual terminal – software version of a software terminal

File transfer, access, and management – access files remotely

Mail services – email forwarding and storage

Directory services – distributed database sources

Sample Protocols at Layer 7

  • HTTP – Hypertext Transfer Protocol
  • FTP – File Transfer Protocol
  • SMTP – Simple Mail Transfer Protocol
  • POP3 – Post Office Protocol
  • IMAP – Internet Message Access Protocol
  • DNS – Domain Name Service
  • Telnet

OSI Layers

Physical

Data Link

Network

Transport

Session

Presentation

Application

To transmit bits over a medium; to provide mechanical and electrical specifications

To organize bits into frames; to provide hop-to-hop delivery

To move packets from source to destination; to provide internetworking

To provide reliable process-to-process message delivery and error recovery

To establish, manage and terminate sessions

To translate, encrypt and compress data

To allow access to network resources

TCP/IP Protocol Suite

Developed prior to the OSI model

Originally defined as having four layers: host-to-network, internet, transport, application

DCN views it as having five layers: physical, data link, network, transport, application

Hierarchical protocol made up of interactive modules – upper-level protocol is supported by one or more lower-level protocols

TCP/IP Protocol Suite

TCP/IP: Physical and Data Link

Does not define any specific protocol

Supports all standard and proprietary protocols

May be LAN or WAN

TCP/IP: Internetwork Layer

Internet Protocol (IP)

  • unreliable and connectionless, best-effort delivery
  • Packets are called datagrams
  • Can follow different paths and arrive out of sequence or be duplicated
  • Limited functionality is not a weakness: provides bare-bones transmission functions (efficiency)

TCP/IP: Internetwork Layer

  • Associates a logical address with a physical address
  • Used to find the physical address of the node when its Internet address is known

Address Resolution Protocol (ARP)

Reverse Address Resolution Protocol (RARP)

  • Used to find the Internet address given the physical address
  • Used when a computer is connected to a network for the first time or when a diskless computer is booted

TCP/IP: Internetwork Layer

Internet Control Message Protocol (ICMP)

  • Used by hosts and gateways to send notification of datagram problems back to sender
  • Sends query and reporting messages

Internet Group Message Protocol (IGMP)

  • Facilitates simultaneous transmission of a message to a group of recipients

TCP/IP: Transport Layer

IP is host-to-host, UDP and TCP are process-to-process

User Datagram Protocol (UDP)

  • Adds only port addresses, checksum error control, and length information
  • Connectionless, unreliable

Transmission Control Protocol (TCP)

  • Provides full transport-layer services
  • Connection-oriented, reliable stream transport
  • Streams are divided into segments with sequence numbers

TCP/IP: Transport Layer

Stream Control Transmission Protocol (SCTP)

  • best of TCP and UDP
    • message oriented like UDP
    • ensures reliable, in-sequence  transport of message like TCP
  • VoIP

TCP/IP: Application Layer

Combined Session, Presentation, and Application layers in the OSI model

TCP/IP Addressing

Physical Layer

Data Link Layer

Network Layer

Transport Layer

Application Layer

Underlying

physical

network

IP and

other protocols

SCP       TCP       UDP

Processes

Physical Addresses

Logical Addresses

Port Addresses

Specific Addresses

Physical Addresses

Lowest-level address defined by a node's LAN or WAN

Size and format varies depending on network

Example: Ethernet uses 6-byte physical addresses written as 12 hexadecimal digits separated by a colon
68:a3:c4:ce:8c:e2

Physical Addresses

Organizationally Unique Identifier (OUI)

Dell: 00-14-22
Nortel: 00-04-DC
Cisco: 00-40-96
Belkin: 00-30-BD

Physical Addresses

Logical Addresses

Necessary for universal communications – internetworks

Internet (IP) address is 32 bits

No two publicly addressed and visible host on the Internet can have the same IP address

Logical Addresses

Logical Addresses

A/10

Data

A|P

10 | 20

T2

F/20

Data

A|P

Data

Upper Layers

Network Layer

Data Link Layer

LAN 1

T/99

10 | 20

A|P

Data

T2

A|P

Data

To another network

X/44

Router 1

Sender

Logical Addresses

F/20

A|P

A|P

Data

Data

T2

LAN 2

T/99

N/33

A|P

Data

Data

A|P

T2

Z/66

A|P

A|P

Data

Data

T2

T2

Data

A|P

66 | 95

10 | 20

99 | 33

99 | 33

A|P

Data

Router 1

Router 2

Logical Addresses

LAN 3

Z/66

P/95

Data

Data

Data

Data

A|P

A|P

A|P

A|P

66 | 95

66 | 95

T2

T2

Data

Receiver

Router 2

objective of Internet communication is a process communicating with another (peer) process running on a remote computer

Port Addresses

TCP/IP Port address – assigned to a process,16 bits

Port Addresses

Protocol TCP/UDP Port Number
FTP TCP 20 (data)
21 (control)
SSH TCP 22
DNS TCP/UDP 53
DHCP UDP 67 (server)
68 (client)
HTTP TCP 80
POP3 TCP 110
NTP UDP 123

Port Addresses

Specific Addresses

User-friendly addresses
Example: email addresses, URL


Changed to corresponding port and logical addresses

References

  • Forouzan, B.A. 2007. Data Communications and Networking, 4th Ed.McGraw-Hill, New York
  • http://whatismyipaddress.com/mac-address

DCN - Network Models

By Rozano Maniaol

DCN - Network Models

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