Open Systems Interconnection

1. (open systems interconnection)
By: SAIF ULLAH KHAN

2.  The Open Systems Interconnection (OSI) model (ISO/IEC
7498-1) is a product of the Open Systems
Interconnection effort at the International Organization for
Standardization
 Similar communication functions
are grouped into logical layers

3.  Work on a layered model of network architecture was
started and the International Organization for
Standardization (ISO) began to develop its OSI framework
architecture
 The concept of a seven-layer model was provided by the
work of Charles Bachman, Honeywell Information Services
 Each entity interacted directly only with the layer
immediately beneath it, and provided facilities for use by
the layer above it
 Protocols enabled an entity in one host to interact with a
corresponding entity at the same layer in another host

4.  According to recommendation X.200, there are seven
layers, labeled 1 to 7, with layer 1 at the bottom
 Each layer is generically known as an N layer
 At each level, two entities (N-entity peers) interact by
means of the N protocol by transmitting protocol data
units (PDU)
 A Service Data Unit (SDU) is a specific unit of data that has
been passed down from an OSI layer to a lower layer, and
which the lower layer has not yet encapsulated into a
protocol data unit (PDU)
 An SDU is a set of data that is sent by a user of the services
of a given layer, and is transmitted semantically unchanged
to a peer service user

5.  The PDU at a layer N is the SDU of layer N-1
 In effect the SDU is the 'payload' of a given PDU
 That is, the process of changing an SDU to a PDU, consists
of an encapsulation process, performed by the lower layer
 All the data contained in the SDU becomes encapsulated
within the PDU
 The layer N-1 adds headers or footers, or both, to the SDU,
transforming it into a PDU of layer N-1
 The added headers or footers are part of the process used
to make it possible to get data from a source to a
destination

6.  The physical layer defines electrical and physical
specifications for devices
 it defines the relationship between a device and
a transmission medium, such as a copper or fiber optical
cable
 This includes the layout
of pins, voltages, line impedance, cable specifications, sig
nal timing, hubs, repeaters, network adapters, host bus
adapters (HBA used in storage area networks) and more
 Parallel SCSI buses operate in this layer
 The same applies to other local-area networks, such
as token ring, FDDI, ITU-T G.hn and IEEE 802.11, as well as
personal area networks such as Bluetooth and IEEE
802.15.4

7. Network
Layer Functions Protocols
Component
Repeater
IEEE 802
Multiplexer Hubs
ISO 2110
Transmits raw bit stream over physical cable
ISDN
Passive
EIA/TIA-232
Active
EIA/TIA-449
Defines cables, cards, and physical aspects ITU-T V-Series TDR
I.430
Establishment and termination of a connection to I.431 Oscilloscope
a communications medium PDH
Physical
Defines techniques to transfer bit stream to SONET/SDH
Hardware; Amplifier
cable PON
Raw bit
OTN
Stream Participation in the process whereby the DSL
communication resources are effectively shared IEEE 1394
among multiple users. For ITU-T G.hn
example, contention resolution and flow control PHY
Modulation or conversion between the USB
representation of digital data in user equipment Bluetooth
and the corresponding signals transmitted over a RS-232
communications channel RS-449

8.  The data link layer provides the functional and procedural
means to transfer data between network entities and to
detect and possibly correct errors that may occur in the
physical layer
 Data Link layer defines the format of data on the network
 In modern practice, only error detection, not flow control
using sliding window, is present in data link protocols such
as Point-to-Point Protocol (PPP), and, on local area
networks, the IEEE 802.2 LLC layer is not used for most
protocols on the Ethernet, and on other local area
networks, its flow control and acknowledgment
mechanisms are rarely used
 A network data frame, aka packet, includes checksum,
source and destination address, and data

9.  The largest packet that can be sent through a data link
layer defines the Maximum Transmission Unit (MTU)
 Sliding window flow control and acknowledgment is used
at the transport layer by protocols such as TCP, but is still
used in niches where X.25 offers performance advantages
 The data link layer handles the physical and logical
connections to the packet's destination, using a network
interface
 The ITU-T G.hn standard, which provides high-speed local
area networking over existing wires (power lines, phone
lines and coaxial cables), includes a complete data link
layer which provides both error correction and flow control
by means of a selective repeat Sliding Window Protocol
 Both WAN and LAN service arranges bits, from the physical
layer, into logical sequences called frames

10.  A host connected to an Ethernet would have an Ethernet
interface to handle connections to the outside world, and
a loopback interface to send packets to itself
 Ethernet addresses a host using a unique, 48-bit address
called its Ethernet address or Media Access Control (MAC)
address
 MAC addresses are usually represented as six colon-
separated pairs of hex digits, e.g., 8:0:20:11:ac:85
 This number is unique and is associated with a particular
Ethernet device

11. Network
Layer Function Protocol
Component
Logical Link Control:
Turns packets into raw bits 100101 and at • Error correction and flow Bridge
the receiving end turns bits into packets control Switch
• Control and defines SAPs
Handles data frames between the Network
802.2 Logical Link Control ISDN Router
and Physical layers
Media Access Control
The receiving end packages raw data from • communicates with the
Intelligent
the Physical layer into data frames for adapter card
Hub
Data layer delivery to the Network layer • controls the type of media
Link Data being used:
frames to 802.3 CSMA/CD (Ethernet)
bits Responsible for error-free transfer of frames 802.4 Token Bus (ARC net)
NIC
to other computer via the Physical Layer 802.5 Token Ring
802.12 Demand Priority
This layer defines the methods used to
transmit and receive data on the network. It ATM , SDLC, HDLC, CSLIP
consists of the wiring, the devices use to SLIP , GFP, PLIP, IEEE802.2
Advanced
connect the NIC to the wiring, the signaling LLC , L2TP, IEEE 802.3
Cable Tester
involved to transmit / receive data and the Frame Relay, ITU-T G.hn
ability to detect signaling errors on the DLL , PPP, X.25
network media

12.  The network layer provides the functional and procedural
means of transferring variable length data sequences from
a source host on one network to a destination host on a
different network (in contrast to the data link layer which
connects hosts within the same network), while
maintaining the quality of service requested by the
transport layer
 The network layer performs network routing functions, and
might also perform fragmentation and reassembly, and
report delivery errors
 Routers operate at this layer, sending data throughout the
extended network and making the Internet possible.
 IP is responsible for routing, directing datagrams from one
network to another

13.  The network layer may have to break large
datagrams, larger than MTU, into smaller packets and host
receiving the packet will have to reassemble the
fragmented datagram
 The Internetwork Protocol identifies each host with a 32-
bit IP address
 IP addresses are written as four dot-separated decimal
numbers between 0 and 255, e.g., 129.79.16.40
 The leading 1-3 bytes of the IP identify the network and
the remaining bytes identifies the host on that network.
 The Address Resolution Protocol (ARP) is used to map the
IP address to it hardware address
 A number of layer-management protocols, a function
defined in the Management Annex, ISO 7498/4, belong to
the network layer

14. The network layer may be divided into three sub-layers:
 Sub-network access
 that considers protocols that deal with the interface to networks,
such as X.25
 Sub-network (dependent convergence)
 when it is necessary to bring the level of a transit network up to
the level of networks on either side
 Sub-network (independent convergence)
 handles transfer across multiple networks

15. Network
Layer Function Protocols
Component
Translates logical network address and names to their
Router Frame
physical address (e.g. computer name ==> MAC address)
IP
ARP
RARP
Responsible for ICMP
• Addressing RIP
Network
• Determining routes for sending OSFP Relay Device
Addressing;
• Managing network problems such as packet switching, IGMP
Routing
data congestion and routing IPX
NWLink
NetBEUI
DDP ATM Switch
If router can’t send data frame as large as the source
DECnet
computer sends, the network layer compensates by
breaking the data into smaller units. At the receiving end, Advanced
the network layer reassembles the data Cable Tester

16.  Transport layer subdivides user-buffer into network-buffer
sized datagrams and enforces desired transmission control
 The transport layer provides transparent transfer of data
between end users, providing reliable data transfer
services to the upper layers
 The transport layer controls the reliability of a given link
through flow control, segmentation/de-segmentation, and
error control
 Some protocols are state and connection-oriented
 This means that the transport layer can keep track of
the segments and retransmit those that fail.
 Two transport protocols, Transmission Control Protocol
(TCP) and User Datagram Protocol (UDP), sits at the
transport layer

17.  Reliability and speed are the primary difference between
these two protocols
 TCP:
 TCP establishes connections between two hosts on the
network through 'sockets' which are determined by the
IP address and port number
 TCP keeps track of the packet delivery order and the
packets that must be resent
 UDP:
 UDP on the other hand provides a low overhead
transmission service, but with less error checking.
 NFS (network file system) is built on top of UDP because of
its speed and statelessness. Statelessness simplifies the
crash recovery

18.  OSI defines five classes of connection-mode transport protocols ranging from class 0
(which is also known as TP0 and provides the least features) to class 4
(TP4, designed for less reliable networks, similar to the Internet)
Feature Name TP0 TP1 TP2 TP3 TP4
Connection oriented network YES YES YES YES YES
Connectionless network NO NO NO NO YES
Concatenation and separation NO YES YES YES YES
Segmentation and reassembly YES YES YES YES YES
Error Recovery NO YES YES YES YES
Reinitiate connection (if an excessive number
NO YES NO YES NO
of PDUs are unacknowledged)
Multiplexing and de-multiplexing over a single virtual
NO NO YES YES YES
circuit
Explicit flow control NO NO YES YES YES
Retransmission on timeout NO NO NO NO YES
Reliable Transport Service NO YES NO YES YES

19. Network
Layer Function Protocol
Component
Manages the flow control of data between parties TCP
ARP Gateway
across the network
RARP
Divides streams of data into chunks or packets; the SPX Brouter
Transport transport layer of the receiving computer NWLink (Bridge Router)
Packets; Flow reassembles the message from packets NetBIOS
control & Provides error-checking to guarantee error-free data NetBEUI Advanced Cable
Error-handling delivery, with on losses or duplications ATP Tester
UDP
Provides acknowledgment of successful transmissions; SCTP
requests retransmission if some packets don’t arrive DCCP
error-free SPX

20.  The session protocol defines the format of the data sent over the
connections
 The NFS uses the Remote Procedure Call (RPC) for its session
protocol
 RPC may be built on either TCP or UDP
 Login sessions uses TCP whereas NFS and broadcast use UDP
 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, and
establishes check pointing, adjournment, termination, and restart
procedures
 The OSI model made this layer responsible for graceful close of
sessions, which is a property of the Transmission Control Protocol,
and also for session check pointing and recovery, which is not usually
used in the Internet Protocol Suite

21. Network
Layer Function Protocol
Component
Establishes, maintains and ends sessions across the network
NetBIOS
Responsible for name recognition (identification) so only Named
the designated parties can participate in the session Pipes
Provides synchronization services by planning check points Mail
in the data stream => if session fails, only data after the Slots
Session
most recent checkpoint need be transmitted RPC
Syncs and Gateway
SAP
Sessions Manages who can transmit data at a certain time and for PPTP
how long RTP
Examples are interactive login and file transfer SOCKS
connections, the session would connect and re-connect if SPDY
there was an interruption; recognize names in sessions and TLS/SSL
register names in history

22.  The presentation layer establishes context between
application-layer entities, in which the higher-layer
entities may use different syntax and semantics if the
presentation service provides a mapping between them
 External Data Representation (XDR) sits at the presentation
level
 It converts local representation of data to its canonical
form and vice versa
 The canonical uses a standard byte ordering and structure
packing convention, independent of the host
 If a mapping is available, presentation service data units
are encapsulated into session protocol data units, and
passed down the stack

23.  This layer provides independence from data representation
(e.g., encryption) by translating between application and
network formats
 The presentation layer transforms data into the form that
the application accepts
 This layer formats and encrypts data to be sent across a
network
 It is sometimes called the syntax layer
 The original presentation structure used the basic encoding
rules of Abstract Syntax Notation One (ASN.1), with
capabilities such as converting an EBCDIC-coded
text file to an ASCII-coded file, or
serialization of objects and other data structures from and
to XML

24. Network
Layer Function Protocol
Component
Translates from application to network format and vice-
versa
All different formats from all sources are made into a
common uniform format that the rest of the OSI model can
Presentation understand MIME Gateway
Translation Responsible for protocol conversion, character conversion, XDR Redirector
data encryption / decryption, expanding graphics
commands, data compression
Sets standards for different systems to provide seamless
communication from multiple protocol stacks

25.  Provide network services to the end-users
 Mail, ftp, telnet, DNS, NIS, NFS are examples of network
applications
 The application layer is the OSI layer closest to the end user,
which means that both the OSI application layer and the user
interact directly with the software application
 This layer interacts with software applications that
implement a communicating component
 Application-layer functions typically include identifying
communication partners, determining resource availability,
and synchronizing communication
 When identifying communication partners, the application
layer determines the identity and availability of
communication partners for an application with data to
transmit

26.  When determining resource availability, the application
layer must decide whether sufficient network or the
requested communication exist
 In synchronizing communication, all communication
between applications requires cooperation that is managed
by the application layer
 Some examples of application-layer implementations
also include:
 On OSI stack:
 FTAM File Transfer and Access Management Protocol
 X.400 Mail
 Common Management Information Protocol (CMIP)
 On TCP/IP stack:
 Hypertext Transfer Protocol (HTTP),
 File Transfer Protocol (FTP),
 Simple Mail Transfer Protocol (SMTP)
 Simple Network Management Protocol (SNMP).

27. Network
Layer Function Protocol Componen
t
Used for applications specifically written to run over DNS, FTP
the network TFTP, BOOTP
SNMP, RLOGIN
Allows access to network services that support
SMTP, MIME
applications
NFS, FINGER
Application Directly represents the services that directly support TELNET, NCP
User user applications APPC, AFP Gateway
Interface SMB, NNTP
Handles network access, flow control and error
SIP, SSI
recovery
Gopher, HTTP
NTP, SMPP
Example apps are file transfer, e-mail, NetBIOS-
Netconf,
based applications
(more)

29.  There are some functions or services that are not tied to a given layer, but they can affect
more than one layer
 Examples include the following:
 Security service (telecommunication) as defined by ITU-T X.800 Recommendation
 management functions:
 functions that permit to configure, instantiate, monitor, terminate the
communications of two or more entities: there is a specific application layer
protocol, common management information protocol (CMIP) and its corresponding
service, common management information service (CMIS), they need to interact
with every layer in order to deal with their instances
 ARP:
 It is used to translate IPv4 addresses (OSI layer 3) into Ethernet MAC addresses (OSI
layer 2)
 Multiprotocol Label Switching (MPLS):
 It operates at an OSI-model layer that is generally considered to lie between
traditional definitions of layer 2 (data link layer) and layer 3 (network layer), and
thus is often referred to as a "layer-2.5" protocol. It was designed to provide a
unified data-carrying service for both circuit-based clients and packet-switching
clients which provide a datagram service model. It can be used to carry many
different kinds of traffic, including IP packets, as well as native ATM, SONET, and
Ethernet frames

30.  In the TCP/IP model of the Internet, protocols are
deliberately not as rigidly designed into strict layers as in
the OSI model
 However, TCP/IP does recognize four broad layers of
functionality which are derived from the operating scope
of their contained protocols, namely the scope of the
software application, the end-to-end transport connection,
the internetworking range, and the scope of the direct
links to other nodes on the local network
 Even though the concept is different from the OSI model,
these layers are nevertheless often compared with the OSI
layering scheme in the following way:
 The Internet application layer includes the OSI application layer,
presentation layer, and most of the session layer

31.  Its end-to-end transport layer includes the graceful close
function of the OSI session layer as well as the OSI
transport layer.
 The internetworking layer (Internet layer) is a subset of
the OSI network layer, while the link layer includes the OSI
data link and physical layers, as well as parts of OSI's
network layer.
 These comparisons are based on the original seven-layer
protocol model as defined in ISO 7498, rather than
refinements in such things as the internal organization of
the network layer document

32. Thank you for your time