The OSI Model

Top>>Down: All People Seem To Need Data Processing

Bottom>> Up: Please Do Not Throw Sausage Pizza Away

The standard model for networking protocols and distributed applications is the International
Standard Organization's Open System Interconnect (ISO/OSI) model. It defines seven network
layers. Short for Open System Interconnection, an ISO standard for worldwide
communications that defines a networking framework for implementing protocols in seven
layers. Control is passed from one layer to the next, starting at the application layer in one
station, and proceeding to the bottom layer, over the channel to the next station and back up the
hierarchy.

At one time, most vendors agreed to support OSI in one form or another, but OSI was too loosely
defined and proprietary standards were too entrenched. Except for the OSI-compliant X.400 and
X.500 e-mail and directory standards, which are widely used, what was once thought to become
the universal communications standard now serves as the teaching model for all other protocols.

Control is passed from one layer to the next, starting at the application layer in one station, and
proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

Layer 1 - Physical
Physical layer defines the cable or physical medium itself, e.g., thinnet, thicknet, unshielded
twisted pairs (UTP). All media are functionally equivalent. The main difference is in
convenience and cost of installation and maintenance. Converters from one media to another
operate at this level.

Layer 2 - Data Link

Data Link layer defines the format of data on the network. A network data frame, aka packet,
includes checksum, source and destination address, and data. The largest packet that can be sent
through a data link layer defines the Maximum Transmission Unit (MTU). The data link layer
handles the physical and logical connections to the packet's destination, using a network
interface. 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. Hosts with multiple network interfaces should use the same MAC
address on each. The data link layer's protocol-specific header specifies the MAC address of the
packet's source and destination. When a packet is sent to all hosts (broadcast), a special MAC
address (ff:ff:ff:ff:ff:ff) is used.
Layer 3 - Network
NFS uses Internetwork Protocol (IP) as its network layer interface. IP is responsible for routing,
directing datagrams from one network to another. 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 network portion of the IP is assigned by InterNIC
Registration Services, under the contract to the National Science Foundation, and the host
portion of the IP is assigned by the local network administrators. For large sites, the first two
bytes represents the network portion of the IP, and the third and fourth bytes identify the subnet
and host respectively.

Even though IP packets are addressed using IP addresses, hardware addresses must be used to
actually transport data from one host to another. The Address Resolution Protocol (ARP) is used
to map the IP address to it hardware address.

Layer 4 - Transport
Transport layer subdivides user-buffer into network-buffer sized datagrams and enforces desired
transmission control. Two transport protocols, Transmission Control Protocol (TCP) and User
Datagram Protocol (UDP), sits at the transport layer. Reliability and speed are the primary
difference between these two protocols. 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. Maintaining this
information for each connection makes TCP a stateful protocol. UDP on the other hand provides
a low overhead transmission service, but with less error checking. NFS is built on top of UDP
because of its speed and statelessness. Statelessness simplifies the crash recovery.

Layer 5 - Session
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.

Layer 6 - Presentation
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.

Layer 7 - Application
Provides network services to the end-users. Mail, ftp, telnet, DNS, NIS, NFS are examples of
network applications.
 OSI Model Reference Table
Layer Function Protocols Network
Components
Application  Used for applications DNS; FTP; TFTP; Gateway
specifically written to BOOTP;
User Interface run over the network SNMP;RLOGIN;
 Allows access to SMTP; MIME; NFS;
network services that FINGER; TELNET;
support applications; NCP; APPC; AFP; SMB
 Directly represents the
services that directly
support user
applications
 Handles network
access, flow control and
error recovery
 Example apps are file
transfer,e-mail,
NetBIOS-based 
applications           

Presentation  Translates from   Gateway

application to network
Translation format and vice-versa Redirector
 All different formats
from all sources are
made into a common
uniform format that the
rest of the OSI model
can understand
 Responsible for
protocol conversion,
character
conversion,data
encryption / decryption,
expanding graphics
commands, data
compression
 Sets standards for
different systems to
provide seamless
communication from
multiple protocol stacks
 Not always
implemented in a
network protocol
 Session  Establishes, maintains NetBIOS Gateway
and ends sessions
Syncs and across the network Names Pipes
Sessions  Responsible for name
recognition Mail Slots
(identification) so only
the designated parties RPC
can participate in the
session
 Provides
synchronization
services by planning
check points in the data
stream => if session
fails, only data after the
most recent checkpoint
need be transmitted
 Manages who can
transmit data at a
certain time and for
how long
 Examples are
interactive login and
file transfer
connections, the session
would connect and re-
connect if there was an
interruption; recognize
names in sessions and
register names in
history

Transport  Additional connection TCP, ARP, RARP; Gateway

below the session layer
Packets; Flow  Manages the flow SPX Advanced Cable
control & Error- control of data between Tester
handling parties across the NWLink
network Brouter
 Divides streams of data NetBIOS / NetBEUI
into chunks or packets;
the transport layer of ATP
the receiving computer
reassembles the
message from packets
 A train is a good
analogy => the data is
divided into identical
units
 Provides error-checking
 to guarantee error-free
data delivery, with on
losses or duplications
 Provides
acknowledgment of
successful
transmissions; requests
retransmission if some
packets don’t arrive
error-free
 Provides flow control
and error-handling

Network  Translates logical IP; ARP; RARP, ICMP; Brouter

network address and RIP; OSFP;
Addressing; names to their physical Router
Routing address (e.g. IGMP;
computername ==> Frame Relay
MAC address) IPX Device
 Responsible for
o addressing NWLink ATM Switch
o determining
routes for NetBEUI Advanced Cable
sending Tester
o managing OSI
network
problems such DDP
as packet
switching, data DECnet
congestion and
routing
 If router can’t send data
frame as large as the
source computer sends,
the network layer
compensates by
breaking the data into
smaller units. At the
receiving end, the
network layer
reassembles the data
 Think of this layer
stamping the addresses
on each train car
 Data Link  Turns packets into raw  Logical Link Bridge
bits 100101 and at the Control
Data frames to receiving end turns bits  error correction Switch
bits into packets. and flow control
 Handles data frames  manages link ISDN Router
between the Network control and
and Physical layers defines SAPs Intelligent Hub
 The receiving end  802.1 OSI Model
packages raw data from  802.2 Logical NIC
the Physical layer into Link Control
data frames for delivery Advanced Cable
to the Network layer Tester
 Responsible for error- Media Access Control
free transfer of frames
to other computer via  communicates
the Physical Layer with the adapter
 This layer defines the card
methods used to  controls the type
transmit and receive of media being
data on the network. It used:
consists of the wiring,
the devices use to 802.3 CSMA/CD
connect the NIC to the (Ethernet)
wiring, the signaling
involved to transmit / 802.4 Token Bus
receive data and the (ARCnet)
ability to detect
signaling errors on the 802.5 Token Ring
network media
802.12 Demand Priority
Physical  Transmits raw bit IEEE 802 Repeater
stream over physical
Hardware; Raw cable IEEE 802.2 Multiplexer
bit stream  Defines cables, cards,
and physical aspects ISO 2110 Hubs
 Defines NIC
attachments to ISDN  Passive
hardware, how cable is  Active
attached to NIC
 Defines techniques to TDR
transfer bit stream to
cable Oscilloscope

Amplifier