OSI Reference Model

OSI Reference Model
Although the OSI model is a structure for developing standards, it is not in
itself a complete networking protocol. Its purpose is to establish a framework that
developers can use to devise protocols that can be used independent of manufacturers’
proprietary protocols. Most applications today communicate with TCP/IP,
which has become a de facto international standard. Although TCP/IP does not conform
exactly to the OSI model in the upper layers, its modular design and multitude
of coordinating protocols adhere to the same principles as OSI. Virtually all IP networks
run on top of Ethernet, which implements OSI’s physical and datalink layers.
Layer 1—Physical
Layer 1 transfers bits across a circuit, which can be any transmission medium
including wire, fiber optics, coaxial cable, or wireless. The physical layer contains
the rules for the transmission of bits between endpoints and standardizes pin
connections, modulation methods, and multiplexing over the physical medium.
Two devices can communicate using only the physical layer. For example, the
serial ports of two computers can be connected through an adapter known as a
null modem, which connects the transmitting data and signaling leads of each
computer to the receiving leads of the other. SONET/SDH, which we discussed in
the previous chapter, is a family of coordinating layer 1 protocols.
Layer 2—Datalink
Datalink protocols provide link integrity to transmit frames of data between
endpoints. The protocol accepts bits from the physical layer, assembles them
CHAPTER 6 Data Communications Protocols 95
TA B L E 6-1
Representative ISO, ITU-T, and IETF Protocol Standards at Their Corresponding
OSI Layers
Layer Common Standards
1 Physical EIA-232, EIA-422, V.35
2 Datalink High-Level Data-Link Control (HDLC), Balanced Link Access
Procedure (LAPB), Designated Link Access Procedure (LAPD)
3 Network X.25 Packet Level Protocol, Internet Protocol (IP), ISO 8473
Connectionless Network Protocol (CLNP)
4 Transport Transport Control Protocol (TCP), User Datagram Protocol (UDP),
ISO 8473 Connectionless Transport Service
5 Session ISO 8306 and 8037, ITU-T X.215 and X.225
6 Presentation ISO 8822 and 8823, ITU-T X.216 and X.226
7 Application Virtual Terminal Protocol, X.400 Message-Handling Service (MHS),
X.500 Directory Service, X.700 Common Management
Information Protocol (CMIP), File Transfer, Access, and
Management (FTAM)
into a frame, and calculates CRC. If a network protocol is used, the link protocol
passes the data block with control and CRC blocks attached. As Figure 4-5
shows, flags of a specific bit pattern mark the beginning and ending of the frame.
A header contains address and control information, followed by an information
field and a trailer containing CRC bits for error correction. The datalink protocol
at the receiving end calculates the CRC of the received frame. It acknowledges
correctly received frames and requests retransmission if necessary.
The principal international standard, HDLC, has numerous subsets, of
which Balanced Link Access Procedure (LAPB) and Designated Link Access
Procedure (LAPD) are common. The former is used in packet switched data
networks, and the latter is the access protocol for ISDN. Frame relay is a layer 2
protocol that transmits HDLC frames across a network path that is defined in
software.
Layer 3—Network
The network layer routes packets between endpoints on a network. A packet is a
frame with a header that contains addressing and other information. For example,
the IP header, shown in Figure 6-2, contains a time-to-live (TTL) field. If a packet
is not delivered before the time expires, it is killed. The type of service (TOS) field
informs downstream applications of what type of information the packet contains.
The network layer can be either connectionless or connection oriented.
A connectionless protocol such as IP relies on a higher level protocol to assure
data delivery and integrity. A connection-oriented protocol such as X.25 sets up a
connection across the network before packet exchange can begin.
Layer 4—Transport
The transport layer assures integrity between endpoints. Transport protocols
establish and terminate connections, segment data into manageable PDUs, and
reassemble them at the receiving end. Layer 4 is responsible for flow control,
sequencing, and end-to-end error correction. TCP, which is discussed later in this
chapter, is the most widely used transport layer protocol, although it is not an OSI
protocol. UDP is a connectionless transport protocol that is used by VoIP, Simple
Network Management Protocol (SNMP), and other applications that do not
require data integrity.
Layer 5—Session
Session service is an optional function that may be embedded in the application
as opposed to employing a separate protocol. The session layer establishes and
terminates connections between users of session services and synchronizes data
transfer between them. It negotiates the use of session layer tokens, which it
requires users to have before they can communicate. It provides synchronization
points in the data being transferred so that if the session is interrupted, the
applications may be able to recover without retransmitting everything.
Layer 6—Presentation
This layer provides the syntax for the session. For example, it might translate
between ASCII and EBDIC if the two systems use a different format. If data
compression and encryption are used, they communicate through this layer.
Layer 7—Application
The application layer is the interface between the network and the application
running on the computer. Examples of application layer functions now in use are
ITU-T’s X.400 Electronic Mail Protocol and its companion X.500 Directory Services
Protocol. Message Handling System (MHS) is an important protocol for enabling
X.400 e-mail systems to communicate. ISO’s File Transfer, Access, and Management
(FTAM) is a protocol for managing and manipulating files across a network.
Other protocols include Virtual Terminal (VT), which provides a standard terminal
interface, and Electronic Document Interchange (EDI), which uses the MHS
platform for transferring electronic documents across networks.
Most vendors at one time or another agreed to support OSI, which is ITU-T
recommendation X.200, in a quest for international standardization. It forms the
basis for Ethernet, which is discussed in the next section, but with variations. At
the higher layers, protocols such as X.400 and X.500 are applied today, but many
of the layer 3 to 6 protocols are displaced by TCP/IP, which is more agile and
better suited to intermachine communications.