Internetworking Basics 1-3
Characteristics of the OSI Layers
The following list details the seven layers of the Open System Interconnection (OSI) reference
model:
• Layer 7—Application layer
• Layer 6—Presentation layer
• Layer 5—Session layer
• Layer 4—Transport layer
• Layer 3—Network layer
• Layer 2—Data Link layer
• Layer 1—Physical layer
Figure 1-2 illustrates the seven-layer OSI reference model.
Figure 1-2 The OSI reference model contains seven independent layers.
Characteristics of the OSI Layers
The seven layers of the OSI reference model can be divided into two categories: upper layers and
lower layers.
The upper layers of the OSI model deal with application issues and generally are implemented only
in software. The highest layer, application, is closest to theend user. Both usersand application-layer
processes interact with software applications that contain a communications component. The term
upper layer is sometimes used to refer to any layer above another layer in the OSI model.
The lower layers of the OSI model handle data transport issues. The physical layer and data link
layer are implemented in hardware and software. The other lower layers generally are implemented
only in software. The lowest layer, the physical layer, is closest to the physical network medium (the
network cabling, for example) , and is responsible for actually placing information on the medium.
Figure 1-3 illustrates the division between the upper and lower OSI layers.
ith0102
Network
Physical
Application
Presentation
Session
Transport
Data Link
3
1
7
6
5
4
2
Open Systems Interconnection (OSI) Reference Model
Internetworking Technology Overview, June 1999
1-4
Figure 1-3 Two sets of layers make up the OSI layers.
Protocols
The OSI model provides a conceptual framework for communication between computers, but the
model itself is not a method of communication. Actual communication is made possible by using
communication protocols. In the context of data networking, a protocol is a formal set of rules and
conventions that governs how computers exchange information over a network medium. A protocol
implements the functions of one or more of the OSI layers. A wide variety of communication
protocols exist, but all tend to fall into one of the following groups: LAN protocols, WAN protocols,
network protocols, and routing protocols. LAN protocols operate at the network and data link layers
of the OSI model and define communication over the various LAN media. WAN protocols operate
at the lowest three layers of the OSI model and define communication over the various wide-area
media. Routing protocols are network-layer protocolsthat are responsible forpath determination and
trafficswitching. Finally, networkprotocols are thevarious upper-layer protocols thatexist ina given
protocol suite.
OSI Model and Communication Between Systems
Information being transferred from a software application in one computer system to a software
application in another must pass through each of the OSI layers. If, for example, a software
application in System A has information to transmit to a software application in System B, the
application program in System A will pass its information to the application layer (Layer 7) of
System A. The application layer then passes the information to the presentation layer (Layer 6),
which relays the data to the session layer (Layer 5), and so on down to the physical layer (Layer 1).
At the physical layer, the information is placed on the physical network medium and is sent across
the medium to System B.The physical layer of System B removes the information from the physical
medium, and then its physical layer passes the information up to the data link layer (Layer 2), which
passes it to the network layer (Layer 3), and so on until it reaches the application layer (Layer 7) of
System B. Finally, the application layer of System B passes the information to the recipient
application program to complete the communication process.
t
h0103
Network
Physical
Application
Presentation
Session
Transport
Data Link
Data Transport
Application
Internetworking Basics 1-5
OSI Model and Communication Between Systems
Interaction Between OSI Model Layers
A given layer in the OSI layers generally communicates with three other OSI layers: the layer
directly above it, the layer directly below it, and its peer layer in other networked computer systems.
The data link layer in System A, for example, communicates with the network layer of System A,
the physical layer of System A, and the data link layer in System B. Figure 1-4 illustrates this
example.
Figure 1-4 OSI model layers communicate with other layers.
OSI-Layer Services
One OSI layer communicates with another layer to make use of the services provided by the second
layer. The services provided by adjacent layers help a given OSI layer communicate with its peer
layer in other computer systems. Three basic elements are involved in layer services: the service
user, the service provider, and the service access point (SAP).
In this context, the service user is the OSI layer that requests services from an adjacent OSI layer.
The service provider is the OSI layer that provides services to service users. OSI layers can provide
services to multiple service users. The SAP is a conceptual location at which one OSI layer can
request the services of another OSI layer.
Figure 1-5 illustrates how these three elements interact at the network and data link layers.
A
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Presentation
Session
Transport
Network
Data Link
Physical
B
12309
Open Systems Interconnection (OSI) Reference Model
Internetworking Technology Overview, June 1999
1-6
Figure 1-5 Service users, providers, and SAPs interact at the network and data link
layers.
OSI Model Layers and Information Exchange
The seven OSI layersuse various forms ofcontrol information to communicate with their peer layers
in other computer systems. This control information consists of specific requests and instructions
that are exchanged between peer OSI layers.
Control information typically takes one of two forms: headers and trailers. Headers are prepended
to data that has been passed down from upper layers.Trailers are appended to data that has been
passed down from upper layers. An OSI layer is not required to attach a header or trailer to data from
upper layers.
Headers, trailers, and data are relative concepts, dependingon the layerthat analyzes theinformation
unit. At the network layer, an information unit, for example, consists of a Layer 3 header and data.
At the data link layer, however, all the information passed down by the network layer (the Layer 3
header and the data) is treated as data.
In other words, the data portion of an information unit at a given OSI layer potentially can contain
headers, trailers, and data from all the higher layers. This is known as encapsulation.Figure 1-6
shows how the header and data from one layer are encapsulated into the header of the next lowest
layer.
Service User
Network Layer Protocol
Service User
Network Layer Protocol
Service Provider
(Data Link Layer Protocol)
SAPs
Network
Layer
Data Link
Layer
12310
Internetworking Basics 1-7
OSI Model Physical Layer
Figure 1-6 Headers and data can be encapsulated during information exchange.
Information Exchange Process
The information exchange process occurs between peer OSI layers. Each layer in the source system
adds control information to data and each layer in the destination system analyzes and removes the
control information from that data.
If System A has data from a software application to send to System B, the data is passed to the
application layer. The application layer in System A then communicates any control information
required by the application layer in System B The prepending a header to the data. The resulting
information unit (a header and the data) is passed to the presentation layer, which prepends its own
header containing control information intended for the presentation layer in System B. The
information unit grows in size as each layer prepends its own header (and in some cases a trailer)
that contains control information to be used by its peer layer in System B. At the physical layer, the
entire information unit is placed onto the network medium.
The physical layer in System B receives the information unit and passes it to the data link layer. The
data link layer in System B then reads the control information contained in the header prepended by
the data link layer in System A. The header is then removed, and the remainder of the information
unit is passed to the network layer. Each layer performs the same actions: The layer reads the header
from its peer layer, strips it off, and passes the remaining information unit to the next highest layer.
After the application layer performs these actions, the data is passed to the recipient software
application in System B, in exactly the form in which it was transmitted by the application in
System A.
OSI Model Physical Layer
The physical layer defines the electrical, mechanical, procedural, and functional specifications for
activating, maintaining, and deactivating the physical link between communicating network
systems. Physical layer specifications define characteristics such as voltage levels, timing of voltage
changes, physical data rates, maximum transmission distances, and physical connectors.
Physical-layer implementations can be categorized as either LAN or WAN specifications. Figure 1-7
illustrates some common LAN and WAN physical-layer implementations.
Information Units
7
6
5
4
3
2
1
7
6
5
4
3
2
1
System A System B
•
•
•
Network
ith0106
Data
Data
Data
DataHeader 4
Header 2
Header 3
Open Systems Interconnection (OSI) Reference Model
Internetworking Technology Overview, June 1999
1-8
Figure 1-7 Physical-layer implementations can be LAN or WAN specifications.
OSI Model Data Link Layer
The datalink layer provides reliable transit of data acrossa physical network link. Different data link
layer specifications define different network and protocol characteristics, including physical
addressing, network topology, error notification, sequencing of frames, and flow control. Physical
addressing (as opposed to network addressing) defines how devices are addressed at the data link
layer. Network topology consists of the data link layer specifications that often define how devices
are to be physically connected, such as in a bus or a ring topology. Error notification alerts
upper-layer protocols that a transmission error has occurred, and the sequencing of data frames
reorders frames that are transmitted out of sequence. Finally, flow control moderates the
transmission of data so that the receiving device is not overwhelmed with more traffic than it can
handle at one time.
The Institute of Electrical and Electronics Engineers (IEEE) has subdivided the data link layer into
two sublayers: LogicalLink Control (LLC)and Media Access Control (MAC). Figure 1-8 illustrates
the IEEE sublayers of the data link layer.
Figure 1-8 The data link layer contains two sublayers.
ith0107
Physical
Layer
Ethernet
IEEE 802.3
100BaseT
Token Ring/
IEEE 802.5
FDDI
EIA/TIA-232
EIA/TIA-449
V.24 V.35
HSSI G.703
EIA-530
X.21bis SIP
WANLAN
Physical Layer Implementations
OSI Layer
Data Link
Layer
LLC
Sublayer
MAC
Sublayer
ith0108
Data Link
Layer
Internetworking Basics 1-9
OSI Model Network Layer
The Logical Link Control (LLC) sublayer of the data link layer manages communications between
devices over a single link of a network. LLC is defined in the IEEE 802.2 specification and supports
both connectionless and connection-oriented services used by higher-layer protocols. IEEE 802.2
defines a number of fields in data link layer frames that enable multiple higher-layer protocols to
share a single physical data link. The Media Access Control (MAC) sublayer of the data link layer
manages protocol access to the physical network medium. The IEEE MAC specification defines
MAC addresses, which enable multiple devices to uniquely identify one another at the data link
layer.
OSI Model Network Layer
The network layer provides routing and related functions that enable multiple data links to be
combined into an internetwork. This is accomplished by the logical addressing (as opposed to the
physical addressing) of devices. The network layer supports both connection-oriented and
connectionless service from higher-layer protocols. Network-layer protocols typically are routing
protocols, but other types of protocols are implemented at the network layer as well. Some common
routing protocols includeBorderGateway Protocol (BGP),an Internet interdomain routingprotocol;
Open Shortest Path First (OSPF), a link-state, interior gateway protocol developed for use in TCP/IP
networks; and Routing Information Protocol (RIP), an Internet routing protocol that uses hop count
as its metric.
OSI Model Transport Layer
The transport layer implements reliable internetwork data transport services that are transparent to
upper layers. Transport-layer functions typically include flow control, multiplexing, virtual circuit
management, and error checking and recovery.
Flow control manages data transmission between devices so that the transmitting device does not
send more data than the receiving device can process. Multiplexing enables data from several
applications to betransmitted onto a singlephysicallink. Virtual circuits areestablished,maintained,
and terminated by the transport layer. Error checking involves creating various mechanisms for
detecting transmission errors, while error recovery involves taking an action, such as requesting that
data be retransmitted, to resolve any errors that occur.
Some transport-layer implementations include Transmission Control Protocol, Name Binding
Protocol, and OSI transport protocols. Transmission Control Protocol (TCP) is the protocol in the
TCP/IP suite that provides reliable transmission of data. Name Binding Protocol (NBP) is the
protocol that associates AppleTalk names with addresses. OSI transport protocols are a series of
transport protocols in the OSI protocol suite.
OSI Model Session Layer
The session layer establishes, manages, and terminates communication sessions between
presentation layer entities. Communication sessions consist of service requests and service
responses that occur between applications located in different network devices. These requests and
responses are coordinated by protocols implemented at the session layer. Some examples of
session-layer implementations include Zone InformationProtocol(ZIP), the AppleTalk protocol that
coordinates the name binding process; and Session Control Protocol (SCP), the DECnet Phase IV
session-layer protocol.
Information Formats
Internetworking Technology Overview, June 1999
1-10
OSI Model Presentation Layer
The presentation layer provides a variety of coding and conversion functions that are applied to
application layer data. These functions ensure thatinformation sent from the application layer ofone
system will be readable by the application layer of another system. Some examples of
presentation-layer coding and conversion schemes include common data representation formats,
conversion of character representation formats, common data compression schemes, and common
data encryption schemes.
Common data representation formats, or theuse of standardimage, sound, andvideo formats, enable
the interchange of application data between different types of computer systems. Conversion
schemes are used to exchange information with systems by using different text and data
representations, such as EBCDIC and ASCII. Standard data compression schemes enable data that
is compressed at the source device to be properly decompressed at the destination. Standard data
encryption schemes enable data encrypted at the source device to be properly deciphered at the
destination.
Presentation-layer implementations are not typically associated with a particular protocol stack.
Some well-known standards for video include QuickTime and Motion Picture Experts Group
(MPEG). QuickTime is an Apple Computer specification for video and audio, and MPEG is a
standard for video compression and coding.
Among the well-known graphic image formats are Graphics Interchange Format (GIF), Joint
Photographic Experts Group (JPEG), and Tagged Image File Format (TIFF). GIF is a standard for
compressing and coding graphic images. JPEG is another compression and coding standard for
graphic images, and TIFF is a standard coding format for graphic images.
OSI Model Application Layer
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. Such
application programs fall outside the scope of the OSI model. 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. When determining
resource availability, the application layer must decide whether sufficient network resources for the
requested communication exist. In synchronizing communication, all communication between
applications requires cooperation that is managed by the application layer.
Two key types of application-layer implementations are TCP/IP applications and OSI applications.
TCP/IP applications are protocols, such as Telnet, File Transfer Protocol (FTP),and Simple Mail
Transfer Protocol (SMTP), that exist in the Internet Protocol suite. OSI applications are protocols,
such as File Transfer, Access, and Management (FTAM), Virtual Terminal Protocol (VTP), and
Common Management Information Protocol (CMIP), that exist in the OSI suite.
Information Formats
The data and control information that is transmitted through internetworks takes a wide variety of
forms. The terms used to refer to these information formats are not used consistently in the
internetworking industry but sometimes are used interchangeably. Common information formats
include frame, packet, datagram, segment, message, cell, and data unit.
Internetworking Basics 1-11
Information Formats
A frame is an information unit whose source and destination are data link layer entities. A frame is
composed of the data-link layer header (and possibly a trailer) and upper-layer data. The header and
trailer contain control information intended for the data-link layer entity in the destination system.
Data from upper-layer entities is encapsulated in the data-link layer header and trailer. Figure 1-9
illustrates the basic components of a data-link layer frame.
Figure 1-9 Data from upper-layer entities makes up the data link layer frame.
A packet is an information unit whose source and destination are network-layer entities. A packet is
composed of the network-layer header (and possibly a trailer) and upper-layer data. The header and
trailer contain control information intended for the network-layer entity in the destination system.
Data from upper-layer entities is encapsulated in the network-layer header and trailer. Figure 1-10
illustrates the basic components of a network-layer packet.
Figure 1-10 Three basic components make up a network-layer packet.
The term datagram usually refers to an information unit whose source and destination are
network-layer entities that use connectionless network service.
The term segment usually refers to an information unit whose source and destination are
transport-layer entities.
A message is an information unit whose source and destinationentities exist above the network layer
(often the application layer).
A cell is an information unit of a fixed size whose source and destination are data-link layer entities.
Cells are used in switched environments, such as Asynchronous Transfer Mode (ATM) and
Switched Multimegabit Data Service (SMDS) networks. A cell is composed of the header and
payload. The header contains control information intended for the destination data-link layer entity
and is typically 5 bytes long. The payload contains upper-layer data that is encapsulated in the cell
header and is typically 48 bytes long.
The length of the headerand the payload fields always are exactly the same for each cell. Figure 1-11
depicts the components of a typical cell.
LLC
Sublayer
ith0109
Frame
Data Link Layer
Header
Upper Layer
Data
Data Link Layer
Trailer
LLC
Sublayer
MAC
Sublayer
ith0110
Packet
Network Layer
Header
Upper Layer
Data
Network Layer
Trailer
ISO Hierarchy of Networks
Internetworking Technology Overview, June 1999
1-12
Figure 1-11 Two components make up a typical cell.
Data unit is a generic term that refers to a variety of information units. Some common data units are
service data units (SDUs), protocol data units, and bridge protocol data units (BPDUs). SDUs are
information units from upper-layer protocols that define a service request to a lower-layer protocol.
PDU is OSI terminology for a packet. BPDUs are used by the spanning-tree algorithm as hello
messages.
ISO Hierarchy of Networks
Large networks typically are organized as hierarchies. A hierarchical organization provides such
advantages as ease of management, flexibility, and a reduction in unnecessary traffic. Thus, the
International Organization for Standardization (ISO) has adopted a number of terminology
conventions for addressing network entities. Key terms, defined in this section, include end system
(ES), intermediate system (IS), area, and autonomous system (AS).
An ES is a network device that does not perform routing or other trafficforwarding functions.
Typical ESs include such devices as terminals, personal computers, and printers. An IS is a network
device that performs routing or other traffic-forwarding functions. Typical ISs include such devices
as routers, switches, and bridges. Two types of IS networks exist: intradomain IS and interdomain
IS. An intradomain IS communicates within a single autonomous system, while an interdomain IS
communicates within and between autonomous systems. An area is a logical group of network
segments and their attached devices. Areas are subdivisions of autonomous systems (ASs). An AS
is a collection of networks under a common administration that share a common routing strategy.
Autonomous systems are subdivided into areas, and an AS is sometimes called a domain.
Figure 1-12illustrates a hierarchical network and its components.
Figure 1-12 A hierarchical network contains numerous components.
ith0111
Cell
53 Bytes
Cell Header
(5 Bytes)
Payload
(48 Bytes)
Area
Area
Area
IS
IS
IS
Autonomus
system
12311
ES
Không có nhận xét nào:
Đăng nhận xét