OSI Physical and Data Link layers
The OSI protocol suite supports numerous standard media-access protocols at the physical and data link
layers. The wide variety of media-access protocols supported in the OSI protocol suite allows other
protocol suites to exist easily alongside OSI on the same network media. Supported media-access
protocols include IEEE 802.2 LLC, IEEE 802.3, Token Ring/IEEE 802.5, Fiber Distributed Data
Interface (FDDI), and X.25.
OSI Network Layer
The OSI protocol suite specifies two routing protocols at the network layer: End System-to-Intermediate
System (ES-IS) and Intermediate System-to-Intermediate System (IS-IS). In addition, the OSI suite
implements two types of network services: connectionless service and connection-oriented service.
OSI Layer Standards
In addition to the standards specifying the OSI network layer protocols and services, the following
documents describe other OSI network layer specifications:
Presentation
Application
Sesssion
Transport
Data link
Physical
OSI reference
model
OSI protocol suite
TPO TP1 TP2 TP3 TP4
IEEE 802.3
hardware
Token Ring
hardware
FDDI
hardware
X.25
hardware
Presentation service/presentation protocol
IEEE
802.2 IEEE 802.3
IEEE 802.5/
Token Ring FDDI X.25
Session service/session protocol
CMIP DS FTAM MHS VTP
ACSE
ASES
ROSE RTSE CCRSE …
IS-IS
Network
CONP/CMNS CLNP/CLNS
ES-IS
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Chapter Open System Interconnection Protocols
OSI Networking Protocols
• ISO 8648—This standard defines the internal organization of the network layer (IONL), which
divides the network layer into three distinct sublayers to support different subnetwork types.
• ISO 8348—This standard defines network layer addressing and describes the connection-oriented
and connectionless services provided by the OSI network layer.
• ISO TR 9575—This standard describes the framework, concepts, and terminology used in relation
to OSI routing protocols.
OSI Connectionless Network Service
OSI connectionless network service is implemented by using the Connectionless Network Protocol
(CLNP) and Connectionless Network Service (CLNS). CLNP and CLNS are described in the ISO
8473 standard.
CLNP is an OSI network layer protocol that carries upper-layer data and error indications over
connectionless links. CLNP provides the interface between the Connectionless Network Service
(CLNS) and upper layers.
CLNS provides network layer services to the transport layer via CLNP.
CLNS does not perform connection setup or termination because paths are determined
independently for each packet that is transmitted through a network. This contrasts with
Connection-Mode Network Service (CMNS).
In addition, CLNS provides best-effort delivery, which means that no guarantee exists that data will
not be lost, corrupted, misordered, or duplicated. CLNS relies on transport layer protocols to
perform error detection and correction.
OSI Connection-Oriented Network Service
OSI connection-oriented network service is implemented by using the Connection-Oriented
Network Protocol (CONP) and Connection-Mode Network Service (CMNS).
CONP is an OSI network layer protocol that carries upper-layer data and error indications over
connection-oriented links. CONP is based on the X.25 Packet-Layer Protocol (PLP) and is described
in the ISO 8208 standard, “X.25 Packet-Layer Protocol for DTE.”
CONP provides the interface between CMNS and upper layers. It is a network layer service that acts
as the interface between the transport layer and CONP, and it is described in the ISO 8878 standard.
CMNS performs functions related to the explicit establishment of paths between communicating
transport layer entities. These functions include connection setup, maintenance, and termination.
CMNS also provides a mechanism for requesting a specific quality of service (QoS). This contrasts
with CLNS.
Network Layer Addressing
OSI network layer addressing is implemented by using two types of hierarchical addresses: network
service access point addresses and network entity titles.
A network service access point (NSAP) is a conceptual point on the boundary between the network
and the transport layers. The NSAP is the location at which OSI network services are provided to
the transport layer. Each transport layer entity is assigned a single NSAP, which is individually
addressed in an OSI internetwork using NSAP addresses.
Figure 30-2 illustrates the format of the OSI NSAP address, which identifies individual NSAPs.