Which OSI layers offer reliable connection oriented data communication services?

Cross-layer functions

Cross-layer functions, or services that may affect more than one layer, include the following:

• security service telecommunication as defined by the International Telecommunication Union Standardization Sector (ITU-T) X.800 recommendation;
• management functions that enable the configuration, instantiation, monitoring and terminating of the communications of two or more entities;
• Multiprotocol Label Switching (MPLS), which operates at an OSI model layer that lies between the Layer 2 data-link layer and the Layer 3 network layer -- MPLS can carry a variety of traffic, including Ethernet frames and IP packets;
• Address Resolution Protocol (ARP) translates IPv4 addresses (OSI Layer 3) into Ethernet MAC addresses (OSI Layer 2); and
• domain name system (DNS), which is an application layer service that's used to look up the IP address of a domain name.

Pros and cons of the OSI model

The OSI model has a number of advantages, including the following:

• It's considered a standard model in computer networking.
• The model supports connectionless, as well as connection-oriented, services. Users can take advantage of connectionless services when they need faster data transmissions over the internet and the connection-oriented model when they're looking for reliability.
• It has the flexibility to adapt to many protocols.
• The model is more adaptable and secure than having all services bundled in one layer.

The disadvantages of the OSI model include the following:

• It doesn't define any particular protocol.
• The session layer, which is used for session management, and the presentation layer, which deals with user interaction, aren't as useful as other layers in the OSI model.
• Some services are duplicated at various layers, such as the transport and data-link layers.
• Layers can't work in parallel; each layer must wait to receive data from the previous layer.

OSI model vs. TCP/IP model

The OSI reference model describes the functions of a telecommunication or networking system, while TCP/IP is a suite of communication protocols used to interconnect network devices on the internet. TCP/IP and OSI are the most broadly used networking models for communication.

The OSI and TCP/IP models have similarities and differences. The main similarity is in their construction, as both use layers, although the OSI model consists of seven layers, while TCP/IP consists of just four layers.

Another similarity is that the upper layer for each model is the application layer, which performs the same tasks in each model but may vary according to the information each receives.

The functions performed in each model are also similar because each uses a network and transport layer to operate. The OSI and TCP/IP model are mostly used to transmit data packets, although they each use different means and paths to reach their destinations.

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