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Checkpoint Osi Model

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CheckPoint: OSI Model Due Week 1 Day 4

Describe the OSI model and how it relates to a network in 200 to 300 words. Your response should include answers to the following:

- On which layers of the OSI model do WAN protocols operate? - What are some of these protocols? - On which OSI layers do switches and routers operate? - If routers reside at more than one layer, what is the difference between the OSI layers?

• On which layers of the OSI model do WAN protocols operate?

WAN protocols function at the lower three layers of the OSI reference model: the physical layer, the data-link layer, and the network layer.

• What are some of these protocols?

Frame Relay- is a high performance WAN protocol that operates at the physical and data-link layers of the OSI model. The Frame Relay allows for use across ISDN (Integrated Services Digital Network) interfaces. It is a good example of a packet-switched technology.

LAPB, which stands for, “Link Access Procedure Balanced” – is a data-link layer protocol that manages communication and packet framing between devices. It shares the same frame format, types and field functions as SDLC and HDLC. However, unlike either of those, LAPB is restricted to the ABM transfer mode and is appropriate only for combined stations. LAPB, circuits can be established by either the data terminal equipment, or the data circuit-terminating equipment. The station initiating the call is determined to be the primary, and the responding station is the secondary.

HDLC – High-Level Data Link Control, specifies a method to encapsulate data over synchronous serial links using frame characters and checksums. No authentication can be used with HDLC. The reason each vendor has a proprietary encapsulation of HDLC is that they each have a different way for the HDLC protocol to communicate with the Network layer protocols, and the ISO standard doesn't allow for multiple protocols on a single link. Since, they needed a way for HDLC to communicate with layer-3 protocols the HDLC can carry more than one type of protocol.

PPP- Point-to-Point protocol is a datalink-layer protocol that can be used over asynchronous serial, which is dial-up and synchronous serial, which is Integrated Service Digital Network media and uses the Link Control Protocol to build and maintain data-link connections. The basic purpose of PPP is to transport layer-3 packets over a Data Link layer point-to-point link. PPP consists of two main components, the Link Control Protocol and a family of Network Control Protocols. The PPP allows for the simultaneous use of multiple Network layer protocols.

ATM (Asynchronous Transfer Mode) - It encodes data into small fixed-sized cells and provides data link layer services that run over the OSI physical layer. ATM allows properties from both circuit switched and small packet switched networking, making it suitable for wide area data networking as well as real-time media transport.

• On which OSI layers do switches and routers operate?

A Network Switch's main purpose is to provide connectivity between LAN clients. Any switch that provides this 'basic functionality operates at the second OSI layer also known as the Datalink Layer. Routers operate on the third layer of the OSI Model, the Network-Control Layer.

• If routers reside at more than one layer, what is the difference between OSI layers?

Since routers are identified as Layer-3 devices, because they process logical addressing information in the Network header of a packet, such as IP addresses. Therefore, I do not believe they can operate on more than one layer and if they did, there would not be a difference between the layers.

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