Frame Relay is a packet-switching technology that exists at the Data Link layer of the OSI model and is one that has become increasingly common as a WAN solution since the early 1990s. Unlike with leased lines and circuit-switched networks, the available bandwidth on a provider’s Frame Relay network is shared amongst many subscribers. This […]

ISDN maps to the three lower layers of the OSI model – Network, Data Link, and Physical. However, different protocols exist at the Data Link and Network layers for B and D channels, as illustrated in the figure below. The functions handled at each OSI layer are described below.

Physical Layer. The ISDN Physical layer is […]

WAN technologies are considered to exist and function at the three lower layers of the OSI model – Physical, Data Link, and Network. While not all WAN technologies have elements that function at the Network Layer, some (like X.25 and ISDN) do. The figure below provides an overview of how the WAN technologies that you’ll […]

The Application/Process Layer is where TCP/IP applications and services reside. You’re more than likely familiar with many of these, since you probably interact with many TCP/IP applications on a daily basis – a web browser using HTTP, or your email client connecting to a POP3 server are but two simple examples.
The list below outlines some […]

If you recall from Chapter 1, we’ve already spent some time looking at connection-oriented and connectionless protocols. At the Host-to-host layer of the TCP/IP model, two primary protocols exist – Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
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If you recall, the Internet layer’s primary responsibilities are determining a path between networks (routing), as well as network addressing. The addressing that takes place at the Internet layer is often referred to as logical addressing. These addresses aren’t “burned-in” like Ethernet MAC addresses, but instead are assigned by an administrator. The addressing protocol of […]

The Department of Defence TCP/IP model is a 4-layer model that defines areas of responsibility much like the OSI, while providing insight into the functions of the different protocols that make up the TCP/IP suite. The model provides an excellent point of reference when compared to the OSI. We won’t look at all the details […]

The Physical layer of the OSI model is concerned with the electrical, optical, and mechanical properties of the network, including elements such as voltage, media, connector types, signal regeneration, and so forth. The physical layer doesn’t actually alter packets, but rather acts as the transmission facility over which the actual bits (1’s and 0’s) are […]

The Data Link Layer of the OSI model acts as an interface between the Network and Physical layers. The main responsibilities of the Data Link layer include:

Data framing and physical addressing. When data is passed to the Data Link layer, it is framed for transmission using various LAN and WAN protocols. This allows network protocols […]

The Network layer of the OSI model is commonly referred to as Layer 3, and has the following responsibilities:

Routing. When a host on one network wishes to exchange data with a host on another, packets will be sent to a router interface. After determining where the packet should be forwarded next using information found in […]