Osi Model and Cisco Systems Inc. Essay

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http://www.ciss100.com/lecture-topics-modules/networking-internet/wide-area-networking-wan/

Wide Area Networking
Introduction
A WAN, also known as a long-haul network is a loose term used to distinguish networking technologies from a geographical perspective. In contrast to local area networks (LAN), WANs provide communications over substantially longer distances. Note that the definition of long-distance is a vague term that could entail crossing a state, a country or even an ocean. In contrast to LANs where organizations typically own and manage their network, WAN services are typically a pay-for-service and are provided and managed by regional telecommunications carriers.
As previously cited, a LAN’s primary purpose is to provide network access to hosts using various topologies. In contrast to LANs, WANs are designed to interconnect networks through serial point-to-point links. Recall a physical point-to-point topology connects two nodes directly. In a point-to-point WAN architecture, layer two frames are placed on the communications medium at the source and removed at the destination. This architecture allows the data link
MAC protocols to be very simple since frames can only travel between the two nodes. This simplicity allows the layer two switching to be optimized and performed in hardware achieving an efficient low latency communications channel.
Lastly, it should be noted that in contrast to the quick speeds seen in today’s LANs, WANs historically operate at slower speeds and have a greater delay between connections.
With this basis, WANs may be characterized as a series of specialized computers also known as packet switching nodes (PSN) that connect networks through serial long haul communications lines . In summary, WANs may be distinguished from LANs based on the following three major characteristics: (a) WANs connect networks across wide geographic areas, (b) WANs typically use the services of large network providers that include telephone companies, cable companies and satellite systems, and (c) WANs use serial point-to-point connections . Implicit in this serial communications architecture is the need to share the communications channel and accommodate concurrent dialogs through multiplexing.
WAN Analysis Basis
To serve as a foundation for analysis, WANs can be distinguished based on the following characteristics: (a) packet switched or circuit switched connections, (b) synchronous or asynchronous communications, (c) narrowband or broadband capacity, (d) end-to-end delivery or used solely as an intermediate transport network, (e) dedicated or ondemand connections, and (f) the types of communications media (e.g. fiber optic or co-axial cable). Packet switching and circuit switching will be discussed in detail below. Synchronous serial connections provide synchronization through an external clock. This requires that the synchronous transmissions contain a separate or second signal that allows the destination to discern where transmissions begin and end. Synchronous transmission clocking will be discussed below in the WAN devices section analyzing data communications equipment. Asynchronous serial connections embed the clocking in the signal as previously examined in earlier (e.g. Manchester Encoding).
Narrowband and broadband indicate the communication medium’s capacity. The distinction between narrowband and broadband is also vague however Cisco (2004) states that the demarcation is 128 Kilobits per second (Kbps).
End-to-end delivery can be further distinguished between dedicated end-to-end point-to-point connections, permanent switched circuits (PVC) and virtual switched circuits (SVC). Intermediary transport links are the serial point-to-point links within the WAN. Dedicated circuits provide consistent always on bandwidth whereas on-demand services are provided in response to need. To provide a sound foundation for analysis, it is worthwhile to provide a historical context and trace the history…