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Module 7: WAN Concepts, image, image, image, image, image, image, image,…
Module 7: WAN Concepts
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7.1 Porpuse of WANs
7.1.1 LANs and WANs
Whether at work or at home, we all use Local Area Networks (LANs). However, LANs are limited to a small geographical area.A Wide Area Network (WAN) is required to connect beyond the boundary of the LAN. A WAN is a telecommunications network that spans over a relatively large geographical area. A WAN operates beyond the geographic scope of a LAN.
7.1.2 Private and public WANs
WANs may be built by a variety of different types of organizations, as follows:
- An organization that wants to connect users in different locations
- An ISP that wants to connect customers to the internet
- An ISP or telecommunications that wants to interconnect ISPs
A private WAN is a connection that is dedicated to a single customer. This provides for the following:
- Guaranteed service level
- Consistent bandwidth
- Security
A public WAN connection is typically provided by an ISP or telecommunications service provider using the internet. In this case, the service levels and bandwidth may vary, and the shared connections do not guarantee security.
7.1.3 WAN Topologies
Physical topologies describe the physical network infrastructure used by data when it is travelling from a source to a destination. The physical WAN topology used in WANs is complex and for the most part, unknown to users. Consider a user in New York establishing a video conference call with a user in Tokyo, Japan. Other than the user’s internet connection in New York, it would not be feasible to identify the all of the actual physical connections that are needed to support the video call.
Instead, WAN topologies are described using a logical topology. Logical topologies describe the virtual connection between the source and destination. For example, the video conference call between the user in New York and Japan would be a logical point-to-point connection.
7.1.4 Carrier Connections
Another aspect of WAN design is how an organization connects to the internet. An organization usually signs a service level agreement (SLA) with a service provider. The SLA outlines the expected services relating to the reliability and availability of the connection. The service provider may or may not be the actual carrier. A carrier owns and maintains the physical connection and equipment between the provider and the customer. Typically, an organization will choose either a single-carrier or dual-carrier WAN connection.
7.1.5 Evolving Networks
Network requirements of a company can change dramatically as the company grows over time. Distributing employees saves costs in many ways, but it puts increased demands on the network. Not only must a network meet the day-to-day operational needs of the business, but it must be able to adapt and grow as the company changes. Network designers and administrators meet these challenges by carefully choosing network technologies, protocols, and service providers. They must also optimize their networks by using a variety of network design techniques and architectures.To illustrate differences between network size, we will use a fictitious company called SPAN Engineering as it grows from a small, local, business into a global enterprise. SPAN Engineering, an environmental consulting firm, has developed a special process for converting household waste into electricity and is developing a small pilot project for a municipal government in its local area.
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hen LANs appeared in the 1980s, organizations began to see the need to interconnect with other locations. To do so, they needed their networks to connect to the local loop of a service provider. This was accomplished by using dedicated lines, or by using switched services from a service provider.
Leased lines were used for permanent dedicated connections, providing a pre-established WAN communications path via copper media. These lines, available since the 1950s, were also called leased circuits, serial links, or T1/E1 and T3/E3 lines. Pricing depended on bandwidth and distance.Two systems defined digital capacity over copper:
7.3.3 Circuit-Switched Options
Circuit-switched connections are provided by PSTN carriers, with the local loop using copper media. There are two main options:
PSTN (Dialup WAN Access): Uses modems to convert digital data into analog signals for transmission over voice telephone networks. Speeds are limited to less than 56 kbps. Dialup is now a legacy technology but may still be used where no other WAN options exist.
ISDN: A circuit-switching technology that enables digital transmission over PSTN, offering speeds from 45 Kbps to 2.048 Mbps. ISDN has largely been replaced by high-speed DSL and broadband, with most providers discontinuing the service.
7.3.4 Packet-Switched Options
Packet switching divides data into packets that are routed over a shared network, unlike circuit switching, which requires a dedicated connection. Two legacy packet-switched options are:
Frame Relay: A Layer 2 NBMA WAN technology used to interconnect LANs. It utilizes PVCs identified by DLCIs for bidirectional communication and supports data rates up to 4 Mbps. Frame Relay has been largely replaced by Metro Ethernet and internet-based solutions.
ATM (Asynchronous Transfer Mode): Uses fixed 53-byte cells for efficient voice and video transmission. However, its small cell size leads to higher overhead and less efficiency compared to Frame Relay. ATM has also been replaced by faster alternatives like Metro Ethernet.
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