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Learning Objectives

We learned that a WAN is a data communications network that operates beyond a LAN's geographic scope. A WAN is different from a LAN in other ways as well. To establish a WAN and use WAN carrier network services, one must subscribe to a Regional Bell operating company (RBOC) WAN service provider. A WAN uses data links such as Integrated Services Digital Network (ISDN) and Frame Relay. These are provided by carrier services to access bandwidth over wide-area geographies. A WAN provides connectivity between organizations, services, and remote users. WANs generally carry voice, data, and video.

WANs function at the three lowest layers of the OSI reference model: the physical layer, the data link layer, and the network layer. Telephone and data services are the most commonly used WAN services. Telephone and data services are connected from the building's point of presence (POP) to the WAN provider's central office (CO). The CO is the local telephone company's central office to which all local loops in a given area connect and in which circuit switching of subscriber lines occurs.

A key interface in the customer site occurs between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE). Typically, the DTE is the router. The DCE is the device used to convert the user data from the DTE into a form acceptable to the WAN service's facility. The DCE is either the attached modem, channel service unit/data service unit (CSU/DSU), or terminal adapter/network termination 1 (TA/NT1).

Information from many sources has bandwidth allocation on a single medium. Circuit switching uses signaling to determine the call route, which is a dedicated path between the sender and the receiver. By multiplexing traffic into fixed time slots, Time-division multiplexing (TDM) avoids congested facilities and variable delays. Basic telephone service and ISDN use TDM circuits.

Customer premises equipment (CPE) are devices physically located on the subscriber's premises. Includes both devices owned by the subscriber and devices leased to the subscriber by the service provider.

In a hierarchical structure the network is organized in layers, each of which has one or more specific functions.

We learned that the benefits to using a hierarchical WAN model include the following:
--Scalability -- Networks that follow the hierarchical model can grow much larger without sacrificing control or manageability. Control and manageability are maintained because functionality is localized and potential problems can be recognized more easily. An example of a very large-scale hierarchical network design is the Public Switched Telephone Network.
--Ease of implementation -- A hierarchical design assigns clear functionality to each layer, thereby making network implementation easier.
--Ease of troubleshooting -- Because the functions of the individual layers are well defined, the isolation of problems in the network is less complicated. Temporarily segmenting the network to reduce the scope of a problem also is easier.
--Predictability -- The behavior of a network using functional layers is fairly predictable, which makes capacity planning for growth considerably easier. This design approach also facilitates modeling of network performance for analytical purposes.
--Protocol support -- The mixing of current and future applications and protocols is much easier on networks that follow the principles of hierarchical design because the underlying infrastructure is already logically organized.
--Manageability -- All the benefits listed here contribute to provide greater manageability of the network.

A hierarchical network design includes the following three layers:

1. The core layer, which provides optimal transport between sites
2. The distribution layer, which provides policy-based connectivity
3. The access layer, which provides workgroup and user access to the network

We discovered that the placement of servers relating to host access affects traffic patterns.

We discovered that PPP offers link configuration and link quality testing which is very useful in connecting users in remote locations.
PPP utilizes two authentication protocols: PAP and CHAP.
Password Authentication Protocol (PAP) provides a simple way for a remote host to establish its identity by using a 2-way handshake. After the link establishment phase is complete; a username/password pair is repeatedly sent by the remote node until authentication is acknowledged or when the connection is terminated.

CHAP is a more secure authentication protocol because it repeatedly requires verification of identity from the remote node by using a three-way handshake. This is done upon initial link establishment and can be repeated any time after the link has been established. CHAP offers features such as periodic verification to improve security. This allows CHAP to be more effective than PAP since CHAP requires a challenge before authentication can take place. CHAP provides protection against playback attacks through the use of a variable challenge value that is unique and unpredictable.

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