EWAN v4 Chapter 1 Check Your Understanding: Introduction to WANs

Explanation: Ethernet switches and repeaters typically are found in LANs. Routers can be considered both LAN and WAN devices, used to route packets within a company’s network, to an ISP, or between autonomous systems. An access server concentrates dial-in and dial-out user communications. It may have a mixture of analog and digital interfaces supporting hundreds of simultaneous users connecting to the provider’s WAN.

Explanation: The core layer is also known as the network backbone, designed to switch packets as fast as possible.

Explanation: The DTE transmits the data from a customer network or host computer over the WAN. The DTE connects to the local loop through the DCE. A router is a DTE device and uses a CSU/DSU (DCE device) when connecting to the WAN.

Explanation: A leased line is a dedicated point-to-point link usually leased from a carrier.

Explanation: Frame Relay virtual circuits are uniquely identified by a DLCI, which ensures bidirectional communication from one DTE device to another.

Explanation: Asynchronous Transfer Mode (ATM) technology can transfer voice, video, and data through private and public networks. It is built on a cell-based architecture rather than on a frame-based architecture.

Explanation: The Cisco Enterprise Branch Architecture allows businesses to extend the applications and services found at the campus network to various remote locations and users independent of size and location.

Explanation: The access layer is used to connect users to the network. Layer 2 and Layer 3 switches typically are used at this layer.

Explanation: ISDN PRI in North America delivers 23 B channels with 64 kbps and one D channel with 64 kbps in North America, for a total bit rate of up to 1.544 Mbps.

Explanation: VPNs are used to tunnel data securely between two private networks over a public network such as the Internet.

Explanation: The distribution layer segments workgroups, which isolates any network problems within that subnet or VLAN. Routing between VLANs typically occurs at the distribution layer.

Explanation: The local loop cabling that connects the CPE at the subscriber site to the CO of the service provider. The local loop is also sometimes called the “last mile.”

Explanation: The Enterprise Teleworker Architecture connects individual employees to network resources remotely, typically from their homes.

Access layer:

• Grants user access to network devices.
• In a network campus, the access layer generally incorporates switched LAN devices with ports that provide connectivity to workstations and servers.
• In the WAN environment, it may give teleworkers or remote sites access to the corporate network across WAN technology.

Distribution layer:

• Aggregates the wiring closets, using switches to segment workgroups and isolate network problems in a campus environment.
• Similarly, the distribution layer aggregates WAN connections at the edge of the campus and provides policy-based connectivity.

Core layer (also called the backbone):

• A high-speed backbone that is designed to switch packets as fast as possible.
• Because the core is critical for connectivity, it must provide a high level of availability and adapt to changes very quickly. It also provides scalability and fast convergence.

Enterprise Campus Architecture:

• An enterprise campus network is a building or group of buildings connected into one network that consists of many LANs.
• It is generally limited to a fixed geographic area, but it can span several neighboring buildings.
• The architecture is modular and scalable and can easily expand to include additional buildings or floors as required.

Enterprise Branch Architecture:

• This module allows businesses to extend the applications and services found at the enterprise campus to thousands of remote locations and users or to a small group of branches.

Enterprise Data Center Architecture:

• Data centers are responsible for managing and maintaining the many data systems that are vital to modern business operations.
• This module centrally houses the data and resources to enable users to effectively create, collaborate, and interact.

Enterprise Teleworker Architecture:

• This module leverages the network resources of the enterprise from home using broadband services such as cable modem or DSL to connect to the corporate network.
• Typically implemented using remote-access VPNs.

Enterprise Edge Architecture:

• This module often functions as a liaison between the campus module and the other modules in the Enterprise Architecture.

Customer Premises Equipment (CPE):

• The devices and inside wiring located at the subscriber’s premises and connected with a carrier’s telecommunication channel.
• The subscriber either owns the CPE or leases it from the service provider.

Central office (CO):

• A local service provider facility or building where local telephone cables link to longhaul, all digital, fiber-optic communications lines through a system of switches and other equipment.

Local loop:

• Often called the “last mile,” it is the copper or fiber telephone cable that connects the CPE at the subscriber site to the CO of the service provider.

Data Communications Equipment (DCE):

• Also called data circuit-terminating equipment, the DCE consists of devices that put data on the local loop.
• The DCE primarily provides an interface to connect subscribers to a communication link in the WAN cloud.

Data Terminal Equipment (DTE):

• The customer devices that pass the data from a customer network or host computer for transmission over the WAN.
• The DTE connects to the local loop through the DCE.

Demarcation point:

• Physically, the demarcation point is the cabling junction box, located on the customer premises, that connects the CPE wiring to the local loop and officially separates the customer equipment from service provider equipment.
• It is the place where the responsibility for the connection changes from the user to the service provider.

Modem:

• A voiceband modem converts and reconverts the digital signals produced by a computer into voice frequencies that can be transmitted over the analog lines of the public telephone network.
• Faster modems, such as cable modems and DSL modems, transmit using higher broadband frequencies.

CSU/DSU:

• Digital lines, such as T1 or T3 carrier lines, require a channel service unit (CSU) and a data service unit (DSU).
• The two are often combined into a single piece of equipment called the CSU/DSU.
• The CSU provides termination for the digital signal and ensures connection integrity through error correction and line monitoring. The DSU converts the T-carrier line frames into frames that the LAN can interpret.

Access server:

• Concentrates dial-in and dial-out user communications. It may have a mixture of analog and digital interfaces and support hundreds of simultaneous users.

WAN switch:

• A multiport internetworking device used in carrier networks to support Frame Relay, ATM, or X.25.

Router:

• Provides internetworking and WAN access interface ports that are used to connect to the service provider network.
• These interfaces may be serial connections or other WAN interfaces. They may require an external device such as a DSU/CSU or modem (analog, cable, or DSL) to connect to the service provider.

X.25:

• An older, low-capacity WAN technology with a maximum speed of 48 kbps. Typically used in dialup mode with point-of-sale card readers to validate transactions on a central computer.
• For these applications, the low bandwidth and high latency are not a concern, and the low cost makes X.25 affordable.
• Frame Relay has replaced X.25 at many service provider locations.

Frame Relay:

• A Layer 2 WAN protocol that typically offers data rates of 4 Mbps or higher.
• It provides permanent, shared, medium-bandwidth connectivity using virtual circuits that can carry both voice and data traffic.
• VCs are uniquely identified by a DLCI, which ensures bidirectional communication from one DTE device to another.

ATM:

• Asynchronous Transfer Mode technology is based on a cell-based architecture rather than a frame-based architecture, using fixed-length cells of 53 bytes.
• These small, fixed-length cells are well suited for carrying delay-sensitive voice and video traffic.