CCNA v1.1 Exam Review: Wide Area Networks (WAN)

WAN Connection Types

Here’s a look at the connections and their characteristics.

Leased Line:

  • A pre-established, private connection from one site to another through a provider’s network, also called a dedicated circuit or a dedicated connection
  • Always a point-to-point connection between two end points
  • Used when there is a constant flow of data, or when a dedicated amount of bandwidth is required
  • One router interface is connected to one destination site
  • Example encapsulations: PPP, HDLC

Circuit Switching:

  • A dial-up connection through a provider’s voice-grade network, either using an analog modem or an ISDN connection
  • Used when only a slow-speed connection is needed, or when there is not much of a need to transfer a lot of data
  • One call establishes a circuit to one destination site
  • Example encapsulations: PPP, HDLC, SLIP

Packet Switching:

  • Each site only uses one physical connection to the provider’s network, however there may be multiple virtual circuits to various destinations
  • Typically less expensive than leased lines, because various data streams are mixed across a single link
  • Used when a dedicated connection is needed, but cost savings is important
  • Examples: Frame Relay, X.25

Cell Switching:

  • Each site only uses one physical connection into the provider’s network, however there may be multiple virtual circuits to various destinations
  • Typically less expensive than leased lines, because various data streams are mixed across a single link
  • Uses fixed-size frames called cells to achieve faster and more predicable transport through the network
  • Examples: ATM, SMDS

Internet (using a VPN):

  • Each site uses a broadband connection (DSL, cable or broadband wireless) to connect to an ISP, and uses VPN technology to ensure secure communication.

WAN Encapsulation Types

High-Level Data Link Control (HDLC):

  • A Cisco-proprietary serial encapsulation
  • Allows multiple network-layer protocols to be sent
  • Default encapsulation for all serial interfaces on a Cisco router
  • One router interface only goes to one destination

Point-to-Point Protocol (PPP):

  • An open-standard serial encapsulation
  • Allows multiple network-layer protocols to be sent
  • Allows optional link-layer authentication (CHAP or PAP)
  • One router interface only goes to one destination

Serial Line Internet Protocol (SLIP):

  • An open-standard serial encapsulation
  • Allows only IP to travel across
  • One router interface only goes to one destination

Frame Relay:

  • Uses switched virtual circuits (SVCs) or permanent virtual circuits (PVCs)
  • Allows multiple network-layer protocols to be sent
  • Each virtual circuit is a private channel between two end points
  • One router interface may have many virtual circuits, going to the same location or various locations

X.25:

  • An old, but still available, packet switching standard
  • Uses switched virtual circuits (SVCs) or permanent virtual circuits (PVCs)
  • Allows multiple network-layer protocols to be sent
  • Each virtual circuit is a private channel between two end points
  • One router interface may have many virtual circuits, going to the same location or various locations

WAN Terminology

  • Customer Premise Equipment (CPE) — Network devices/equipment physically located at the customer’s location/site. Customer is typically required to procure and maintain this equipment. Equipment could include routers and CSU/DSUs.
  • Central Office (CO) — The facility that provides WAN services to the customer. Source of analog phone service, ISDN service, DSL service, frame relay connections, X.25 connections, and leased lines.
  • Local Loop — The link from the provider’s CO to the customer’s demarcation point. Also called the “last mile”, this is normally not more than a few miles long.
  • Demarcation Point (Demarc) — The line between the customer site and the provider network. The CPE is inside the demarc; the local loop is outside the demarc.
  • Toll Network — The provider’s network, inside the WAN cloud.

WAN Devices

WAN devices include:

  • Routers, to route between LANs and WANs
  • Access servers, to terminate dial-in connections
  • Modems, to convert between digital and analog signals
  • Channel Service Unit/ Data Service Unit (CSU/DSU), to provide termination for digital signals
  • WAN switches, such as Frame Relay switches and public switched telephone network (PSTN) switches
  • Core routers, such as those in a multiprotocol label switching (MPLS) network

WAN Cables

WAN cables connect a data terminal equipment (DTE) device (which is typically a router) to a data circuit-terminating equipment (DCE) device (for example, a CSU/DSU or modem). The DTE end for a Cisco router has a 60 pin connector or a 26 pin connector. The types of DCE interfaces include: EIA/TIA-232, EIA/TIA-449, V.35, and X.21.

Frame Relay Terminology

  • Local Access Rate — Connection rate between a frame relay site and the frame relay provider.
  • Virtual Circuit — Logical connection between two end points.
  • Permanent Virtual Circuit (PVC) — A circuit that is always available. The bandwidth for the circuit is always allocated.
  • Switched Virtual Circuit (SVC) — A circuit that is built when needed. The bandwidth is returned when the circuit is closed.
  • Data Link Connection Identifier (DLCI) — The locally significant reference to one end of a virtual circuit. The DLCI numbers are assigned by the frame relay provider.
  • Committed Information Rate (CIR) — The maximum average data rate that the network tries to deliver, through the PVC from one end to the other. Each PVC may have a unique CIR.
  • Inverse Address Resolution Protocol (IARP) — The process used by a frame relay device, such as a router, to discover the network-layer information of the devices at the other end of the PVCs.
  • B Signaling between the frame relay device (the router) and the frame relay switch (the provider). LMI messages do not travel across the entire PVC.

How Protocol Layers Interact

The Type field or 802.2 header in an Ethernet frame indicates the type of layer 3 (network layer) information contained in the frame. The protocol field in an IP packet indicates the type of layer 4 (transport layer) information contained in the packet. The port number field in a TCP or UDP segment indicates the type of application layer information contained in the segment.

Excerpted and available for download from Global Knowledge White Paper: CCNA v1.1 Exam Review: Critical Concepts of the 640 – 802 CCNA Exam

Resources:
CCNA Cert Check
CCNA Exam Prep Mobile App

Related Courses:
ICND1  —  Interconnecting Cisco Network Devices 1
ICND2  —  Interconnecting Cisco Network Devices 2
CCNAX  —  CCNA Boot Camp v1.1

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