Wide Area Network (WAN)
So far we have talked exclusively about LANs but now we're going to switch over to WAN links. Wide Area Networks are capable of connecting LANs over large geographical distances. Unlike LANs, which are usually owned by the operator, WANs are typically leased from a service provider. Cisco supports many different WAN protocols but for the CCNA we're only going to discuss High-level Data-link Control (HDLC), Point to point (PPP), and Frame Relay. Before we get into the individual protocols we need to discuss some of the WAN concepts.
Typically the WAN links are connected to LAN links through a Channel Service Unit/Digital Service Unit (CSU/DSU). This device is similar to a modem. The CSU/DSU connects the service providers Central Office (CO) to the customers Customer Premises Equipment (CPE). It is important to familiarize yourself with the WAN terminology, the follow lists some of the terms that you will need to know for the CCNA exam -
- Central Office (CO) - The Central Office is the office of the service provider that connects your LAN to the WAN.
- Customer Premises Equipment (CPE) - This is the equipment in your LAN. All of the CPE is owned and maintained by the customer.
- Demarcation Point - The demarcation point is the point where your LAN ends and the WAN link begins. This is the point where traffic is transmitted from the customers equipment to the service providers equipment.
- Local Loop - The local loop is the link that connects the demarcation point to the central office.
The different connection types that you will encounter when dealing with WAN links are -
- Leased Line - This type of connection is a dedicated link connecting remote LANs together through the service providers network. Leased lines are typically more expensive but the customer gets an exclusive connection to themselves..
- Circuit-Switched - This type of connection is a dedicated circuit that remains active during the duration of the data transfer. Think of this line similar to a telephone conversation. When you place a call the line becomes active until you hang up the phone. When the call is terminated that line becomes available for use by someone else.
- Packet-Switched - This type of connection is similar to a leased line but not effective if you're constantly transferring traffic. Packet-switched connections are cheaper than leased lines however and are a good alternative if you don't need to transfer data over the WAN constantly.
Typically when you are working with WANs you will be working with aysnchronous serial at layer 1. This means that when connecting WAN links you will be connected layer 1 serial connections. When connecting devices through a serial connection it is important to ensure the appropriate device is connected to DCE and DTE.
DCE devices -
DTE devices -
DCE devices -
- Are defined as data terminating equipment - This equipment represent the end of the service provider.
- Is responsible for determining the clock rate.
DTE devices -
- Are defined as data terminal equipment - This equipment respresents the end of the users.
High-Level Data Link Control (HDLC)