12-04-2014, 12:10 PM
WAN Technologies
WAN technology/terminology
Devices on the subscriber premises are called customer premises equipment (CPE).
The subscriber owns the CPE or leases the CPE from the service provider.
A copper or fiber cable connects the CPE to the service provider’s nearest exchange or central office (CO).
This cabling is often called the local loop, or "last-mile".
Devices that put data on the local loop are called data circuit-terminating equipment, or data communications equipment (DCE).
The customer devices that pass the data to the DCE are called data terminal equipment (DTE).
The DCE primarily provides an interface for the DTE into the communication link on the WAN cloud.
Connecting a Modem To a Router
Connecting to a serial interface:
physical-layer async interface command
The Picture shows a connection between a Cisco 2620 series router and an external modem using an EIA/TIA-232 Smart Serial cable.
Modems
Modems transmit data over voice-grade telephone lines by modulating and demodulating the signal.
The digital signals are superimposed on an analog voice signal that is modulated for transmission.
The modulated signal can be heard as a series of whistles by turning on the internal modem speaker.
At the receiving end the analog signals are returned to their digital form, or demodulated.
HDLC Framing
The choice of encapsulation protocols depends on the WAN technology and the equipment.
Most framing is based on the HDLC standard.
The address field is not needed for WAN links, which are almost always point-to-point. The address field is still present and may be one or two bytes long.
Several data link protocols are used, including sub-sets and proprietary versions of HDLC.
Vendors usually use their own proprietary version of HDLC.
Both PPP and the Cisco version of HDLC have an extra field in the header to identify the network layer protocol of the encapsulated data.
Using Leased lines to the WAN Cloud
To connect to a packet-switched network, a subscriber needs a local loop to the nearest location where the provider makes the service available.
This is called the point-of-presence (POP) of the service.
Normally this will be a dedicated leased line.
This line will be much shorter than a leased line directly connected to the subscriber locations, and often carries several VCs.
Since it is likely that not all the VCs will require maximum demand simultaneously, the capacity of the leased line can be smaller than the sum of the individual VCs.