WANs

WAN Overview

Today most organization’s computer networks need to stretch beyond the simple local area network. Businesses need to support multiple branches, offices, and remote workers, they need secure data connections, and continuous data connectivity for things like videoconferencing and teleconferencing. Today many of these services can be delivered over the public internet with data secured and protected through VPN tunnels, however sometimes, specialized dedicated WAN services are necessary to support a company’s infrastructure. Understanding WAN protocols and services becomes a necessary skill for any IT networking specialist. 

Important WAN Concepts

– WAN protocols operate at Layer 2 of the OSI networking model. When we talk about WAN protocols we are talking about Layer 2 encapsulation and framing. The WAN frame type depends on the WAN technology and hardware implemented.

– WAN links are typically point-to-point connections, the frames do not have source and destination addresses in the header like Ethernet frames.

– WAN protocols use various types of serial connections, as opposed to Ethernet LANs that use different types of Ethernet connections

– ISPs typically provide customers and organizations with a WAN device that will connect back to the ISP’s central office. A WAN device is typically a modem or a CSU/DSU. The WAN device can also be called the DCE which handles the clock rate. In the Cisco curriculum and labs a router is used to emulate a WAN device. The serial interface is configured with a clock rate and a DCE cable connection.

– The router is the DTE and the modem or CSU/DSU is the DCE

– The demarcation point is place where the ISP’s line enters your building. The line that goes back to the ISP’s office is called the local loop.

HDLC (High-level Data Link Control)

Based on IBM’s SDLC protocol, HDLC is one of the oldest WAN protocol frame types. Cisco’s version of HDLC protocol is called Cisco HDLC and it is the default frame type for serial interfaces on Cisco routers. Many other WAN protocol frames are based on the  HDLC protocol and frame including:

Protocols Based on HDLC

Protocol WAN Technology
LAPB – Link Access Procedure Balanced X.25
LAPD – Link Access Procedure D Channel ISDN
LAPF – Link Access Procedure Frame Frame Relay
PPP – Point-To-Point Protocol Serial Switched
Cisco HDLC – High-level Data Link Control Cisco Serial

HDLC Frame Fields – (Cisco HDLC adds the protocol field)

Flag Header Data FCS Flag
Address Control Protocol
8 bit delimiter 01111110 8 or 16 bits Variable length, 0 or more bits 16 or 32 bits 8 bit delimiter 01111110

Circuit Switched Networks

Circuit switched networks like phone lines PSTN (public switched telephone network) or POTS (plain old telephone system) and ISDN (Integrated Services Digital Network) have a continuous connection or circuit from one end point to another. The circuit can be divided into multiple conversations through multiplexing. Modems provide the ability to convert the traffic to digital information. Typically circuit switched networks have slow speeds and a low cost.

Packet Switched Networks

Packet switched wide area networks (WANs) such as V.35, Frame Relay, and ATM use network relay switches to create paths to destination networks. The paths across the networks are called virtual circuits. Virtual circuits can be permanent virtual circuits (PVC) or switched virtual circuits (SVC) that are created temporarily to send data to destination networks and then are tore down and erased.  

  • X.25 – is an older outdated technology, a precursor to Frame Relay and is still used in some countries.
  • Frame Relay – cost effective to implement in hardware because multiple circuits can go out of the same interface. Uses DLCIs to create switched paths to destination networks. DLCIs are locally significant only, not unique. Frame Relay is capable of high speeds.
  • ATM –  expensive to implement in hardware, because it uses multiple interfaces and hardware devices. Uses cells instead of frames. ATM cells are a fixed size of 53 bytes, 48 of which is for data. ATM is capable of high speeds, and can delivering video and voice, as well as data.

Dedicated Line Networks

  • T1 and E1 – bandwidth is dedicated and therefore is always available and delivers consistent speeds. Dedicated lines have expensive installation and month to month costs. A T1 line is 1.5 Mbps.

Public Internet Networks

  • DSL
  • Cable modem
  • Wireless – WiMax

Author: Dan

Dan teaches computer networking and security classes at Central Oregon Community College.

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