Lesson 6: WAN Basics

WAN Basics | Transmission Options | WAN Requirements & Solutions

Circuit Switching

 - Dedicated physical circuit established, maintained, and terminated through a carrier network for    each communication session

 - Datagram and data stream transmissions

 - Operates like a normal telephone call

 - Example: ISDN

Service providers typically offer both circuit switching packet switching services.
Circuit switching is a WAN switching method in which a dedicated physical circuit is established, maintained, and terminated through a carrier network for each communication session. Circuit switching accommodates two types of transmissions: datagram transmissions and data-stream transmissions. Used extensively in telephone company networks, circuit switching operates much like a normal telephone call. Integrated Services Digital Network (ISDN) is an example of a circuit-switched WAN technology.

Packet Switching

Packet switching is a WAN switching method in which network devices share a single point-to-point link to transport packets from a source to a destination across a carrier network. Statistical multiplexing is used to enable devices to share these circuits. Asynchronous Transfer Mode (ATM), Frame Relay, Switched Multimegabit Data Service (SMDS), and X.25 are examples of packet-switched WAN technologies.

 - Network devices share a point-to-point link to transport packets from a source to a destination across    a carrier network

 - Statistical multiplexing is used to enable devices to share these circuits

 - Examples: ATM, Frame Relay, SMDS, X.25

WAN Virtual Circuits

 - A logical circuit ensuring reliable communication between two devices

 - Switched virtual circuits (SVCs)

     - Dynamically established on demand
     - Torn down when transmission is complete
     - Used when data transmission is sporadic

 - Permanent virtual circuits (PVCs)

     - Permanently established
     - Save bandwidth for cases where certain virtual circuits must exist all the time

 - Used in Frame Relay, X.25, and ATM

A virtual circuit is a logical circuit created to ensure reliable communication between two network devices. Two types of virtual circuits exist: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs). Virtual circuits are used in Frame Relay and X.25 and ATM.
SVCs are dynamically established on demand and are torn down when transmission is complete. SVCs are used in situations where data transmission is sporadic.
PVCs are permanently established. PVCs save bandwidth associated with circuit establishment and tear down in situations where certain virtual circuits must exist all the time.

WAN Protocols

The OSI model provides a conceptual framework for communication between computers, but the model itself is not a method of communication. Actual communication is made possible by using communication protocols. A protocol implements the functions of one or more of the OSI layers. A wide variety of communication protocols exist, but all tend to fall into one of the following groups:

 - LAN protocols: operate at the physical and data link layers and define communication over the various    LAN media

 - WAN protocols: operate at the lowest three layers and define communication over the various wide-area    media.

 - Network protocols: are the various upper-layer protocols in a given protocol suite.

 - Routing protocols: network-layer protocols responsible for path determination and traffic switching.

SDLC:-
Synchronous Data Link Control. IBM’s SNA data link layer communications protocol. SDLC is a bit-oriented, full-duplex serial protocol that has spawned numerous similar protocols, including HDLC and LAPB.

HDLC:-
High-Level Data Link Control. Bit-oriented synchronous data link layer protocol developed by ISO. Specifies a data encapsulation method on synchronous serial links using frame characters and checksums.

LAPB:-
Link Access Procedure, Balanced. Data link layer protocol in the X.25 protocol stack. LAPB is a bit-oriented protocol derived from HDLC.

PPP:-
Point-to-Point Protocol. Provides router-to-router and host-to-network connections over synchronous and asynchronous circuits with built-in security features. Works with several network layer protocols, such as IP, IPX, & ARA.

X.25 PTP:-
Packet level protocol. Network layer protocol in the X.25 protocol stack. Defines how connections are maintained for remote terminal access and computer communications in PDNs. Frame Relay is superseding X.25.

ISDN:-
Integrated Services Digital Network. Communication protocol, offered by telephone companies, that permits telephone networks to carry data, voice, and other source traffic.

Frame Relay:-
Industry-standard, switched data link layer protocol that handles multiple virtual circuits using HDLC encapsulation between connected devices. Frame Relay is more efficient than X.25, and generally replaces it.

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