Introduction to WANs (Wide Area Network)

This topic is to discuss the following lesson:

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For cell switching Data add to the cell have to be fix size and the same for all cell or can be different? like first cell 100 second cell 150.

Hi Heng,

The cell size is a fixed length, it doesn’t change.


Hi Rene,
If possible so please define and explain Circuit switching, I am confused.

Hello Muhammad

An excellent example of circuit switching can be found in the traditional telephony network. Until the mid 1980s, before digital telephone switches were in widespread use, telephony systems used mechanical switching. When you picked up the phone and dialed, telephone switches along the path between you and the party you called would whir into action and psychically connect wires and connectors in such a way that a physical circuit would be created between your phone and the called party’s phone. The following diagram illustrates this:

In such a situation, the physical wire is used for the duration of the call and cannot be shared with any other telephone conversations. These telephone switches, also known as switching offices, create this temporary circuit from end to end until the call is finished. Once it ends, the circuit is released and is made available for other calls to be made. This method of operation is called circuit switching.

Later telephony technologies such as ISDN, although digital, are still considered circuit switched technologies because, instead of reserving a physical wire for exclusive use of a conversation, a specific time slot in the transmission of information is reserved. The concept is the same, the implementation is somewhat different.

In any case, circuit switching is less efficient than packet switching because wires or time slots are reserved unconditionally during a conversation or during data transfer, even if no information is being exchanged. Packet switching on the other hand will simply not send any packets if there is no information to be exchanged.

I hope this has been helpful!


Thanks Lazaros, Nice explanation.

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Can you explain the difference between dark-fiber and DWDM.

Hello Giovanni

Dark fiber is a term used to refer to fiber optic cables that are installed for future use. It is called “dark” because it is initially not in use, thus it is not “lit up” by diodes or lasers. Dark fiber is often installed within “right of way” paths (roads, railroads, sewer systems, electrical cabling runs etc) whenever those infrastructures are being upgraded. It costs less to put in cabling when trenches are being dug up anyway for other works, rather than digging up trenches solely for the installation of the cables. The cables are installed, and then “lit up” as needed depending on future requirements. Dark fiber is simply a strategy used for infrastructure deployment in order to reduce installation costs.

Dense Wavelength Division Multiplexing (DWDM) is a completely different concept. It is a technology that is used to allow multiple data streams to be sent over the same fiber strand simply by using different wavelengths for each signal. Because light doesn’t interfere with itself, these multiple wavelengths can be simultaneously sent, thus drastically increasing the bandwidth of any particular fiber optic strand. DWDM depends upon the capability of light emitters and receivers to generate as well as distinguish between, wavelength pulses of differing wavelengths.

I hope this has been helpful!


But I think that customer that uses a dark-fiber connection use too A DWDM to send more signals on a single link, is it true??

Hello Giovanni

Now the term Dark Fiber “officially” means what I have mentioned in my previous post. However, customers and teclos do use the term for different things as well.

For example, dark fiber can refer to a physical fiber link that you buy from a teclo. They bring the fiber to your premises, but you are responsible for “lighting it up” so to speak. In other words, you are responsible for connecting the end devices that provide the signalling. It’s similar to a leased line in that the telco provides you with the physical infrastructure, and you have to connect the equipment on either end to send your signalling.

So in your case, your customer purchased the physical fiber link (which is dark because the telco doesn’t light it up), and they have put in their own DWDM equipment on either end to send their data.

For marketing purposes, telcos will often use “cool sounding” terminology like dark fiber (kind of reminds me of dark matter which is kinda cool) in their own ways in order to promote services. I believe that this is one of those situations.

A lot of times, the actual real meaning of a term actually changes over time, depending on how often it is used. This is the case with the term “cloud” which initially was just a symbol depicting a network about 30 years ago or more. Now it means something completely different. But I digress…

I hope this has been helpful!


Hello Team,

My question is about physical connections for the WAN and WAN Technologies.
I am pretty confuse on this section because we usually don’t see the physical connection type for the different WAN technologies.

  • I wanted to know the different physical connections types available for the WAN?
  • Can Ethernet be used for WAN?
  • What is DWDM?
  • When we say Ethernet for WAN, does it mean only Fiber?

For instance, MPLS is a WAN Technology, at the edge we have the Customer Edge Router and Provider Edge Router. What kind of physical connection can we use from the CE to the PE router?

For a PPP (Point-to-Point Protocol), I know we can use Serial Links, are there any other physical connection available? Does Ethernet work? I remember there is an extension for this protocol which is PPPoE. It encapsulates PPP over Ethernet, right? When they say it encapsulates PPP over an Ethernet frame, it means they will use SRC and DST MACs?

WAN Technologies : MPLS, PPP, DMVPN
Old WAN Technologies: Frame-Relay, ATM

  • Are there any other WAN Technologies?
  • Which WAN Technologies do we use for VoIP and Fiber Channel (Storage) traffic?


Hello Luis

First of all, it is important to understand what it is that specific terms are referring to. Some terms refer to a specific technology, others to the physical infrastructure, and others to Datalink (Layer 2) or even Network (Layer 3) functionalities, but all are involved, in some way, with the WAN.

The term WAN is most often used for two things.

  1. It refers to the connection to the Internet from the enterprise network
  2. It refers to the network that connects remote sites

Now the various terms that you have mentioned in your post exist at different layers of the OSI model, so some of these may actually co-exist in a particular implementation. I’ll try to clarify the role of each one below.

Physical WAN connections, like all network connections, can take one of three forms: copper, fiber, or wireless. Strictly on the physical layer, for each of these we have:


  • Physical layer - typically two-wire connections come into the customer premises, although eight wire UTP connections are rare but not unheard of. Coaxial cable is also prevalent. Technologies that typically use copper include xDSL, Cable (DOCSIS) and Serial.


  • Physical layer - Fiber Optic cables can be terminated on the customer premises. These cables are most often single mode fibers that employ Dense Wavelength Division Multiplexing (DWDM) which is strictly a physical layer technique for fiber optic cables, that allows multiple carrier signals to run over a single fiber, thus increasing data capacity.


  • Physical Layer - Technologies such as microwave, Wi-MAX, and broadband cellular are all physical layer wireless technologies that use different frequencies and encoding techniques.

The Data Link layer is a place where some more commonly heard WAN terms live. These are technologies which, with very few exceptions, can run over any of the above physical layer WAN infrastructures. These include:

  • Ethernet - Even though Ethernet was designed to be a LAN technology, it is so well designed, that it has the flexibility to be expanded into a WAN technology. Specifically, Metro Ethernet is the “type” of Ethernet that is used for WAN. Metro Ethernet can run over copper, fiber, as well as wireless technologies.
  • PPP is typically used over, and has traditionally been associated with, a serial connection. But PPP is used over many types of physical networks including serial cable, phone lines, trunk line, cellular telephone, specialized radio links, and fiber optic links such as SONET. Its authentication mechanisms are used especially for xDSL and Cable (and dialup in the past) in specific arrangemements such as PPPoE. For more info about PPPoE, take a look at the related lesson.
  • Frame Relay is a Layer 2 technology that can run over various types of physical infrastructure, but most commonly over copper. It is most often used to interconnect remote sites, rather than connect a site to the Internet.
  • ATM was developed for high speed transfer of video, voice, and data, but never took off.

Somewhere between the Data Link and Network Layers, is MPLS. This is a technology primarily used to interconnect remote sites. It can run over various combinations of physical and data link layer technologies.

At the Network layer we have technologies such as DMVPN which is essentially a technology that allows for a hub and spoke topology over any combination of the above mentioned WAN technologies.

Are there any other WAN technologies? The only ones I can think of at this time are X.25, ISDN, dialup, and satellite.

VoIP doesn’t need a particular WAN technology to function, its primary requirement is a steady rate of transmission, something that can be achieved over any WAN technology, as long as QoS mechanisms are employed. Fiber channel storage across WANs can use what is called Fiber Channel over IP to overcome the distance limitations that fiber channel has. FCoIP can be routed over any WAN technology described above. More about this can be seen at RFC3821

I hope this has been helpful!


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Great explanation!!! Thanks!

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