This topic is to discuss the following lesson:
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Hello
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.
Rene
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!
Laz
Thanks Lazaros, Nice explanation.
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HI,
Can you explain the difference between dark-fiber and DWDM.
Thanks
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!
Laz
Thanks,
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!
Laz
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?
Regards,
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.
- It refers to the connection to the Internet from the enterprise network
- 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:
Copper
- 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.
Fiber
- 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.
Wireless
- 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!
Laz
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Great explanation!!! Thanks!
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Hi,
Do I understand it correctly that we don’t have a DTE/DCE (DCE also being a CSU/DSU) in all cases, but only when the connection between the CPE and the Service Provider’s equipment is a Serial connection? So in case of a fibre optic leased line for example, there would be neither of those things.
Also, why can’t we just have a single fiber-optic cable between the SP’s router and the customer’s?
Why do we have any Serial connections at all in the age of fiber-optic cables?
Thanks.
Attila
Hello Atila
The terms Channel Service Unit/Data Service Unit (CSU/DSU), Data Terminal Equipment (DTE) and Data Communication Equipment (DCE) are all terms used to refer to equipment that has been traditionally installed at or near the demarcation point of the customer’s premises.
This equipment was necessary primarily because the WAN technologies (serial, DS1, T1, E1, OC-X) used by telcos were different from the LAN technologies used by the customer. The CSU/DSU was used to convert the WAN technology to LAN technology such as Ethernet.
The DTE is typically a customer device, on the customer side of the demarcation point, such as a router.
The DCE, which is typically a telco-owned device, is the device on the telco side of the demarcation point. It is a CSU/DSU that converts from WAN technology to LAN technology.
Now today, these terms are slowly falling out of use because we have more and more cases where the WAN and LAN technologies are the same (i.e. Metro Ethernet, fiber optics, wireless links etc…). So although the demarcation point still exists, the equipment that may or may not be there depends highly upon the arrangement that you have made with the telco.
So you can have many different arrangements such as:
- a single fiber optic cable coming onto your premises that connects to your router’s SFP port and delivers Ethernet
- a DSL modem (that can be considered a CSU/DSU, DTE and DCE all in one device) that delivers wired and wireless communication on the customer premises
- a more traditional serial connection with the necessary DTE unit
- a wireless microwave link that may need specialized equipment to terminate the microwave link before delivering Ethernet internally
Specifically, in the case you mention, if you have a fiber optic leased line, you can terminate that on your own equipment, so you wouldn’t need any other telco-owned equipment in such a case.
You can, if that is the arrangement you have with your ISP.
They are scarce nowadays but are mentioned here for completeness as they may be mentioned in an exam. But remember, there are still some places in the world where these technologies still exist, especially in remote and rural areas.
I hope this has been helpful!
Laz
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Hello Laz,
Thank you. 
So if there’s a “DSL modem (that can be considered a CSU/DSU, DTE and DCE all in one device),” then the customer’s router connects to that modem via a Serial cable?
Attila
Hello Attila
No, of course not. You would connect it using an Ethernet cable.
The point I was trying to make in my previous post is that the distinction between all of these edge network devices was important in the past when WAN and LAN technologies were worlds apart. Today, the role played by edge equipment has changed somewhat, so you are not obligated to use those same technologies as before.
In any case, I haven’t come across any consumer-grade telco-provided xDSL devices that also have a serial interface. And even if you find one, I wouldn’t recommend it… 
I hope this has been helpful!
Laz
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