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Physical Side of WANs

The following diagram shows a typical WAN structure. Let's take a look at some physical items:


  • Data Terminal Equipment (DTE) is an end instrument that serves as a data source or a data sink. DTE devices are typically owned by your organization. The most common DTE is a router.

  • Data Communications Equipment (DCE) is a device that sits between the DTE and a data transmission circuit. The DCE converts user data into the service provider's preferred format, DCE may be a part of the DTE or intermediate equipment. A DCE is typically a switch inside a service provider's network.

  • Channel Service Units / Data Service Units (CSU/DSU) adapts the signal provided by the carrier to the interface used by a DTE. In analog lines, modems convert the digital signal of the sending device into analog format for transmission over an analog line and vice visa. On the other hand, CSU/DSU is for digital lines. A CSU/DSU looks somewhat similar to an external modem. The difference is that a modem is a digital/analog signal converter, while a CSU/DSU is a digital signal processing device. The DSU portion is responsible for timing, and actually connects to the DTE (in this case the router) via its serial port. It typically connects to a DTE router via a V.35 serial cable. The CSU portion is responsible for terminating the service provider’s link, and handles transmitting and receiving data over the WAN link.

  • DTE requires external clocking from the DCE or CSU/DSU.

  • Customer premises equipment (CPE) is equipment that's owned by the subscriber and located on the subscriber's premises. CPE generally refers to telephones, DSL modems, cable modems, CSU/DSU and routers.

  • Demarcation point (Demarc) is the point at which the service provider's responsibility ends and the CPE begins. It's generally a device in a telecommunications closet. The customer need to provide a cable connection from this closet to the CPE via CSU/DSU or ISDN interface on the DTE router.

  • Toll network: is a collection of trunks inside the provider's network (WAN cloud). It contains switches and facilities owned by the ISP. Besides the switch acting as the DCE, there are other WAN networking devices, such as ATM switches, Frame Relay switches, public switched telephone network (PSTN) switches, and core routers, which are transparent to end users.

The following diagram is an overall architecture of internet.


  • Local Loop also called "last mile", is a copper or fiber cable that connects the demarc to the closest switching office, called a central office. In the diagram, the local loop is a T1 line.

  • Central office (CO) also called point of presence (POP), connects the customers to the provider's switching network. It is the entry point to the WAN cloud and the exit point from the WAN for called devices.

  • Network Access Points (NAP) is a public network exchange facility where Internet Service Providers (ISPs) can connect with one another in peering arrangements. The NAPs are a key component of the Internet backbone because the connections within them determine how traffic is routed. They are also the points of most Internet congestion.

Bandwidth refers to the rate at which data is transfered over the communication link.

There are two major bandwidth measure system:

  1. T1, T2, T3...lines belong to T-carrier system originally developed by Bell Labs and used in North America, Japan, and Korea. In North America, bandwidth is usually expressed as a "DS" number (DS0, DS1, and so forth) that technically refers to the rate and format of the signal. The bandwidth on a serial connection can be incrementally increased to accommodate the need for faster transmission. For example, Digital signal 1 (DS1), also known as T1  runs at the original 1.544 Mbit/s line rate. is is made up of 24 8-bit channels (also known as timeslots or DS0s), each channel being a 64 kbit/s DS0 multiplexed carrier circuit. T2 and T3 circuit channels carry multiple T1 channels multiplexed, resulting in connection speed of 6.312 and 44.736 Mbit/s, respectively.

  2. The E-carrier system, where 'E' stands for European, is incompatible with the T-carrier system and is used in locations outside of Northe America, Japan, and Korea. It typically uses the E1 line rate and the E3 line rate.

Many physical implementations carry traffic across the WAN. The following figure shows the four major WAN connectivity options.


There are three major WAN connections (services) provided by ISPs on a subscription basis:

Option:DescriptionAdvantagesDisadvantagesBandwidth rangeSample protocols used
Leased lineA pre-established Point-to-Point or dedicated connection path between the service provider's network (WAN) to a remote network (usually an  organization's LAN).Provide reserved connection for the cliet, also the most secure connections.ExpensiveSynchronous serial lines with bandwidth up to 45 Mbps (E3)HDLC, PPP
Circuit switchingSame as phone call. A dedicated circuit path is created between end points. Circuit switching allows multiple sites to connect to the switched network of a carrier and communicate with each other.Less Expensive, you pay only for the time you actually use.Need to establish an end-to-end connection before data can be transfered.Use dial-up modems or ISDN and is used for low-bandwidth data transfers with speed range from 28 to 144 kbpsPPP, ISDN
Packet switchingDevices transport packets via a shared single point-to-point or point-to-multipoint link across a carrier internetwork. Variable length packets are transmitted over Virtual Circuits (VC). Progammed switching devices prodvide physical connections. Packet headers are used to identify the destination.Allows you to share bandwidth with other companies to save money. Packet switching offers leased-line-type services, but at the cost like circuit switching.Shared media across link, subject to uncertanty. good for data transfers with bursty nature, not good for data transfers with constant nature.typically use serial connections with speeds ranging from 56Kbps to T3 (45Mbps)X.25, Frame-Relay

Of course, there are many other WAN connection types such as satellite internet, wireless... 

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