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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:

WAN structure
WAN structure

  • 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.

  • DS0 -- the smallest-capacity digital circuit with the digital signaling rate of 64 kbit/s. It is equivalent to one channel. Europe uses E0, and Japan uses J0 to refer to the same channel speed.
  • T1 or DS1 -- 24 DS0 circuits bundled together for a total bandwidth of 1.544 Mbps.
  • E1 -- 30 DS0 circuits bundled together for a total bandwidth of 2.048 Mbps.
  • T3 or DS3 -- 672 DS0 circuits bundled together for a total bandwidth of 44.736 Mbps.
Latest OC (Optical carrier) uses fiber for large bandwidth.
  • OC-3 -- have capacity of 2016 DS0 for a total bandwidth of 155.52 Mbps.
  • OC-12 -- 8064 DS0 (4 OC-3) bandwidth of 622.08 Mbps.
  • OC-48 -- 32256 DS0  (16 OC-3) bandwidth of 2488.32 Mbps.
  • OC-192 -- 129,024 DS0  (64 OC-3) bandwidth of 9953.28 Mbps.

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

WAN connectivity options
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 
ExpensiveSynchronous serial lines
 with bandwidth up to 45 Mbps (E3)
Circuit SwichingSame 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 kbps
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
Allows you to share 
bandwidth with other
 companies to save 
money. Packet 
switching offers
 services, but 
at the cost 
like circuit
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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