WAN Services & Transmission Mediums | CompTIA Network+ N10-007 | 2.5a

In this video you will learn about WAN services such as: ISDN, T1/T3 circuits, E1/E3 circuits, OC-3 & OC-192 circuits, DSL, Metropolitan Ethernet, cable broadband, dial-up, PRI, & various transmission mediums.

Service Type

ISDN (Integrated Services Digital Network)

ISDN is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network. ISDN is used on PSTN lines with a terminal adapter (TA) on multiple channels of 64Kbps each. A TA resembles a conventional analog modem. Internal models plug into the same PCIe or PCI slot used by analog modems, and external models are USB or serial ports.

There are two types of ISDN connections provided by local telephone companies in which both types of connections enable the use of the internet, talk or fax communications through a phone line at the same time:

  • Primary Rate Interface (PRI):  A PRI connection provides 1.536Mbps of bandwidth, whereas a BRI interface provides 64Kbps (single-channel or 128Kbps dual-channel) of bandwidth.  PRI is sold to large organizations.
  • Basic Rate Interface (BRI):  BRI was sold to small businesses and home offices before DSL and cable became widely available.  BRI offers up to 128Kbps of bandwidth.

Most telephone companies have largely phased out ISDN in favor of DSL, which is much faster and less expensive for internet connections.


A T1 carrier is a dedicated phone connection supporting data rates of 1.544Mb/s.  A T-1 line consists of 24 individual channels, each of which support 64Kbps.  Each 64Kbps channel can be configured to carry voice or data traffic.  Most telephone companies allow you to buy just some of these individual channels, known as fractional T-1 access.  T-1 lines are a popular leased line option for businesses connecting to the Internet and for ISPs connecting to the Internet backbone.  The Internet backbone itself consists of faster T-3 connections.  T-1 lines are sometimes referred to as DS1 lines.  When someone says they are running T1, they may be saying a few different things such as:  they may mean that they have a network that is passing data at 1.544Mbps; they may mean that they have a network that conforms to the T1 electrical interface specification, or that they have a network that passes data that conforms to one of the several framing formats (D4, ESF, etc.).  More likely than not, they mean all three but their concentration may be only one of these items.

A T3 is a dedicated phone connection supporting data rates of about 43Mbps.  A T-3 line actually consists of 672 individual channels, each of which supports 64Kbps.  T-3 lines are used mainly by ISPs connecting to the Internet backbone and for the backbone itself.  T-3 lines are sometimes referred to as DS3 lines.


The E-carrier system enables the transmission of several (multiplexed) voice/data channels simultaneously on the same transmission facility or line.  Of the various levels of the E-carrier system, the E1 & E3 levels are the only ones that are used.  An E1 circuit contains 32 channels, in contrast to the 24 channels on a T1 circuit.  Only 30 of those 32 channels, however, can transmit data (or voice or video).  Specifically, the first of those 32 channels is reserved for framing and synchronization, and the seventeenth channel is reserved for signaling (that is, setting up, maintaining, and tearing down a call).  E1 circuits also have a bandwidth capacity of 2.048Mbps.  E1 circuits are typically popular outside of North America & Japan.

Just as a T3 circuit provides more bandwidth than a T1 circuit, and E3 circuit’s available bandwidth of 34.4Mbps is significantly more than the 2.046Mbps of bandwidth offered by an E1 circuit.

OC-3 & OC-192

Optical Carrier transmission rates are a standardized set of specifications of transmission bandwidth for digital signals that can be carried on Synchronous Optical Networking (SONET) fiber optic networks.  Transmission rates are defined by rate of the bitstream of the digital signal and are designated by hyphenation of the acronym OC and an integer value of the multiple of the basic unit of rate (e.g. OC-3).

OC-3 is a network line with transmission data rates of up to 155.52Mbps (payload:  148.608Mbps; overhead:  6.912Mbps, including path overhead) using fiber optics.

OC-192 is a network line with transmission speeds of up to 9953.28Mbps (payload:  9510.912Mbps; overhead:  442.368Mbps).

DSL (Digital Subscriber Line)

DSL is a communications medium used to transfer digital signals over standard telephone lines.  Along with cable internet, DSL is one of the most popular ways ISPs provide broadband internet access.  DSL used by telephone companies to provide internet services at speeds much faster than that allowed by analog (dial-up) internet services while allowing traditional analog telephones and devices such as fax machines to share the same connection.  To make this possible, devices called DSL microfilters are attached between telephones and telephony devices such as answering machines, voicemail, and fax machines to prevent interference with DSL signals.

Traditional DSL runs over telephone lines originally developed for voice services only, it is far slower than a cable internet connection.  Traditional DSL speeds range from 768Kbps/384Kbps (downstream/upstream) to 6Mbps/768Kbps. Speeds decrease as the distance between the DSL service location and the central switch (central office) providing the DSL connection increases.  Most DSL providers do not have data caps.

DSL advantages:

  • Inconsistent internet connection speeds.
  • Available in limited distances from service provider.
  • Speeds drop the greater the distance from the connection to the internet (also known as the central office or central switch).
  • Slower than cable or fiber connections.

Metropolitan Ethernet

A metropolitan-area Ethernet (Metro Ethernet) is a metropolitan area network (MAN) that is based on Ethernet standards.  It is commonly used to connect subscribers to a larger service network or the Internet.  Service providers use Metro Ethernet to:

  • Interconnect business offices or data centers.  Metro Ethernet can connect 2 sites or hundreds of sites.
  • Connect residential subscribers or businesses to the Internet.
  • Provide connectivity to public or private cloud data centers.
  • Provide wholesale mobile backhaul services.
  • Provide multicast delivery used by business customers for video conferencing,and used by residential subscribers for IPTV & video applications.

Cable Broadband

Cable Internet access (or cable Internet) is a form of broadband Internet access which uses the same infrastructure as cable television.  Like DSL & fiber, cable Internet access provides network edge connectivity (known as last mile access) from the ISP to an end user.  It is integrated into the cable television infrastructure analogously to DSL which uses the existing telephone network.  Data signals can be sent over a network in 2 ways:  baseband & broadband.  Baseband signaling sends a signal at any given instant, whereas broadband signaling sends multiple signals on separate frequencies.  A single piece of coaxial cable carrying multiple signals comes into your home and a small box enables you to select specific channels.  Broadband creates these separate channels through a process called frequency division multiplexing.  The term broadband is widely used to refer to high speed Internet access.  Broadband provides users with always-on, high speed connections to access the Internet and transfer data.  Unlike narrowband, broadband does not tie up a consumer’s phone line, plus it enables access to high bandwidth applications, such as voice and video calling, online video streaming, interacting gaming, and more.


Dial-up internet access is a form of internet access that uses the facilities of the public switched telephone network (PSTN) to establish a connection to an ISP by dialing a telephone number on a conventional telephone line. Dial-up connections use modems to decode audio signals into data to send to a router or computer, and to encode signals from the latter two devices to send to another modem. Until the late 1990s, dial-up networking was the most common way for home & small businesses to connect to the internet. Today, dial-up connections are used when no other internet connection is available. Dial-up has rates ranging from 28.8Kbps to 56Kbps and is available anyplace that has a landline telephone system. A disadvantage of telephone dial-up is that voice and data cannot share the wire simultaneously.

PRI (Primary Rate Interface)

PRI is a telecommunications interface used mostly in an Integrated Services Digital Network (ISDN).  A PRI is a form of time division multiplexing (TDM), allowing you to send up to 23 separate communications via voice, data, or video.  Channels are typically used by larger enterprise companies with a PBX system to help with access to the PSTN (public switched telephone network).

Transmission Mediums

Various WAN transmission mediums such as satellite, copper, fiber, & wireless have been covered already & can be found in section 2.7 of CompTIA A+ titled ‘Internet Connections & Network Types’.