• Circuit Switching
• Packet Switching

An example of a circuit switched network is a telephone network.  The a remote access link for each major node in the circuit associated with a phone call is established for the duration of the call.  It is also the case that the pathway taken might be different from one call to the next.

An example of a packet switching network is the standard routing operations on the Internet.  When you send an e-mail it is broken down into packets.  Each packet might take a different route to get to the destination.  Then the packets are reassembled at the other end.

Circuit Switching Networks

This page surveys several circuit switching technologies in each of the following subsections.  A lsit of these technologies follows.

• POTS/PSTN
• ISDN
• DSL
• Leased Lines
• Digital data service (DDS)
• T-carriers
• Switched 56

POTS/PSTN.  The POTS - Plain Old Telephone System or PSTN - Public Switched Telephone Network is the analog phone lines that are installed in most residences and businesses.  It has two big advantages.

• it is available almost all over the world
• it is relatively inexpensive

Using the existing telephone infrastructure to implement computer networking should make sense.  Just about all that is required is a modem, to bridge the gap between digital computers and analog telephone lines, which are basically commodities.

Unfortunately, the original phone lines were not intended to carry the data rates required for many modern computer uses.  This implies there are inherent limits to their capacity.  Even with 56 Kbps modems, the capacity of many lines is 40 Kbps to 45 Kbps.  The quality of the lines and transmissions is also a factor.

ISDN.  ISDN - Integrated Services Digital Network was developed to replace the POTS - Plain Old Telephone Service and provide a reliable digital connection for both voice and data.  The reality has proven to be much different.  It hasn't replaced POTS and almost certainly won't because of the development of higher speed and still lower cost alternatives.

Some of the major characteristics of ISDN are contained in the following list.

• it is a digital link
  • no need to convert data from digital to analog and back
  • improves reliability
  • improves performance
• it is more readily available than many of its newer competitors such as DSL
• it can be used as an always on link
• it is more expensive than the PSTN
  • requires specialized equipment at both ends
• it is made up of one or more channels that carry data
  • B channels for bearer channels
  • each channel carries 64 kbps worth of data
  • these channels can be aggregated using inverse multiplexing to create multiple channels in one high speed connection
• it has a single control channel
  • D channel or delta channel
  • this channel provides 16 kbps or 64kbps depending on the implementation

ISDN is offered by the telephone companies in two standard access interfaces:

• BRI - Basic Rate ISDN
  • two 64 kbps B channels
    • aggregate bandwidth of 128 kbps
  • one 16 kbps D channel
• PRI - Primary Rate ISDN
  • 23 different 64 kbps B channels
    • aggregate bandwidth of 1.472 Mbps
  • one 64 kbps D channel

BRI is implemented for residential or small business data transfer.  PRI is typically used for digital voice transmission in conjunction with PBX - Private Branch Exchange telephone systems.

DSL.  DSL - Digital Subscriber Line is a newer technology than ISDN.  It was developed as an add on service over existing copper wire lines.  Some of the major characteristics of DSL are given in the following list:

• it offers speeds up to and exceeding those of T-1 lines at a fraction of the cost
  • in many areas it costs less than ISDN
• it provides an always on link
• both voice and data can be transmitted over the same line simultaneously
• you must be within a certain distance of a central office in order to get DSL service if your phone company provides it

DSL comes in several varieties:

• ADSL - Assymmetric DSL
  • most common
  • higher rate downstream
    • 384 kbps to 6 Mbps
  • lower rate upstream
  • usually must be within 17,500 feet of a central office
• SDSL - Symmetric DSL
  • same speed for uploads and downloads
• HDSL - High Data Rate DSL
  • typically provides 768 kbps in both directions
  • not widely available
• VDSL - Very High Rate DSL
  • capable of between 13 Mbps and 52 Mbps
  • used for live video and audio
  • not widely available
• IDSL - DSL over ISDN lines
  • capable of 144 kbps
  • available in a wider variety of areas than the standard DSLs

xDSL is used to represent DSL in all of its possible implementations.

ADSL is based on the theory and experience that most customers require a much higher rate for downloads than uploads.  Unfortunately, this will not be sufficient if you wish to have a web or FTP server.  Most ADSL implementations make use of FDM - Frequency Division Multiplexing.  Some make use of a different approach called echo cancellation.  Echo cancellation is more efficient, but more expensive.

Leased Lines.  If an organization wants to guarantee high performance and reliability they may end up choosing to lease lines from the telephone company.  Using leased lines it is possible to get a direct connection from one location to another.

DDS.  DDS is a digital service and was one of the first to be made publicly available.  DDS provides a 56 kbps transfer rate.  It has been superceded by T-carrier technologies because of their capabilities for higher capacity at lower cost.

T-Carriers.  T-Carriers are dedicated digital circuits that can be leased to provide high speed data, voice and video over a relatively reliable point-to-point connection.  These are usually implemented over copper wiring.  They can also be implemented over fiber optic, coax and wireless technologies.

A CSU/DSU - Channel Service Unit/Digital Service Unit is used at each end of the connection to encode the data sent over a T-Carrier.

The T in T-Carrier refers to the transmission channel.  The data signal transfer rate is referred to as the DS rate.  Many people use an acronym such as DS-1 and T-1 to refer to the same type of connection.  The following table outlines the most common T-Carrier implementations.

But, even though prices for these options have fallen dramatically, it is much cheaper to purchase DSL service than comparable T-1 service providing about the same bandwidth.  Unfortunately, there aren't the same options for the higher bandwidth T-Carriers.  On the other hand, you can purchase guaranteed bandwidth with the T-Carriers which still can make them more worthwhile.

Switched 56.  Switched 56 is an enhanced version of the PSTN.  It is a digital switched circuit connection which has a 56 kbps transfer rate over one channel.  While inexpensive, its capacity is extremely limited.  It is also a dialup technology.

Its biggest advantage over the PSTN is that it actually provides the 56 kbps that the PSTN doesn't really provide.  Also, because it is digital it usually has lower error rates.

Both ISDN and DSL have proven to be more popular than Switched 56.

Packet Switching Networks

Packet switching networks are networks in which packets of data can take different routes to reach the same destination.  At the receiving end the packets are put back together to reconstruct the original transmission.  These sorts of networks are often depicted using a cloud because the exact routes of travel are unknown.

The three main types of packet switching technologies we will discuss are in the following list.

• X.25
• Frame Relay
• ATM - Asynchronous Transfer Mode

X.25.  X.25 was one of the first packet switching networks.  It was designed to work with IBM mainframes.  It was originally called the ARPAnet 1822 protocol.  The X.25 comes from these specifications established by the CCITT - International Telephone and Telegraph Consultative Committee.  The CCITT changed its name to ITU - International Telecommunications Union in 1993.

X.25 is actually made up of several protocols operating at different layers of the OSI model.

• PSDN - which uses a protocol called X.21 at the physical layer.
• The LAPB - Link Access Procedure Balanced protocol is used at the data link layer.
• The PLP - Packet Layer Protocol is used at the network layer.
  • used to assemble frames from the data link layer into packets

Apparently, the primary objective in designing the X.25 protocol suite was reliability.  At the time it was developed in the 1970s, both computers and telephone lines were more subject to errors.  Thus, the PSDN running in X.25 includes redundant error checking.  Thus reliability is improved, but transfer rates are slwoed by the extra error checking activity.  The PSDN usually transfers at 64 kbps or less.

X.25 networks are still in use today.  To make a WAN connection using this approach you can do one of the following.

• Dial in to a PAD - Packet Assembler/Disassembler with an asynchronous modem
• Make a synchronous connection using the X.32 protocol
• Use an X.25 smartcard to connect directly to the PSDN

Frame Relay.  Frame Relay is an improvement of the older X.25 technology.  It runs at only the two lowest layers of the OSI model, the physical and data link layers.  It makes better use of newer more reliable telephone infrastructure and doesn't implement the extensive error checking of X.25.  Frames with errors are discarded and the destination is required to detect missing packets and request retransmission.

Frame relay switches packets of a shared packet switching network owned by a regional telephone company such as MCI or AT&T.  Frame Relays usually sit on top of T1 or T3 trunks operated by the provider.  Long distance connections are over infrastructure owned by the frame relay provider.  These connections are shared among a number of other customers. 

Most customers rent PVCs - Permanent Virtual Circuits.  These give a customer a continuous dedicated connection without paying for a leased line.  Frame Relay customers are charged according to level of usage.  It is usually used in the following ways

• for high speed, low latency mesh or hub-and-spoke topologies between sites
• for both private and carrier provided networks

ATM.  ATM - Asynchronous Transfer Mode was designed to support high speed applications such as video and/or audio streaming.  ATM is actually specified as a Data Link Layer technology on layer 2 within the OSI Reference Model.  ATM sends 53-byte cells instead of variable length packets.  Using this fixed length results in certain advantages.

• cell based networks run better in point-to-point mode in which the receiving station is ready to actively receive and process the cells
• speed is improved because hardware knows exactly where the header ends and data starts in every cell
• quality of service can be more readily controlled and traffic prioritized

ATM is a switching technology in which virtual circuits are set up before a transmission starts.  These can be either PVCs - Permanent Virtual Circuits or SVCs - Switched Virtual Circuits.  A virtual circuit is built over a path of various hubs, switches and routers.  Each end of the virtual circuit must agree to the path before the transmission can happen.  This differs from Ethernet and Token Ring which transmit without prior notification or routing determination.

ATM is hardware based and the hardware, from NICs to hubs is costly.  Standard transfer rates are

• 25 Mbps
• 155.520 Mbps
• 622.080 Mbps
• capable of 10 Gbps

ATM can be used for both LANs and WANs.  ATM is often associated with certain types of prioritizations to guarantee particular levels of QoS - Quality of Service.  QoS involves controlling the allocation of network bandwidth for specific applications.  ATM is heavily identified with multicasting of transmissions that have latency and/or priority sensitivities.  In general, its fixed cell format and virtual circuits are much better at dealing with these kinds of issues.

Because of these approaches there are also a fair number of issues to resolve when using ATM as a backbone or WAN link because it must interact with other approaches.

ATM is designed to run over fiber optic cable operating the SONET - Synchronous Optical Network specification.  We will discuss OC-SONET in the next section.

OC-SONET.  SONET - Synchronous Optical Network specifications are set up for various cable speeds or OC - optical carrier levels.  The following table displays the standard speeds.

Description

51.84 Mbps fiber optic cable

Broadband ISDN.  BISDN - Broadband ISDN is a technology designed to make use of fiber optic cable and radio waves to transmit data at high speeds over SONET, FDDI and Frame Relay.  It is a broadband approach that can transmit multiple channels of voice, data and/or video over the same medium.

SMDS.  SMDS - Switched Multiple Data Service is a packet switching technology that has been designed especially for WANs that experience bursts of traffic.  It is connectionless, so it doesn't make use of circuits like some other approaches.  It uses relatively large packets of up to 7168 bytes.  Data transfer speeds are usually in the range of 1.544 Mbps up to 45 Mbps.

SMDS makes use of its own addressing scheme which uses 10 digit numbers to identify SMDS subnetworks.  SMDS links are typically connected to SMDS switches on a telephone company's network.  These are usually done on SONET OC-3 links.

SMDS was designed for public networks to provide LAN type services with the reach of a MAN.  Unfortunately, it isn't that widely available and the equipment can be difficult to find.