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WAN Technologies

WAN Technologies

A wide-area network (WAN) is a network that covers a broad geographic area and often uses transmission facilities provided by common carriers. WAN functionality occurs at Layers 1-3 in the Open System Interconnection (OSI) reference model. The bicoastal company in the example would contact its service provider to purchase WAN connectivity between the offices. WAN services are leased from service providers with either a monthly flat fee or a per-usage charge. More bandwidth attracts more costly charges.

WAN Services
The three types of transports used for WANs are the following:

  • Point-to-point - Also known as leased line, a point-to-point connection is a pre-established link from one site, across a carrier’s network, to a remote site. The carrier establishes the point-to-point link for the private use of the customer.
  • Circuit switching - A service provider establishes a dedicated physical circuit into a carrier network for two or more connections. Unlike point-to-point, which has exactly two sites connected to a single connection, multiple sites privately connect into a carrier’s switched network to communicate with each other. Circuit switching operates like a normal telephone call. ISDN is an example of circuit-switched WAN technology.
  • Packet switching - This type of transport is similar to circuit switching in that multiple sites privately connect into a carrier-switched network. However, packet switching involves the statistical multiplexing of packets across shared circuits. Frame Relay is an example of packet switching.

Some WAN technologies, such as Frame Relay and Asynchronous Transfer Mode (ATM), use virtual circuits to ensure reliable communication between two network devices. The two types of virtual circuits are switched virtual circuits (SVCs) and permanent virtual circuits (PVCs). A SVC is dynamically established on demand and torn down when transmission is complete. A connection uses SVCs when data transmission between devices is sporadic. A PVC is a permanently established logical circuit and is useful for connections between two devices in which data transfer is constant.

WAN dialup services are available as alternative backup technologies for traditional WAN services. As the name implies, dialup services use plain old telephone service (POTS) and are inexpensive alternatives when the main WAN service goes down. Cisco routers offer two popular types of dialup services: dial-on-demand routing (DDR) and dial backup. DDR works like this: A router has an interface connected to a modem. If an interesting frame of traffic arrives, the router dials up another router over the phone line. The router transmits the packet and, after a period of inactivity, hangs up the phone line. Dial backup initiates a dial connection to another router after it determines that the primary WAN service is unavailable. The dial connection remains active until the WAN service returns.

High-Speed Serial Interface
High-Speed Serial Interface (HSSI) is a standard for high-speed, point-to-point data communications over serial WAN links. HSSI operates at speeds up to 52 Mbps.

Frame Relay
Frame Relay is a packet-switched WAN service that operates at the physical and logical layers of the OSI reference model. Frame Relay was originally designed to operate over I5DN but today operates over a variety of network interfaces. Typical communication speeds for Frame Relay are between 56 kbps and 2 Mbps (although lower and higher speeds are supported).

Frame Relay provides connection-oriented services using virtual circuits. A Frame Relay virtual circuit is a logical connection between two data terminal equipment (DTE) devices across a Frame Relay packet-switched network. A data-link connection identifier (DLCI) uniquely identifies each virtual circuit. You can multiplex multiple virtual circuits on a single physical circuit.

Frame Relay switched networks provide simple congestion-notification mechanisms. Frame Relay switching equipment can mark a Frame Relay packet with a front-end congestion notification (FECN) or back-end congestion notification (BECN). The equipment marks the packets with a FECN or BECN if congestion occurs during the transmission of the packet. The DTE equipment at the other end of a circuit notices whether a packet has experienced congestion and notifies a higher layer that congestion has occurred.

Additionally, the equipment can mark a packet as Discard Eligible (DE) to indicate that it is less important, which means it can be dropped if congestion occurs.

ATM
ATM is a standard for cell-based relay that carries voice, video, and data in small, fixed-size cells. ATM networks are connection-oriented networks that combine the benefits of circuit switching (guaranteed capacity and constant transmission delay) with those of packet switching (flexibility and efficiency for intermittent traffic). ATM transmits at speeds from a few Mbps to many Gbps.

Traditional circuit-based networks use time-division multiplexing (TDM), in which users are assigned a predetermined time slot; no other device can transmit during this time slot. If a station has a lot of data to send, it can only transmit during its time slot, even if the other time slots are empty. Conversely, if the station has nothing to transmit, the time slot is sent empty and is wasted. This arrangement is called synchronous transmission.

ATM is asynchronous, meaning time slots are available on demand. This allows for a more efficient use of available bandwidth. ATM uses single-sized cells (as opposed to variable-sized frames in Frame Relay), which are 53 bytes in size. A size of 53 bytes is for anything involving computers (which usually define things in powers of 2 or 8). The cell size represents a compromise between the phone-standards folks and the data-standards folks.

ATM networks have two devices: ATM switches and ATM endpoints. ATM switches accept cells from an endpoint or another switch, evaluate the cell header, and quickly forward the cell out another interface toward the destination. An ATM endpoint contains an ATM network interface adapter and is responsible for converting digital data into cells and back again. Examples of ATM endpoints include workstations, LAN switches, routers, and video coder-decoders (codecs).

ATM networks can mark traffic, after it is converted from its original data format, to require different types of handling. Some traffic, such as voice and video, must be transferred through the network in regular intervals with little variation of delay. Otherwise, the destination receives low-quality voice or video transmission. Data traffic is less sensitive to network delays and can be handled differently.

To ensure the appropriate delivery for each of these traffic types, ATM devices employ quality of service (QoS) mechanisms that involve the reservation of bandwidth, the shaping of traffic to meet the reserved bandwidth, and the policing of traffic that exceeds the reservation.

WAN Devices
Numerous types of devices are associated with WAN service delivery.

The first is a WAN switch. Usually located in a carrier’s network, a WAN switch is a multiport internetworking device whose job is moving traffic from the source to destination. Routers at the customer sites attach to the edges of the carrier’s switched network. Examples of switched traffic include Frame Relay and ATM. WAN switches operate at Layer 2, the data link layer, in the OSI model.

A modem is a device that converts between digital and analog signals, enabling data to be transmitted across phone lines. At the source, a modem converts computer data from digital to analog and transmits the signal across a phone line. At the other end, a modem receives the analog signal, converts it back to digital, and forwards the information onto a computer. The word "modem" is a contraction of the terms that describe this conversion: MOdulation and DEModulation.

An access server is a device that acts as a concentration point for dial-in and dial-out services. An example of an access server is dialing into an Internet service provider from home: a computer at home dials through a modem across a phone line. The modem at the service provider answers the call and establishes a network connection across the phone line. Multiple modems are connected to the access server.

A channel service unit/digital service unit (CSU/DSU) is similar to a modem in that it adapts the physical interface on a DTE device, such as a router, to the interface of a data circuit-terminating device (DCE), such as a carrier switch. The CSU/DSU provides signal timing between the DTE and DCE.




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