CertiGuide to Network+  9  Chapter 0111: Wide Area Networking       9  VII  Wide Area Networking Technologies

Frame Relay 1 Packet Switching vs. Circuit-Switching

ATM (Asynchronous Transfer Mode) is an even higher speed technology that, like X.25 and frame relay, is used to connect LAN’s to create a WAN, as well as being used within individual LAN’s. Although the increasing speeds of Ethernet in LAN environments are calling into question ATM’s future in the local office, it is alive and well in WAN environments.

In ATM, data is transferred in cells or packets of relatively small size. The small, predictable size of ATM packets makes for efficient, reliable transmission of newer types of time-sensitive Internet data such as audio and video, while at the same time making sure that transmission of these larger types of data doesn’t hog the network and make it difficult for those just requesting small file transfers or print jobs to get their work done in a timely fashion. This is made possible through ATM’s implementation of QoS (Quality of service) functions, much like those in TCP/IP.

ATM transfer rates range from 25Mbps to 622Mbps. These speeds are higher than most office LAN’s run today, allowing for efficient interconnection of multiple (3, 4 or even more) sites around the country (or world!) into a WAN. ATM can run as a layer on top of SONET, and can also run over other technologies.

The different types of ATM

You can purchase ATM connectivity based on several different throughput/pricing models. The different models are each appropriate for different uses of your network and corporate requirements for consistency of transmission speeds. The options are:

• Constant Bit Rate (CBR), which provides your ATM link with a guaranteed reliable, constant transmission speed at all times, much like a private leased line. If you have critical systems such as electronic manufacturing monitoring systems running over your WAN, this may be the most appropriate choice.
  
  
• Variable Bit Rate (VBR), a step down from CBR, which provides a specified average throughput over time, but which does not send data as evenly as CBR. This option costs less than CBR, because it allows the ATM network capacity to be more efficiently shared among multiple customers by providing the ATM carrier with a small amount of flexibility regarding when individual data packets are sent. This model is often used for audio and video transmission.
  
  
• Unspecified Bit Rate (UBR) that provides no guaranteed throughput levels. This is a cost-effective ATM throughput model for applications, which can tolerate unpredictable throughput rates and packet delays, such as file transfer or email.
  
  
• Available Bit Rate (ABR), which guarantees a minimum level of throughput, but which can optionally burst data through at higher rates when excess network capacity is available. This model is analogous to frame relay’s CIR.
  
  
• Guaranteed Frame Rate (GFR), similar to ABR, which can access additional bandwidth that might become available on the network after the connection has been established, by dynamically adjusting its frame rate.

Frame Relay 1 Packet Switching vs. Circuit-Switching