Data Communications Networking Devices - 4th Ed.pdf

200 __________________________________________________________ WIDE AREA NETWORKSIn addition to direct frame relay provider charges, you will also incur a localaccess charge that may be billed by the provider or a local communications carrier.The local access charge is a monthly charge for the circuit that connects yourlocation to the nearest Point of Presence POP) where the circuit becomesconnected to the frame relay provider's network access port.In spite of a considerable number of metrics upon which frame relay charges arebased, its use may result in considerable savings in comparison to the costassociated with a private leased line based network. This potential cost savingresults from the fact that similar to an X.25 network, a public frame relay networkenables network resources to be shared among many users. This in turn providesgreater transmission ef®ciencies and allows providers to set rates that can be 20±50% less than the cost associated with a private leased line network. Of course, ifyour organization has a substantial communications requirement between twolocations and does not require the use of a mesh structure network topology, it isquite possible that a leased line will prove to be more economical. The only way totell is to determine your transmission requirements and obtain price quotations fordifferent transmission services.Voice over frame relayAlthough not originally designed as a digitized voice transport mechanism,advances in voice digitization technology and the incorporation of prioritizationtechniques into vendor products resulted in many organizations turning to framerelay as a transmission scheme to support their voice, data, and fax requirements.To do so usually requires a separate device capable of presenting a voice or faxdigitized data stream to a FRAD since most FRADs are limited to supporting dataprotocols. This conversion device commonly attaches to a PBX to accept an analogvoice or fax or a digitized voice signal and compresses and digitizes the signal,generating a data ¯ow in a protocol the FRAD supports. Concerning voicedigitization, PCM produces a 64 kbps digital data stream which is more susceptibleto delays through a frame relay network when carried in a large number of timedependentframes than a lower digitization rate carried in a lesser number of frames.Thus, many equipment developers now offer compression methods that can resultin a voice conversation being carried by an 8 kbps to 16 kbps data stream.In addition to stand-alone converters that provide an interface to FRADS, somevendors now market multifunctional FRADS that perform analog to digitalconversion and compression. Included in many products is a silence detectionprocess which further enhances voice compression and a frame sizing algorithmwhich limits the length of digitized voice, fax, and data frames delivered to thenetwork. This frame limiting algorithm minimizes end-to-end delay through theframe relay network being used and ensures that voice packets which are timesensitive are not unacceptable delayed behind fax and data packets that can toleratea relatively lengthy delay.Figure 2.19 illustrates the use of a multifunctional FRAD to fragment andprioritize the ¯ow of voice, data, and fax packets into a frame relay network. After aframe is created based upon a maximum length associated with the data it