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In other words, a not real-time application can receive the audio and video signal in some known sequence, then store the information for any service provider and/or user for a specified period of time before playback. With this approach, the order of the cells as they arrive at their destination is unimportant since they are assembled/stored in their original sequence by the equipment at the destination.

So, a video services provider can ship movies over the ATM network to homeowners through copper wire to the ADSL set-top boxes. If there is some storage device either at the customer premises or at some intermediate point (perhaps the CO), the audio and/or video signal can be saved for playback at the convenience of the customer.


Figure 7-4.  ATM switches

Finally, if the ADSL set-top boxes include buffers with sufficient size to hold 50 milliseconds (approximately 425 Kbits) worth of ATM cells, then all the issues with real-time video signals and transmission time essentially become non-problems.

ATM Switches

There are two distinct types of ATM switches. For the network backbones, there are the fast core switches with speeds in the 20 Gbps to 50 Gbps range. The highest speed core switch on the market in 1998/1999 has a maximum port speed of OC-48. Core switches are physically located on service providers’ premises and form the service provider’s network node to network node interface (NNI). Typical prices of core switches range from $100,000 to $250,000.

ATM switches located on customer premises are called edge or access switches and form the User Network Interface (UNI) (see Figure 7-4). These switches are typically utilized to connect LAN networks to the ATM backbone. These switches also have the ability to concentrate various network traffic from multiple sources at the customer premises and convert the non-native data streams into a combined ATM bit stream for egress and disassemble an ATM bit stream into separate native bit streams on ingress. Typical edge switch speeds range from 12 Gbps to 50 Gbps. The maximum port speed for 1998/1999 is OC-12. The number of ports available for a typical ATM switch is detailed in Table 7-2. The table shows the maximum number of ports a 12.8 Gbps switch can have for several port speeds.

Typical ATM Switch Capacity (Switch speed 12.8 Gb/s)
Port Type Maximum Ports per Switch
DS-3 Ports 282
OC-3 Ports 94
OC-12 Ports 15
DS-1/NxDS-1 Ports 752+

Table 7-2. Typical ATM switch capacity

ATM switch prices in Table 7-3 are general. The user must include 6 percent for maintenance if a maintenance contract is purchased and about $1K per person per day (average 5-day training period) for training if the equipment is self-maintained.

Switch Type Application Speed Price
Service Provider High capacity backbone transport for public carriers OC-3 + $100K-$2M
Enterprise WAN High capacity backbone transport for private ATM networks DS-3 + $50K-$250K
WAN Access Multiplexing multiple technology bit streams onto public service provider’s network NxDS-1 + $10K-$100K
LAN Access Concentrate multiple technology bit stream for workgroups NxDS1 + $10K-$135K
ISP Switch ISP traffic onto public ATM network DS-3 + $55K-$500K

Table 7-3. ATM switch price ranges

Inverse Multiplexing Over ATM (IMA)

DS-1 access is about one-sixth the cost of DS-3. DS-1 is too slow for many applications and DS-3 is much too fast. If the service provider only offers those two choices and the application requires something in between, well, we all have to make tough decisions from time to time. Do you install the more expensive DS-3 access and buy more bandwidth than necessary and waste the resources (how about becoming a reseller?), or don’t buy enough and cope somehow with the ensuing traffic problems (oversubscription and 24 hour per day transmission)? Or do you sign up with a service provider who offers IMA? IMA is the ability to multiplex traffic in multiples of DS-1, such as 2xDS-1 or 2.088 Mbps. Typically, IMA is offered by service providers in multiples of DS-1 up to 8xDS-1. With a DS-3 access line six times more expensive than a DS-1, the user can multiplex up to five DS-1s and still be ahead.

There are some modest price differences between ATM access equipment that have IMA capability and access equipment lacking the IMA functionality. A financial analysis will demonstrate the most cost effective solution. DS-3 prices are expected to drop over the next two years. Determine the price differentials between DS-1 and DS-3 access in the geographical area of interest before choosing a solution.

Network Node Interface (NNI)

The NNI issues relative to the user are networking monitoring, management, and link reroute. Network monitoring and management consists of someone or something plugged into the network and making some kind of intelligent decisions based upon what is happening. Intelligent network circuits deal with the mundane moment-to-moment, that is, the predictable, situations. Congestion control comes to mind when network management is mentioned.


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