ATM Traffic Contracts
When an ATM circuit is set up each ATM switch is informed of the traffic class of the connection. These ATM contracts constitute part of ATM's Quality of Service (QoS) mechanisms. There are four basic types of contracts: constant bit rate, variable bit rate, available bit rate and unspecified bit rate.
ATM Traffic Shaping and Policing
Now we get to a couple of the basic underpinning capabilities of ATM that ISPs can use at their “godly” discretion to put the brakes on your Internet Surfing and downloading pleasures.
Firstly however; I must point out that when ATM was being developed the developers may have been viewing the world through rose-colored glasses as their primary reasons for incorporating these particular aspects in the ATM protocol were all about network service, performance and quality of service. High ideals indeed but none-the-less crucial for real-time voice and video communications along with many of today's streaming media applications.
Unfortunately, as is so often the case, many a good intention/idea has been later subverted. Let us have a quick look at them here:
Traffic Shaping - The intended objective of traffic shaping is to ensure that cell flow will meet its traffic contract and is usually done at the entry point to an ATM network.
Traffic Policing - To maintain network performance it is possible to “police” virtual circuits against their traffic contracts. Basic policing works on a cell by cell basis, but this is sub-optimal for encapsulated packet traffic.
Remember traffic contracts are negotiated between customer and service provider on a service-by-service basis. The customer requires a certain amount of bandwidth and pays the service provider accordingly. The service provider for their part agrees to make available and deliver that agreed minimum bandwidth.
For example if a customer required a certain minimum data bandwidth the service provider could ensure that this was met by electing to implement a set specific constant bit rate traffic contract. The ATM switches at either end are configured appropriately and the automatic traffic policing should ensure that this is the way things are.
Service level delivery guarantees are thus monitored automatically and any undesired anomalies or variations can be dealt with by the ATM switches logic. No prizes for guessing that this is not a labor-intensive activity and hence is a favorite with ISPs.
If a circuit is exceeding its traffic contract, the network can either drop the cells or mark the Cell Loss Priority (CLP) bit (to identify a cell as being discardable farther down the line). In instances where that part of the ISP's network carrying the traffic contract bandwidth exceeding traffic is not very busy the ISP can elect not to drop those cells with the CLP bit marked on. Similarly if network traffic is high the ISP can elect to drop said cells.
The problem here is that basic policing works on a cell-by-cell basis. When transferring most data packets ATM will as discussed above chop the frames into 48 byte size chunks, add its 5 byte header and then multiplex them all as a stream. This means that large frames will be carried as a very large number of uniformly-sized 53 byte cells.
Failure of any one of these cells to arrive at the destination will be interpreted as a corrupt frame upon analysis or reassembly at the recipient end and hence the entire frame will be automatically dropped. Simple CRC checking mechanism at the Data Link layer will pick up these errors and automatically drop the frame, even before the entire frame has been received by the Layer 2 device (switches for example).
Thus discarding a single cell will invalidate the whole packet of which the single cell is but one component. The implications are enormous. Traffic policing of the bandwidth constricting type can be effectively and efficiently enforced with a minimum of processing and filtering overheads on the part of those performing the traffic policing (ISP). It also means that cells can be marked as droppable by many different organizations at various points as it travels the Internet.
If you are into conspiracy theories then there is considerable food for thought here.
Another point worthy of consideration here is that ATM uses ATM switches and once the virtual circuits and traffic contracts have been established and configured by the service provider it is pretty much auto-pilot from there on. This means that network traffic policing; from the ISP's point of view, occurs automatically as part of the normal functional processes of the hardware (the ATM switch). This is most definitely a cost-effective minimal labor required solution.
Your problems start when errors occur in the administrative functionalities particularly between ISP peers. For example, you may decide to upgrade your service traffic contract or are experiencing data transfer corruption problems. Some of these data transfer issues can be the result of a Denial-of-Service attack (DoS) that is not necessarily directed against you specifically. You are just being caught in the backwash.
Unfortunately, another scenario that may adversely be affecting you is if an ATM switch on another network is marking some of the cells comprising your packets as droppable by turning on the CLP bit as they pass through that carrier's network.
This could arise from one of their customers exceeding their specific traffic contracts and since you are trying to upload or download from that customer traffic intended for you that was initiated by you is adversely affected. Keep in mind that it only takes one cell to be dropped to invalidate the entire frame.
Final Thoughts
One final word is that if you are in the situation of implementing a VoIP solution using a broadband modem router with inbuilt LAN switch via an ADSL2+ service that offers both PPPoE and PPPoA you may be best advised to implement a traffic segmentation solution by using PPPoE for standard data traffic and PPPoA for voice.
My home D-Link DSL-504 ADSL modem/router has been doing precisely this without any troubles for neigh on six months now. I guess just sometimes the world was meant to be easy. I will present another article shortly showing precisely how this is done. Screen shots will be included as I have just decided that I will make it a walk-through style presentation.
