How Internet Service Providers (ISPs) implement and conduct such activities as network traffic policing, shaping and quotas along with bandwidth capacity, throttling and availability in conjunction with download capping and Quality of Service (QoS) issues are all covered.
ATM Distinguished Service Record
To do this I will present a short summary style review of ATM as a protocol and then discuss the implications of this technology as it applies to the end-user (consumer).
The reason I have elected to use ATM to demonstrate and illustrate these contentious issues is that most consumer ADSL2+ implementations offer PPPoE or PPPoA as their transport protocols (at least here in Australia). PPPoA stands for Point-to-Point Protocol over Asynchronous Transfer Mode while PPPoE means Point-to-Point Protocol over Ethernet.
ATM has a long and distinguished service record for voice communications, is ideally suited to multiplexing environments and can be readily configured to carry VoIP traffic streams. Now for a little background about ATM and all will become clear.
Introducing Asynchronous Transfer Mode (ATM)
Originally intended to be a unified networking strategy Asynchronous Transfer Mode (ATM) is a connection-oriented, circuit-switched, cell relay “Jack-of-all-trades” transport protocol that uses small uniform fixed-sized cells to redress Quality of Service (QoS) issues so important to voice/video communications and the multitude of streaming applications upon which we are all so dependant.
ATM Origins and Development
During development of the standards for the Asynchronous Transfer Mode (ATM), in the mid 1980s, the goals were to create a unified networking strategy that could act as an all-round transport system for real-time video and audio as well as for image, text and email. ATM is pretty much a “Jack-of-all-trades” transport system.
The two groups primarily responsible for the development of the ATM standards were the International Telecommunications Union [ITU 2004] and the ATM Forum [ATM 2004].
Over time we have seen that the majority of implementations and uses that ATM has fulfilled have been primary concerned with telephony and IP networks. Ethernet and the Internet Protocol (IP) are packet-switched network technologies that use packets of variable size referred to as frames.
Asynchronous Transfer Mode (ATM) Protocol Basics
In marked contrast to packet-switched networking technologies; ATM is a connection-oriented, Data Link Layer (OSI Reference Model Layer 2), circuit-switched, cell relay protocol that runs over Synchronous Optical Network (SONET) Physical Layer links (OSI Reference Model Layer 1) using cells of identical and never varying size. Consistent predictability is the underlying ethos here.
Being a connection-oriented channel-based technology means that ATM must always establish a “logical” connection between the two endpoints prior to commencement of data exchange. Significantly, ATM encodes data traffic into small uniform fixed-sized cells. ATM cells are always 53 bytes in size and are comprised of 48 bytes of data and 5 bytes of header information.
ATM Cell Structure
Regardless of the original size of the packets to be transmitted ATM breaks all packets, data, and voice streams into 48-byte chunks and then adds a 5-byte routing header to each one thereby making a total of 53-bytes for each and every cell. The 5-byte header is essential for later reassembly. During development of ATM it was considered that 10% (5 bytes) of each cell (payload) being dedicated to the header for routing information was more than sufficient.
ATM multiplexes these 53-byte cells instead of the larger packets and in so doing reduces the worst-case queuing jitter by a factor of almost 30, thereby removing the need for echo cancellers.
ATM defines two different cell formats: the Network-Network Interface (NNI) and the User-Network Interface (UNI). Most ATM links use the UNI cell format.
ATM Adaption Layers (AAL)
ATM Adaptation Layers (AAL) are the rules for segmenting and reassembling packets and streams into cells. It is the AALs that provide the support for the various services delivered by ATM. AALs may provide constant bit rate services, circuit emulation, variable bit rate services or general data transport services.
Currently, there are five different AALs and the information concerning which one is being used for each cell on a cell-by-cell basis is not contained within the cell or in the cell header. Rather, this information is negotiated by or configured at the endpoints on a per-virtual-connection basis.
ATM Connectivity
Because ATM is a connection-oriented channel-based technology it must establish a “logical” connection between the two endpoints prior to commencement of data exchange. ATM does this by implementing Virtual Circuits, Channels, Paths and Identifiers as follows:
- Virtual Circuits (VC) - Virtual Circuits (VC) are admirably suited to multiplexing scenarios. Simply by including an 8-bit or 12-bit Virtual Path Identifier (VPI) and a 16-bit Virtual Channel Identifier (VCI) pair in every ATM frame's header each Virtual Circuit (VC) is uniquely identifiable.
- Virtual Channel - An ATM Virtual Channel represents the basic means of communication between two end-points. Cells are given a unique identifier called the Virtual Channel Identifier (VCI) which is placed into the ATM cells' header. All ATM cells containing identical VCIs are transported in the same Virtual Channel.
- Virtual Path (VP) - A Virtual Path (VP) denotes the transport of ATM cells belonging to virtual channels which share a common identifier called a Virtual Path Identifier (VPI). The VPI is included in the header of every ATM frame. In other words a Virtual Path (VP) is a bunch of Virtual Channels (VC) connecting the same end-points. These will also have a common traffic allocation.
- Virtual Path Identifier (VPI) - The Virtual Path Identifier's (VPI) length varies depending on the interface it is sent on (inside the network or on the edge of the network).
