so in ATM nomenclature, there are different ATM Adaptation Layers. The process defined for most data is named ATM Adaptation Layer No. 5 or AAL5. Understanding throughput on ATM links requires a knowledge of which ATM adaptation layer has been used for the data being carried. MPLS Multiprotocol Label Switching (MPLS) adds a standard tag or header known as a 'label' to existing packets of data. In certain situations it is possible to use MPLS in a 'stacked' manner, so that labels are added to packets that have already been labelled. Connections between MPLS systems can also be 'native', with no underlying transport protocol, or MPLS labelled packets can be carried inside frame relay or HDLC packets as payloads. Correct throughput calculations need to take such configurations into account. For example, a data packet could have two MPLS labels attached via 'label-stacking', then be placed as payload inside an HDLC frame. This generates more overhead that has to be taken into account that a single MPLS label attached to a packet which is then sent 'natively', with no underlying protocol to a receiving system. Higher-level protocols Few systems transfer files and data by simply copying the contents of the file into the 'Data' field of HDLC or PPP frames — another protocol layer is used to format the data inside the 'Data' field of the HDLC or PPP frame. The most commonly used such protocol is Internet Protocol (IP), defined by RFC 791. This imposes its own overheads. Again, few systems simply copy the contents of files into IP packets, but use yet another protocol that manages the connection between two systems — TCP (Transmission Control Protocol), defined by RFC 1812. This adds its own overhead. Finally, a final protocol layer manages the actual data transfer process. A commonly used protocol for this is the "file transfer protocol Tools available for measuring throughput This section cannot be exhaustive because there are many tools available, some of which are proprietary and specific to vendor applications. Some of these tools can be used for nefarious purposes. The use of these tools can possibly lead to harmful consequences even if the tester has only good intentions. So use of these should be, at a minimum, done with caution and a good understanding of the tools. HTTPing Iperf bandwidth measuring tool Pingb bandwidth estimating tool IxChariot and IxChariot Pro endpoints BandCheck See also Asymptotic