Editing Voice over IP (section)

===DSL and ATM===
DSL modems typically provide Ethernet connections to local equipment, but inside they may actually be [[Asynchronous Transfer Mode]] (ATM) modems.{{efn|Technologies such as [[802.3ah]] can be used for DSL connectivity without using ATM.}} They use [[ATM Adaptation Layer 5]] (AAL5) to segment each Ethernet packet into a series of 53-byte ATM cells for transmission, reassembling them back into Ethernet frames at the receiving end.

Using a separate [[virtual circuit identifier]] (VCI) for voice over IP has the potential to reduce latency on shared connections. ATM's potential for latency reduction is greatest on slow links because worst-case latency decreases with increasing link speed. A full-size (1500 byte) Ethernet frame takes 94&nbsp;ms to transmit at 128&nbsp;kbit/s but only 8&nbsp;ms at 1.5&nbsp;Mbit/s. If this is the bottleneck link, this latency is probably small enough to ensure good VoIP performance without MTU reductions or multiple ATM VCs. The latest generations of DSL, [[VDSL]] and [[VDSL2]], carry Ethernet without intermediate ATM/AAL5 layers, and they generally support [[IEEE 802.1p]] priority tagging so that VoIP can be queued ahead of less time-critical traffic.<ref name=cisco/>

ATM has substantial header overhead: 5/53 = 9.4%, roughly twice the total header overhead of a 1500 byte Ethernet frame. This "ATM tax" is incurred by every DSL user whether or not they take advantage of multiple virtual circuits&nbsp;–&nbsp;and few can.<ref name=cisco/>