|reply to dsiemon |
Re: [Internet] Per packet overhead on Bell's VDSL? ATM based?
said by dsiemon:We're talking about loop aggregation (between the modem and DSLAM), while I think squircle is talking about link aggregation.
These seem contradictory to me. However, I think maybe we're talking about overhead at different locations so let me clarify that I'm trying to figure out the number of bytes added to an IP packet.
To answer your original post, Bell does indeed still use ATM for their backhaul (link aggregation), but all the newer remotes use Ethernet. (GigE)
The difference between ATM and PTM overhead is that PTM introduces one flag octet (on average), one address octet, one control octet and two CRC octets, which adds up to five octets. In addition to this, HDLC encapsulation changes every occurrence of the octets 0x7D and 0x7E in a two-octet sequence, to avoid interpreting data octets as closing flags. This causes an average increase of 0.78% for random data. Applying HDLC encapsulation to packets within the Ethernet length range (641522 octets) results in an expansion of 1.11%8.59%.
AAL5 over ATM consists of adding a protocol trailer (1 octet of UserUser information, 1 octet CPI, a 2-octet length field, and a 4-octet CRC), and a padding to make sure the total packet length is an integer multiple of 48 octets (the payload size of an ATM cell). The actual ATM overhead consists of 5 octets per cell. Applying this encapsulation to packets within the Ethernet length range results in an expansion of 12.67%63.90% (using an average padding of 23 octets).