The quoted "speed" is the sync rate between your modem and the DSLAM or RT port. This reflects the raw bitrate upon which everything else flows. Your data is encapsulated into Ethernet frames which are then encapsulated into TCP/IP packets which are then encapsulated into ATM cells which then are transmitted down the wire.
said by salahx  at »SBC DSL bait and swtich is illegal...1500=750? :
Actually the majority of the overhead isn't because of TCP/IP, its because of ATM (53 octet cells - 48 octet payload, 5 octet header). The PPPoE overhead is 8 octets per MTU (normally 1500 - this size of 1 Ethernet frame) The IP overhead is only 20 octets per MTU - PPPoE (normally 1492) and the TCP overhead is only 24 octets per MSS (normally 1452)
So thus:
PPPoE overhead is 0.53 %
IP overhead is 1.30 %
TCP overhead is 1.65 %
ATM overhead, on the other hand is, 9.4 %
For total DSL delivery overhead of about 12.9 %, or 87.1% efficient.
Then there is the Ethernet overhead:
Ethernet overhead bytes:
12 gap + 8 preamble + 14 header + 4 trailer = 38 bytes/packet w/o 802.1q
12 gap + 8 preamble + 18 header + 4 trailer = 42 bytes/packet with 802.1q
Ethernet Payload data rates are thus:
1500/(38+1500) = 97.5293 % w/o 802.1q tags
1500/(42+1500) = 97.2763 % with 802.1q tags
Best case we're out another 2.5 percent just for using Ethernet...or 97.5% of 87.1%, for a final "efficiency" of 84.9%.
So therefore on a "perfect" 1536/384 line the max payload "speed" would be a tad over 1,300/326 kbps. A 3008/512 (sync) line delivers about 2554/435 (payload) and a 6016/768 (sync) line 5108/652 (payload).
Furthermore, ATM has services on top of it like IP does with TCP and UDP, introducing more overhead, for example, bridging Ethernet over ATM. Also, many of the data protocols within IP/TCP/UDP have even MORE overhead, depending on the protocol.
Every DSL provider does it the same way, the only way for a real improvement is to drop ATM, which isn't going to happen, as it is far too useful to drop, even with that kind of overhead
Minor edits for clarity & additional info by RadioDoc
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last modified: 2009-06-08 20:05:16 |
