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Digital Subscriber Lines (DSL)
- DSL just defines how to transmit data between a customer site and the local CO. Data do not
flow through DS0 channels inside the PSTN, but through some ISP IP networks. It does not need
to be compatible with the core of PSTN and hence is able to provide higher transmission rates.
The speed does not degrade when more users are added to the network.
PC
Ethernet
DSL
Modem or
Router
Analog
Phone
DSL Low-Pass Filter
(LPF)
House / Office
L2 – ATM & PPP
L1 – DSL
Local Loop
ISP IP
Router
Voice
Switch
Local CO
Figure 21-5: DSL Connection
DSLAM
IP Network
PSTN
- The local loop connects to the DSL Access Multiplexer (DSLAM) at the local CO, which is
responsible for splitting the data and voice signals from the local loop.
- DSL allows concurrent data transmission and voice call, as it doesn’t use the voice frequencies.
Phones generate analog signals at 0–4000Hz, while DSL modems generate analog signals higher
than 4000Hz – the interference is minimal. Filters are often being used to prevent interference.
- DSL is considered an always-on Internet connection service. It does not require a circuit to be
setup before data transmission as with modems and ISDN.
- Its downstream speeds range from 1.5–8Mbps, while upstream speeds range from 64–800kbps.
- The ADSL maximum supported distance to home is 18000 feet (or ~5500 meters).
- Other DSL standards or variants include HDSL (High bit rate DSL), IDSL (ISDN DSL),
MSDSL (Multi-rate Symmetric DSL), PDSL (Powerline DSL), RADSL (Rate-Adaptive DSL),
SDSL (Symmetric DSL), SHDSL (Single-Pair High-Speed DSL), UDSL (Uni-DSL), and VDSL
(Very High Speed DSL). Annex A is referred to as ADSL over PSTN (UK) while Annex B is
referred to as ADSL over ISDN (Germany).
- DSL routers usually include the DSL modem features, and other networking features, eg: IP
routing (allows Internet connection for multiple PCs), DHCP server, NAT, and port forwarding.
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