EXFO CableSHARK P3, Руководство пользователя, Страница: 16 / 321

VF / DSL Cable Qualifier
4
ISDN uses a 2B1Q line coding (2 Binary, 1 Quaternary) to represent information. 2B1Q maps two bits of
data into one quaternary symbol. (for example, onto four voltage levels at a regular clocking rate).
High bit-rate Digital Subscriber Line
- HDSL is a symmetric DSL similar to T1 or E1 in that it delivers a bit
rate of 1.544 or 2.048 Mbps of bandwidth. Most systems use two copper twisted pairs, although some early
2.048 Mbps systems required three copper twisted pairs. HDSL is comprised of an HTU-C (HDSL
Transceiver Unit – CO) in the Central Office and a HTU-R (HDSL Transceiver Unit – Remote). Because
HDSL's speeds match those of T1 or E1 pipes, local telcos have been using the technology to provision
local access to T1 / E1 services whenever possible. It should also be noted that traditional T1 transmission
uses 2 copper pairs as well, so the transition from traditional T1 to HDSL T1 is fairly easy. HDSL's operating
range is more limited than that of ADSL since that after 15,000 feet signal repeaters are needed to extend
the service. Because HDSL requires two twisted pairs it is deployed primarily for businesses that require
PBX network connections, Virtual Private Networks (VPNs), Frame Relay Circuits, Internet Access, and
private data networks. HDSL is also preferred over traditional T1 because it is more spectrally compatible
with other technologies in the loop than AMI or B8ZS coded T1 or E1.
Figure 2B – HDSL Architecture
HDSL uses 2B1Q or CAP based line coding.
HDSL2 / SDSL– Similar to HDSL in operation but is capable of achieving HDSL rates over a single pair of
wires. This tends to limit transmission distances but it is still a successful technology. Most HDSL-2 systems
are based on non-standard, proprietary protocols. Little or no interoperability exists between vendors.
Asymmetric Digital Subscriber Line
- ADSL provides delivery of high bit rate digital technology for
consumer based Internet access. ADSL delivers more bandwidth downstream (from the CO to the Customer
Premise) than upstream (Customer Premise to the CO). As most users view far more information than they
create ADSL is optimal. Downstream rates range from 256 kbits/s to 9 Mbps, while upstream bandwidth
ranges from 16 kbps to 640 kbps. ADSL transmissions may work at distances of more than 18,000 feet over
a single copper twisted pair, although it should be noted that only the lowest bit rates are available at these
lengths.