Several schemes for realizing access network

In this paper, several popular access network technologies are described in detail, and the advantages and disadvantages of different technical schemes are analyzed and compared.
Key words: Access XDSL ADSL VDSL Cable Modem LMDS

With the rapid popularization of Internet and the opening of many new services on the Internet, the demand for data transmission rate is increasing. However, the current access to IP network technology is still based on dial-up modem, far from meeting the needs of users, so a variety of broadband access technology emerged.

1, 56K Modem Technology
In the past few years, analog modem technology has been greatly developed, from 9600BIT/S development 14.4kbit/s,28.8kbit/s,33.6kbit/s to the present 56kbit/s.
However, in fact, the so-called 56k modem, its uplink rate and downlink rate is asymmetric, this is due to the use of the 56k modem in the special technology caused. It compresses 8 bits of PCM voice into 7-bit encoding, so the rate changes to 56kbit/s. The general idea of its connection is shown below.

Figure one, 56k modem connection

In the downward flow direction, the number/mode conversion is not affected by the noise to make the 56k connection rate possible. The connection in this direction is direct, and the 128PCM code word produces a voice signal tone. This voice signal can be connected to the PC 56K Modem recognition. For the upstream flow is another thing, in this case, the mode/number conversion noise (quantization noise) is quite serious, so the uplink speed of 56K Modem is limited to 28.8kbit/s or 33.6kbit/s (where the modulus/number must be connected to the digital backbone/switching network).
The biggest advantage of 56k Modem technology is simplicity, low cost, no special requirements on the line.

2. XDSL Digital Subscriber Line Technology
At present, the user loop still plays an important role by the analog twisted pair technology, the reason is not only its economy, in the provision of telephone business, no one technology can be cheaper than it. With the rapid development of communication and computer technology, the modern communication network is moving toward the broadband, however, the low speed transmission of the analog user access part hinders the development and forms the bottleneck effect. In this case, xDSL technology appears. At present, XDSL mainly refers to three kinds of HDSL, ADSL and VDSL.
2.1, HDSL Technology
DSL (digtal subscriber lines, digital Subscriber line technology) is the product of the late 80, mainly used for ISDN basic rate service, on a twisted pair to obtain Full-duplex transmission, using the technology is time compression multiplexing (TCM) and echo cancellation. However, crosstalk and the interference between symbols increase when the transmission rate increases to T1 (1.544mb/s) or E1 (2.048mb/s). In order to improve the communication quality, based on the DSL technology, a high speed digital subscriber line (HDSL) technology is proposed, and the modulation technology is baseband 2BIQ, qam/cap and DMT (discrete multiple audio), which makes the rate of transmission of digital signal from 2B+D (144kb/s) to T1/e1. HDSL can also use two loop pairs, but only in the carrier service Area (CSA) range.

2.2, ADSL Technology
Since the digital services on digital subscriber lines are mostly asymmetrical, 1989 Bellcore first proposed ADSL (asynchronous Digitalsubscriber line, asymmetric digital wire technology), The highest digital transmission rate on a pair of twisted pair copper wires can be as high as 6mb/s.
At present, ADSL technology is divided into general ADSL and UADSL according to the transmission rate. The uplink channel rate of General ADSL is 6mbit/s, and downlink channel is 512kbit/s; UADSL is the G.lite standard of the ITU, the Shuchong rate is 1.5mbit/s and 256kbit/s respectively. ADSL divides the user spectrum of a twisted pair into three frequency bands: 0 ~4khz frequency band is used for telephone service; ~120khz about 100kHz of band is used to transmit low speed data or control information on or off the line; near 1MHz of High-frequency segment (approx 124 ~1000khz) Bandwidth is used to transmit downlink high-speed data.
Current ADSL products mainly used in the line coding technology for CAP (no carrier amplitude/phase modulation) and DMT (discrete multi-tone), the two coding schemes in the impact of ADSL performance and use of different areas have a certain strength. It is generally believed that DMT has advantages in rate adaptation (based on line condition adjustment rate), change loop condition, processing noise and subcarrier (voice application), while Cap's echo cancellation design is simple, the delay is small (it is said that only 25% of the DMT processing delay), the technology is mature (based on QAM, There have been years of history), to achieve simplicity and so on.
No matter what coding technique is adopted, a simple frequency segmentation or echo cancellation technique with uplink and downlink bandwidth is necessary for bidirectional transmission on a pair of twisted pair. Adaptive echo Canceller is used to weaken the echo interference caused by bidirectional transmission. Since there are several pairs of twisted strands in a cable harness, adaptive equalization is needed to reduce crosstalk between lines. Pulse noise is the main damage factor of ADSL, especially when ADSL subscriber loop loss is serious, the forward Error Correction code (FEC) and interleaving coding should be considered in the design of ADSL system to reduce its influence. In order to deal with the background white noise generated by electromagnetic coupling radio frequency interference (RFI) of electronic circuits, ADSL equipment should have good electromagnetic compatibility. It is necessary to separate ADSL signal and pots signal, and to reduce echo and side sound, which requires high quality isolation filter. Considering the transmission characteristics and noise environment of twisted pair, the modulation techniques such as QAM, CAP or DMT are adopted to improve the frequency band utilization efficiency of twisted pair cable.
ADSL as a broadband access network scheme, its advantages are very obvious, it makes full use of the existing twisted-pair resources, for operators, do not need to spend too much investment in the existing network can provide users with broadband services. However, ADSL technology has not yet reached the point of full maturity, there are many problems need to be further resolved, such as the technology for the diameter of the line is required, only 0.4 ~0.5mm cable can be used, but the current world of the laying of a large number of copper stranded line does not comply with the requirements And the data business will have interference with the dialogue band.