Eleven Broadband Access Technologies
As bandwidth requirements increase, optical fiber is continuously extended toward users. FTTB (Fiber to the Building), FTTC (Fiber to the Curb), FTTCab (Fiber to the Cabinet) and FTTZ (Fiber to the Zone) are being implemented, but achieving FTTH (Fiber to the Home) remains relatively difficult at present. The main wired broadband access technologies currently available include: conventional Modem, N-ISDN (Narrowband Integrated Services Digital Network), Cable Modem and HFC (Hybrid Fiber Coax), HDSL (High-bit-rate Digital Subscriber Line) and SDSL (Symmetric Digital Subscriber Line), ADSL (Asymmetric Digital Subscriber Line) and G.lite (splitterless ADSL), VDSL (Very-high-bit-rate Digital Subscriber Line), HomePNA (Home Phoneline Networking Alliance), Ethernet, SDH (Synchronous Digital Hierarchy), PON (Passive Optical Network) and APON (ATM Passive Optical Network), IM-DSL (Inverse Multiplexing DSL), etc. Generally speaking, any broadband access technology has corresponding CO (Central Office) equipment and RT (Remote Terminal) equipment, but the latter is more diverse.
Conventional Modem
The conventional Modem is one of the main methods currently used to achieve narrowband Internet access. The technology is mature, with a maximum transmission rate of 56 kbps. Technically, it does not rely on the optical access network; in terms of products, it includes the Modem used by the subscriber and the Modem pool located in the telecom central office. Due to its relatively low rate, it is being gradually replaced by N-ISDN and other technologies.
N-ISDN
N-ISDN, also known as "One-line Access", is also a mature copper-wire narrowband access technology that relies on the optical access network. It currently mainly uses 2B+D to provide telephone and Internet access, with a typical download speed of up to 64 kbps, which can basically meet the needs of current narrowband browsing. It is an economical and effective choice for Internet users to increase their online speed. It has now been deployed in various cities across the country, with good user feedback, and is gradually replacing the conventional Modem. ISDN equipment includes switches and terminal equipment, among which there are many types of terminal equipment. Functionally speaking, they are mainly flexible combinations of ISDN network terminals, terminal adapters, routers and videophones, while providing different interfaces (such as ISA, PCI, RS232, USB, analog telephone ports, Ethernet ports, etc.) to suit different requirements.
Cable Modem
Cable Modem is a method of using the cable TV network to provide broadband data access for users, and is also one of the key technologies in the Hybrid Fiber Coax network. HFC was the earliest broadband access technology to mature and enter the market, featuring broadband capability and relative cost-effectiveness. Within an optical node coverage area of about 500 households, HFC can provide 60 channels of analog broadcast TV, at least 2 telephone lines per household, and data services at rates of at least up to 10 Mbps (mature 40 Mbps Cable Modems are already available). In the future, the 550 MHz to 750 MHz spectrum can also provide at least 200 channels of MPEG-2 video-on-demand services as well as other two-way telecommunications services. In the long term, the HFC network is planned to provide the so-called FSN (Full Service Network), that is, providing various types of analog and digital services over a single network, and gradually transitioning from multiple users sharing the above bandwidth to a single user having dedicated bandwidth.
SDH
SDH adapted for the access network uses ADM (Add/Drop Multiplexer) equipment installed at the user premises, connected to the STM-N service node via STM-1 channels. This connection can be either point-to-point or through a ring structure. For situations where the bandwidth requirement is far less than 34 Mbps, using lower-rate multiplexers or shared ADM is more economical and effective. For most ordinary enterprise and institutional users, terminal multiplexers located at the roadside (DP point) can provide a large number of users with bandwidth based on 2 Mbps as the basic unit. Users requiring bandwidth less than 2 Mbps can be served by service multiplexers or by a subsequent PON. Using STM-0 sub-rate connections (Sub STM-0) is an economical and effective solution for small-bandwidth users while maintaining all SDH management capabilities and functions. ITU-T G.708 specifies such an interface.
PON and APON
The Passive Optical Network (PON) includes narrowband PON and the ATM-based broadband passive optical network — APON. The former is used to provide data transmission channels at rates of 2 Mbps and below, while the latter can provide downlink transmission channels of up to 622 Mbps. APON mostly adopts a passive double-star or tree structure and uses a special point-to-multipoint multiple access protocol, enabling numerous ONU/ONT (Optical Network Termination) units to share the OLT, and numerous users to share the ONU, thereby reducing initial construction costs. At present, formal APON products exist, but the variety is limited, and the integration level of components still needs further improvement.
HDSL and SDSL
HDSL transmits information symmetrically at high speed over an unrepeatered subscriber loop network using loaded telephone lines, with a typical rate of 2 Mbps and a distance of 3 to 5 km, using two or three pairs of twisted copper wires. It does not require line pair selection, has a low bit error rate, uses line coding, and has good spectrum compatibility. HDSL technology has now become relatively mature and is mainly used to replace traditional T1/E1, solving broadband access for dispersed users, providing leased lines for users, and transmitting multiple channels of voice, video and data. SDSL is a simplified version of HDSL, using a single twisted pair, and can provide two-way high-speed variable bit rate connections with rates ranging from 160 kbps to 2.084 Mbps. On 0.4 mm twisted pair, the maximum transmission distance is 3 km. HDSL/SDSL can be combined with FTTB/FTTC. Functionally speaking, there are not many types of HDSL equipment, and compatibility between different manufacturers' equipment is poor; SDSL matured somewhat later, and its product types are also not very abundant.
ADSL and G.lite
ADSL transmits information asymmetrically at high speed over an unrepeatered subscriber loop network using loaded telephone lines. Compared with HDSL/SDSL, it avoids the problem of subscriber-side interference, increases the transmission rate, and extends the transmission distance. ADSL uses DMT (Discrete Multitone) line coding. The downlink communication can support rates of 1.5 Mbps to 8 Mbps or higher, and the uplink communication rate is 16 kbps to 640 kbps or higher. The analog subscriber voice channel is independent. Currently, a 6 Mbps signal can be transmitted over a distance of 3.6 km on 0.5 mm twisted pair. G.lite is a simplified version of ADSL, designed to reduce cost and facilitate the installation of subscriber-side equipment. Its downlink rate is up to 1.5 Mbps and uplink up to 512 kbps. It can operate without a telephone splitter, and the maximum transmission distance can reach 5 km.
The CO-side equipment of ADSL (including G.lite), the DSLAM (Digital Subscriber Line Access Multiplexer), mainly performs multiplexing/demultiplexing functions and can be placed at the local exchange or in a residential zone. Placing it in a residential zone aims to increase the transmission rate and enable more general users to use ADSL, which requires coordination with the optical access network. There are many types of subscriber-side equipment. Functionally speaking, they mainly include: ADSL Modems with different interfaces (PCI, USB, Ethernet), ADSL routers for different requirements, integrated gateways providing both data and voice, and splitters or low-pass filters.
VDSL
During the development of ADSL, it was found that appropriately reducing the distance greatly increases the transmission rate, which led to the emergence of VDSL. In a VDSL system, the upstream and downstream channel spectra are separated using frequency division multiplexing. The coding schemes include CAP (Carrierless Amplitude/Phase modulation), DMT and DWMT (Discrete Wavelet Multitone). The upstream and downstream rates of VDSL are also asymmetric. The downstream rates have three levels: 13 Mbps, 26 Mbps and 52 Mbps, with corresponding transmission distances of 1500 m, 1000 m and 300 m; the upstream rates generally also have three levels: 1.6 Mbps, 2.3 Mbps and 19.2 Mbps. VDSL must be used in combination with FTTB, FTTC, FTTCab and FTTZ. In terms of products, VDSL is similar to ADSL, but since VDSL technology emerged relatively late, there are not many formal products available.
HomePNA
HomePNA is a technology that uses telephone lines to set up a local area network, solving the problem of connecting multiple devices in a home. It cannot yet be regarded as an independent broadband access technology. In terms of spectrum, the HomePNA physical layer signal is distributed between 5.5 MHz and 9.5 MHz, with a center frequency of 7.5 MHz and a data transmission rate of 1 Mbps. At the media access control layer, HomePNA uses the existing Ethernet protocol. In terms of connection topology, HomePNA technology allows all nodes in the network to be connected in a daisy-chain manner without the need for central concentration or switching. This connection method helps simplify installation and can also cleverly adapt to the random topology of home telephone wiring.
In terms of products, HomePNA terminal equipment is evolving from standalone network interface cards and embedded network cards on PC motherboards toward integration into 10/100M Fast Ethernet cards. To solve the Internet access problem, HomePNA is combined with xDSL and conventional Modems to form intelligent residential gateways.
Ethernet
Because 10M/100M Ethernet is now widespread, 1000M Ethernet technology is mature and low-cost, and users currently mainly need IP services without urgent QoS requirements, the all-Ethernet access solution has attracted wide attention. Its basic concept is: establish a 1000M Ethernet backbone network, deliver 1000M Ethernet to buildings, curbs and residential zones, then extend 100M Ethernet to building floors, small buildings and residential buildings, and finally 10M Ethernet to offices and desktops. Currently, users use Ethernet network interface cards and Category 5 cabling to connect to in-building equipment; the in-building equipment is Layer 2 switches (using VLAN technology), managed out-of-band through conventional Modems, and mostly powered remotely; the residential zone equipment consists of higher-throughput Layer 2 switches (using VLAN technology), which manage and power the in-building switches through hybrid fiber and Modems; multiple residential zones share a Gigabit router and NAT (Network Address Translation) through optical fiber; the backbone network connects multiple routers to form a broadband IP metropolitan area network. Inter-user access should pass through routers, and user management is also completed at the node where the router is located.
IM-DSL
The basic concept of IM-DSL is to establish multiple xDSL links and, through inverse multiplexing technology, form a single high-speed physical link, then use ATM statistical multiplexing technology to enable many users to share this physical channel. Obviously, the transmission distance of IM-DSL is limited to within 2 km, in order to resolve the contradiction between the limited transmission distance of current broadband DSL technologies and the considerable difficulty of relying on broadband access technologies to widely implement FTTB, FTTC and FTTZ. IM-DSL technology makes full use of existing telephone lines, requires minimal engineering, has good capacity scalability and low investment, but currently lacks industry standards and also faces pressure from HFC and the further extension of optical fiber toward users. At present, mature formal products of this technology are still difficult to find.