Adsl 2 modem. How to choose an ADSL modem
These days, almost everyone needs access to the Internet. Be it work, entertainment, communication - the global network has entered our lives everywhere. To provide Internet access at home or in the office, you need a modem that will allow you to connect all the necessary devices to the network. In large cities, providers offer fiber optic and fiber coaxial systems that allow you to get a fast and stable connection. However, to install such cables, it is necessary that the number of users allows filling the entire bandwidth of the cable - otherwise it is simply not profitable. Therefore, the possibility of such a connection is not provided by businesses everywhere. This is especially true for small cities, towns and villages. What to do if such services are not provided, but you still need the Internet?
There are different options, and one of the best is to use twisted pair telephone wires. Many will remember with horror a phone not working while using the Internet. However, technology has long gone far ahead. Today, xDSL technologies are the most common and effective. DSL stands for digital subscriber line. This technology allows you to achieve fairly high data transfer speeds over copper pairs of telephone wires, without occupying the phone. The fact is that voice transmission uses a frequency range from 0 to 4 kHz, while copper telephone cable can transmit signals with a frequency of up to 2.2 MHz, and it is the section from 20 kHz to 2.2 MHz that xDSL technology uses . The speed and stability of such a connection is affected by the length of the cable, that is, the farther the telephone node (or another modem in the case of creating a network) is located from your modem, the lower the data transfer speed will be. The stability of the network is due to the fact that the data flow goes from the user directly to the node, its speed is not affected by other users. Important factor: to provide an xDSL connection there is no need to replace cables, which theoretically does possible connection Internet wherever there is a telephone (depending on the availability of such a service from the provider).
An xDSL modem will be the link between your phone cable and your devices (or router), but when choosing a specific model, you need to consider a number of characteristics that are right for you.
What are the differences between xDSL modems?
xDSL technologies
In the acronym xDSL, the “x” represents the first letter of DSL technology. xDSL technologies differ in signal transmission distance, data transmission speed, and also in the difference in transmission speeds of incoming and outgoing traffic.
ADSL technology translates to asymmetric digital subscriber line. This means that the transfer speed of incoming and outgoing data is different. In this case, the data reception speed is 8 Mbit/s, and the transmission speed is 1.5 Mbit/s. In this case, the maximum distance from the telephone exchange (or another modem in the case of creating a network) is 6 km. But the maximum speed is possible only at a minimum distance from the node: the further away, the lower it is.
ADSL2 technology makes much better use of wire bandwidth. Its main difference is the ability to distribute information over several channels. That is, it uses, for example, an empty outgoing channel when the incoming channel is overloaded, and vice versa. Thanks to this, its data reception speed is 12 Mbit/s. The transmission speed remains the same as in ADSL. In this case, the maximum distance from a telephone exchange (or other modem) is already 7 km.
ADSL2+ technology doubles the speed of the incoming data stream by increasing the usable frequency range to 2.2 MHz. Thus, the data reception speed is already 24 Mbit/s, and the transmission speed is 2 Mbit/s. But such a speed is only possible at a distance of less than 3 km from the node - then it becomes similar to ADSL2 technology. The advantage of ADSL2+ equipment is that it is compatible with previous ADSL standards.
SHDSL technology is a standard for high-speed symmetric data transmission. This means that the reception and upload speeds are the same - 2.3 Mbit/s. Moreover, this technology can work with two copper pairs - then the speed doubles. The maximum distance from the telephone exchange (or other modem) is 7.5 km.
VDSL technology has the maximum data transfer speed, but is significantly limited by the distance from the node. It works in both asymmetric and symmetric modes. In the first option, the data reception speed reaches 52 Mbit/s, and the transmission speed – 2.3 Mbit/s. In symmetric mode, speeds up to 26 Mbps are supported. However, high speeds are available up to 1.3 km from the node.
When choosing an xDSL modem, you need to focus on the distance to the telephone exchange (or other modem). If it is small, you can safely focus on VDSL, but if the node is far away, you should choose ADSL2+. If you have two copper pairs of wires, you can also pay attention to SHDSL.
Annex Standards
Annex is a type of ADSL standards for transmitting high-speed data in conjunction with analog telephony (regular telephone).
The Annex A standard uses frequencies from 25 kHz to 138 kHz to transmit data, and from 200 kHz to 1.1 MHz to receive data. This is the usual standard for ADSL technology.
The Annex L standard allows you to increase the maximum communication distance to 7 km thanks to increased power at low frequencies. But not all providers use this standard due to interference.
The Annex M standard allows you to increase the speed of the outgoing stream to 3.5 Mbit/s. But in practice, connection speeds range from 1.3 to 2.5 Mbit/s. For an uninterrupted connection, this standard requires an undamaged telephone line.
DHCP server
The abbreviation DHCP stands for Dynamic Host Configuration Protocol. A DHCP server is a program that allows you to automatic setup local computers for network work. It provides clients with IP addresses (unique identifiers of the device connected to the local network or Internet), as well as additional parameters necessary for working on the network. This will allow you not to manually register an IP, which will make your work on the network easier. However, please note that for devices such as network printers and permanent remote access to a computer using special programs, it will be desirable to have a statistical rather than a dynamic IP, since constantly changing the IP will cause difficulties.
USB ports
Today, there are two options for organizing an Internet connection using ADSL technology: via a USB port and via an Ethernet port.
An external USB ADSL modem is connected to a computer via a USB port. It receives power from the computer. The advantages of such modems: low cost and ease of use. The disadvantages include not being compatible with all computers, the need to regularly reinstall drivers, and working with only one device.
An ADSL modem connected to the device via an Ethernet port will work more stable. But to be used with multiple devices, it must have a router function or Wi-Fi technology.
Setup and management
Configuring and managing modems is most often carried out using three technologies: Web interface, Telnet and SNMP.
The web interface is a function that allows configuration and management via a computer browser. This option will be sufficient for home use of the modem.
Telnet is a network protocol for remotely accessing a computer using a command interpreter. With its help, you can configure the modem from devices not connected to it. This is useful for small modem circuits at home and in the office.
SNMP is a standard Internet protocol for managing devices on IP networks operating on the TCP/IP architecture (a means for exchanging information between devices connected to a network). Using the SNMP protocol software to manage network devices, it can access information stored on managed devices. Due to this, it is most often used when building office networks.
Criterias of choice
xDSL modems differ in a number of characteristics, the most important of which are the maximum distance from the telephone exchange, the speed of data reception and transmission, the presence of symmetric or asymmetric transmission. Understanding under what conditions and how exactly the modem will be used, you can choose the device that is right for you.
Let us remind you that when choosing an xDSL modem it is important to know the characteristics telephone network: cable length to the telephone exchange, number of copper cable pairs and its quality, offers and capabilities of the provider. It is important that there is no interference on the line, which is caused by the intersection of cable pairs or its poor quality.
We have distributed xDSL modems based on user needs.
To connect to the Internet using xDSL technology on one device It will be enough to purchase an inexpensive USB modem that supports suitable technology (for example, ADSL2+ or VDSL).
To create an Internet network at home or in a small office It is better to pay attention to xDSL modems connected via an Ethernet port. The choice of technology again depends on the capabilities of the telephone network.
To create a large office network with a chain of modems at distances of up to 3 km It is worth choosing among xDSL modems with the latest xDSL standards, symmetric data transmission and support for the SNMP protocol.
I’ll write about an ADSL modem with Wi-Fi, because not everyone has a dedicated line yet. Relatives living in a small town got a laptop. But since there is a computer, then the Internet is also needed, they thought. But since they could only dream of optics or even the so-called dedicated line, they had to be content with what they had and there was only Ukrtelecom and their problematic CSO. But even with Ukrtel there were problems; there were no modems available. Having decided that the wait would be very long, and their modems were expensive, and the laptop also had a Wi-Fi card, we decided to buy the modem ourselves. After wandering through our online stores and looking at the prices, we decided that we could wait for the modem to be delivered from China.
Photo from the website with the purchase price: 
The modem arrived in 22 days, without a box (this is Bika's usual practice). The power plug on the unit was “Chinese” flat, so I also had to splurge on an adapter. The disk included in the package is needed if the connection from the computer to the modem will be done via USB. That is, with a normal “Lan” (yellow cable included in the kit) connection, the disk is not needed. The connection is incredibly simple, but provided that you are not the first to do this;) For those who are doing this for the first time or could not figure it out, I made a couple of screenshots to help set up the adsl modem / edup ep dl520g router.
1.
We connect the modem to the computer with a yellow RJ45 cable.
2.
configure the computer’s network card by entering the following ip data: 192.168.0.2 mask: 255.255.255.0 Gateway and DNS enter 192.168.0.1
3.
We type in the address bar of the browser: 192.168.0.1 and press Enter. We see a window asking you to enter your login and password. Enter login: edup password: leave it blank.
4.
In the top menu. select the “Quick” tab
In the next window, select “No” and click “Next” And we get to the settings menu.
In point 1. Selecting the connection type
In point 2. Enter the login issued by Ukrtel, for example [email protected] and password: for example 1234.
In point 3. Dynamic IP and Dynamic DNS
In point 4. Click “Apply”
In point 5. In the horizontal menu, select "Advansed Setup" in the left vertical menu, select "DSL" and enter in the fields: VPI: 1 VCI: 32 and click "Apply" (VPI and VCI may differ for different providers)
If you did everything correctly, in a minute or two, you will have the Internet :) All that remains is to configure Wi-Fi
1.
Go to the horizontal tab “Wireless Setup”
clause 1= "on"
clause 2 ESSID: write the name of your connection (we will see it in the lists of available networks) for example: dom_wi-fi
clause 3="of"
clause 4 don't touch.
clause 5 Click “Apply” Now, let’s create a password for our connection, because we don’t want to share the Internet with the whole house;) Again, select “Wireless Setup” in the horizontal menu, but select “WPA” in the side menu
In the window that appears, in the “Pre-Shared Key (PSK) for Home Network:” field, write the password you created, for example 12345edrftg. You need to remember the password, or better yet, write it down. At this point, the setup can be considered complete. Enjoy your Internet surfing.
Py.sy This may not be the most optimal method or type of Wi-Fi encryption. But even this is usually difficult for beginners to set up.
I know that the writing style of my posts is somewhat chaotic - please bear with it.
ADSL is still one of the main technologies in many populated areas, because pulling optics is expensive, and, in most cases, not at all liquid. But what is 8 megabits today, in the age of consumption and crazy speeds? Fortunately, there is an ITU G.992.5 standard, also known as ADSL2+, which is used by Rostelecom
ADSL2+ is a telecommunications standard that expands the capabilities of ADSL technology by increasing the frequency for downstream bits (from the network to the user). Thus, using ADSL2+ it is possible to overcome the “ceiling” of the ADSL standard at a speed of 8 megabits.
ADSL2+ modulation, which is widely used by Rostelecom, in theory, can transmit data up to 21 megabits to the client. But in practice, it is rarely possible to connect at speeds above 12 megabits, since the condition of city trunk lines leaves much to be desired.
How do I know if my modem supports ADSL2+?
1. Before buying a modem, we recommend visiting the official website of the developer - at technical specifications It always indicates which DSL standards your modem supports. Here, for example, are the characteristics from the official TP-Link website of the model 8616:
You can also often find this on the box with the device:
Is ADSL2+ enabled on my modem?
In the modem settings there is always a DSL setting, where you can see if ADSL2+ support is enabled. Using the DSL2640U as an example:
Why do you need ADSL2+:
The standard guarantees more high speed connections than regular ADSL. This means that you can use this connection to connect SOHO or even a corporate server. The HP Proliant ML350 Gen9 Tower model is perfect for this role. It is distinguished not only by the quality of its components (the Xeon 8C processor with a 20-MB cache on the port is worth it), but also by the maximum build quality under the Hewlett Packard brand. As many as four cores are responsible for processing graphics data in the ML350 Gen9. Thus, the model fully meets the requirements for high-load systems.
Conditions under which operation on ADSL2+ is possible:
1. Support by end device (modem)
2. Good linear data (noise immunity and attenuation). Replacing the wiring with twisted pair greatly increases the chances of this happening.
3. The desired profile on DSLAM (station equipment)
How to connect to ADSL2+?
You can set the appropriate modulation on your port via technical support Rostelecom - 8 800 100 08 00. You need to inform the operator about this, and he, in turn, will send a request to the regional remote service technological support that have the ability to edit data on access nodes (DSLAM)
Functionality: ADSL/ADSL2/ADSL2+ modem, capable of operating in router or bridge mode, establishing PPP connections, and also allowing you to configure the NAT (Network Address Translation) service in sufficient detail.
The following indicators are located on the router (from left to right):
- power/system status indicator
- LAN port activity indicators
- ADSL connection activity indicator
- PPP connection activity indicator
On the back of the router are located (from left to right):
- 1 × WAN port RJ-11
- 1 × RJ-45 LAN port
- Reset button
- power connector
- power on/off switch
The device comes with the following configuration:
- router
- 2 meter RJ-45 patch cord
- 2 × 2 meter RJ-11 patch cords
- splitter
- CD with instructions
- power adapter
- Quick installation and configuration guide in Russian
Inside view
The device is based on the company's TNETD7300 processor (32-bit RISC processor, with USB and Ethernet support)
The board also has 2 MB of TE28F160 FLASH memory and 8 MB of W986416EH-7 SDRAM memory.
Briefly about ADSL technology
ADSL stands for Asymmetric Digital Subscriber Line. This technology uses standard copper telephone lines to provide broadband, high-speed digital data transmission. ADSL significantly increases the bandwidth of copper telephone line without interfering with normal telephone services. ADSL provides speeds of up to 8 Mbit/s for the forward channel (download from the Internet, WAN -> LAN) and up to 1 Mbit/s for the reverse channel (LAN -> WAN), depending on the quality of the telephone line and the type of modulation used.
Recently, ADSL equipment in our country has been gaining popularity due to the widespread introduction broadband access in Internet.
Communication between the ADSL modem and the provider's DSLSM is carried out using asynchronous data transfer technology (ATM). From the provider to the DSLAM, the signal can go via ATM, Ethernet or some other technology (in our case, the signal goes to the DSLAM via Ethernet).
More information about ADSL technology can be found in the article dedicated to.
Difference between ADSL2/2+ and ADSL
When developing ADSL2 standards, maximum use was made of the experience of introducing ADSL technology. The technology remains backward compatible with the “old” ADSL, improved modulation algorithms are used, the transmission speed is selected adaptively depending on the communication range and channel quality. All this led to an increase in maximum speed to 12 Mbit/s (for a direct channel), as well as an increase in communication range. The technology uses improved diagnostic tools at both ends of the line, which allows for quick troubleshooting (line adaptability).
ADSL2+ doubles the frequency band used, which leads to a 2-fold increase in throughput (up to 24 Mbit/s for a direct channel). The maximum speed of the return channel has also increased from 1 to 2 Mbit/s.
The large-scale introduction of these technologies into the Moscow market is hampered by the widespread use of ADSL technology.
Briefly about ATM technology
ATM is a method of transferring information between devices on a network in small, constant-length packets called cells. The length of each cell is 53 bytes (5 bytes header and 48 bytes data). The use of short cells allows for maximum reduction of delays that typically occur when transmitting large packets. Using a constant cell length also allows for approximately constant transmission delays, which in turn allows the emulation of fixed-rate devices. The implementation of ATM technology requires quite large financial costs, which currently cannot be compared with the costs of implementing the same Ethernet.
ATM technology supports cell prioritization, thereby providing required quality QoS - Quality of Service. Different applications require different levels of quality of service, and QoS and ATM technology can provide this level.
Since those coming from different sources cells can contain voice and video data - it is necessary to ensure control for the transmission of all types of traffic. To solve this problem, the concept of virtual channels is used. A virtual channel is a set of network resources that look like a real connection between users. In the ATM cell header, a virtual circuit is identified by a combination of two fields: VPI (Virtual Path Identifier) and VCI (Virtual Circuit Identifier). These parameters are specified in the parameters of the connection being established.
Device Specifications:
| frame | plastic, horizontal installation or hanging on the wall is allowed | |||
| execution | Indoor | |||
| wired segment | ||||
| WAN | type | ADSL2+ (ITU Annex A) | ||
| number of ports | 1 | |||
| types of connections supported | PPPoE | Yes | ||
| PPPoA | Yes | |||
| Bridge mode | Yes | |||
| Dynamic IP in 1483 Bridge Mode | No | |||
| Fixed IP in 1483 Bridge Mode | Yes | |||
| CLIP (IPoA) | No | |||
| Static IP | Yes | |||
| Dynamic IP (DHCP) | Yes | |||
| LAN | number of ports | 1 | ||
| auto MDI/MDI-X | Yes | |||
| manual blocking of interfaces | No | |||
| ability to set MTU size manually | No | |||
| main features | ||||
| device configuration and client setup | administration | WEB interface | Yes | |
| WEB interface via SSL | No | |||
| own utility | No | |||
| telnet | Yes | |||
| ssh | No | |||
| COM port | No | |||
| SNMP | Yes | |||
| ability to save and load configuration | yes, via FTP | |||
| built-in DHCP server | Yes | |||
| UPnP support | Yes | |||
| method of organizing Internet access | Network Address Translation (NAT technology) | Yes | ||
| NAT capabilities | one-to-many NAT (standard) | Yes | ||
| one-to-one NAT | Yes | |||
| ability to disable NAT (work in router mode) | Yes | |||
| Built-in VPN servers | IPSec | No | ||
| PPTP | No | |||
| L2TP | No | |||
| VPN pass through | IPSec | No | ||
| PPTP | No | |||
| PPPoE | Yes | |||
| L2TP | No | |||
| Traffic shaping (traffic limitation) | yes, limiting outgoing traffic using ATM | |||
| DNS | built-in DNS server (dns-relay) | Yes | ||
| dynamic DNS support | yes, only DynDNS.org | |||
| internal clock | present, manual time setting possible | |||
| clock synchronization | yes (NTP, Time, Daytime), synchronization server address is specified manually | |||
| built-in utilities | ICMP ping | Yes | ||
| traceroute | No | |||
| resolving | No | |||
| event logging | yes, system events, firewall | |||
| logging the execution of firewall rules | Yes | |||
| storage methods | inside the device | Yes | ||
| on an external Syslog server | Yes | |||
| sending by email | No | |||
| SNMP | SNMP Read support | Yes | ||
| SNMP Write support | Yes | |||
| SNMP Traps support | Yes | |||
| Routing | ||||
| static (setting records manually) | yes, but only when controlled via Telnet or console | |||
| dynamic routing | on the WAN interface | possibility of switching off | -- | |
| RIPv1 | No | |||
| RIPv2 | No | |||
| on the LAN interface | possibility of switching off | Yes | ||
| RIPv1 | Yes | |||
| RIPv2 | Yes | |||
| built-in filters and firewall capabilities | ||||
| SPI (Stateful Packet Inspection) support | ?? | |||
| availability of filters/firewall | on the LAN-WAN segment | Yes | ||
| filter types | including SPI | No | ||
| by MAC address | No | |||
| by source IP address | yes, including on a subnet | |||
| by destination IP address | yes, including on a subnet | |||
| according to the protocol | yes, TCP/UDP/ICMP | |||
| by source port | Yes | |||
| by destination port | Yes | |||
| time reference | No | |||
| by URL | No | |||
| by domain | No | |||
| working with URL blocking list services | No | |||
| action type | allow | Yes | ||
| deny | Yes | |||
| log | Yes | |||
| support special applications (netmeeting, quicktime etc) | No | |||
| virtual servers | possibility of creating | Yes | ||
| setting different public/private ports for the virtual server | No | |||
| ability to set DMZ | Yes | |||
| nutrition | ||||
| PSU type | external, 9VDC 1A | |||
| 802.1af (PoE) support | No | |||
| Additional Information | ||||
| firmware version | V3.40(UH.2) from 05/09/2005 | |||
| dimensions, mm | 111 × 106.5 × 35 | |||
| weight, g | 172 | |||
Configuration
Device configuration is carried out via a WEB interface using the Telnet protocol. Screenshots of the WEB interface are provided.
The list of SNMP parameters of the configured ADSL modem is given.
The router in question allows you to configure the NAT (Network Address Translation) service in sufficient detail. You can use two items: "SUA Only" and "Full Feature"
The "SUA Only" (Single User Account) item is used if the WAN interface has 1 IP address. The "Full Feature" item is used when the WAN interface has several addresses.
SUA Only NAT allows you to specify 11 virtual servers; port translation cannot be used.
Full Feature NAT allows you to set 10 advanced NAT rules. In each rule, you can specify the NAT type and forwarding addresses.
The following NAT types are available:
- The "One to one" type matches 1 external IP address to one internal
- The "Many to one" type matches a range of internal IP addresses to one external IP address
- The "Many to many overload" type allows you to map several external IP addresses to several internal IP addresses to distribute the load between them.
- The "Many to many no overload" type works the same as "One to One", but the ranges of external and internal IP addresses are specified, and the ranges must be equal
- The "Server" type works the same as "One to one" but allows you to specify the range of ports used
Saving/loading configuration and firmware can be done via FTP: to do this, you need to log into the device via FTP with the login "root" and the password set to manage the router:
File "ras" is a file with firmware, file "rom-0" is a file with configuration.
The device also supports Telnet management
By configuring a device via Telnet, you can configure a firewall, as well as set static routing entries that cannot be set through the WEB interface.
For some reason, setting up firewall rules is carried out only through the Telnet interface, and in the WEB interface settings there is only the “Internet Security” item, which in its settings can hardly be compared with a full-featured firewall.
Performance testing
Wired segment testing
In addition to ADSL technology, the router we are considering also supports ADSL2 and ADSL2+ technologies. Testing of the wired segment was carried out in router mode (NAT enabled), and no bandwidth limitation rules were applied.
To test the ADSL connection, we used an ADSL switch provided to us by the Russian representative office of the company.
The DSLAM settings allow you to set the data transmission delay time (interleave delay). This time, specified in milliseconds, affects the size of the data block transmitted at a time. If this time is set, for example, at 10 ms, the data received in 10 ms is collected into a single block ms Delay is used to correct transmission errors using the Reed-Solomon algorithm - this algorithm is more efficient when using large blocks of data.Increasing the delay time allows you to increase the size of a single data block just for more efficient work Reed-Solomon algorithm. Increasing the delay time justifies itself when the quality of the telephone line is low and its length is long; on a high-quality telephone line of short length, it is more profitable to minimize delays.
Interleave delay values are set separately for forward and reverse channels. To see how this change affects communication delays, just use the Ping utility. When setting the delays on the forward and reverse channels to 0 ms, Ping shows a round-trip time of about 7~8 ms. As Interleave delay values increase, the reception and transmission time increases.
Since the length of our ADSL line is only about two to three meters, we can limit ourselves to zero latency time (the standard value is 16 ms, on the DSLAM from ZyXEL the default value is 4 ms). Based on previous experience in testing various ADSL -equipment, maximum data transfer rates were achieved using zero delays (Fast mode).
When testing in ADSL mode, G.dmt modulation was used, since it is with its use that you can develop maximum speed in ADSL mode. In ADSL2 and ADSL2+ modes, modulation and speed are selected automatically (adaptive), so no additional settings are specified.
LAN-WAN test- testing was carried out according to .
Maximum speeds:
- ADSL G.dmt: 8.73 Mbps
- ADSL2: 9.69 Mbps
- ADSL2+: 17.48 Mbps
ADSL2 technology differs from ADSL only in the type of signal modulation, the use of which allows you to increase the transmission speed (in our case by about 1 Mbit/s).
In ADSL2+ technology, the range of frequencies used is doubled - this results in an almost 2-fold increase in performance.
The speed of the return channels is almost the same in all cases, although when using ADSL2+ it should reach 2 Mbit/s.
Now let's see how traffic behaves when using smaller packets:
When the packet size is reduced, the difference from using different technologies (ADSL, ADSL2, ADSL2+) is smoothed out, and the maximum speeds do not differ as much as in tests with the maximum packet size.
As the packet size decreases, the transmission speed decreases significantly. The decrease in speed occurs as the overhead of data transfer increases. For example, if data is transmitted in packets of 64 bytes, then each packet for transmission via an ATM connection is divided into 2 cells of 53 bytes (48 bytes - data and 5 bytes - header) - thus, the volume of transmitted traffic increases almost 2 times. As the packet size decreases, the size of the payload within the packet also decreases, while the amount of "overhead" for each packet remains the same. Thus, with the same value of channel capacity, the value of useful bandwidth can vary tens of times depending on the size of the packets used - this is most pronounced when using small-length packets.
The significant drop in speed when using full duplex mode may be due to some aspects of the TCP protocol. TCP is a guaranteed delivery protocol - it requires confirmation of delivery of each packet sent. Taking into account the small width of the reverse channel and its full load, these confirmations may be lost, and if the confirmation is lost, the packet is sent again - therefore, the “useful speed”, which we measure, drops, since the same data is transmitted repeatedly. In addition, the TCP protocol reduces the data transfer rate as losses increase. Thus, the drop in full-duplex forward link speed may be due to aspects of TCP's operation when the reverse link width is small.
Testing traffic limiting capabilities
The router in question supports limiting the total bandwidth using the QoS (Quality of Service) mechanism. For QoS to work correctly, QoS support is required on all ATM devices (in our case, on the router itself and on the DSLAM). QoS allows you to adjust the parameters for sending data - that is, by changing the QoS parameters, we change the width of the reverse channel. When using TCP traffic we also change the throughput of the forward channel - we have already discussed this feature of the TCP protocol above.
There are 3 items in the QoS settings: UBR (Unspecified Bit Rate), CBR (Constant Bit Rate) and VBR (Variable Bit Rate), which means, respectively, transmission at a predetermined rate, transmission at a constant rate and transmission at a variable rate.
UBR makes no guarantees regarding quality of service or bandwidth and assumes that the protocol is more than high level, for example, TCP to correct transmission errors. TCP allows you to adjust the transmission speed depending on the number of lost packets (to reduce losses, the TCP protocol reduces the transmission speed, thereby unloading the line, and the less load on the line, the less losses).
The CBR (Constant Bit Rate) clause means that a given connection will be provided with a predetermined channel bandwidth. When you select this item, the bandwidth value is indicated in the PCR (Peak Cell Rate) field, which specifies the maximum traffic speed.
The VBR (Variable Bit Rate) item means that for a given connection, the channel bandwidth can change over time. When you select this item, 3 values are set: Peak Cell Rate - peak speed, Sustain Cell Rate - average speed and Maximum Burst Size. At maximum speed, a limited amount of traffic can be transmitted, specified in the Maximum Burst Size item, and at average speed, traffic can be transmitted indefinitely.
First, the CBR (Constant Bit Rate) item was selected, which provides a constant bandwidth; during testing, the PCR (Peak Cell Rate) parameter was changed - the peak speed (in the case of CBR, it is also the average speed). Testing was carried out only in half-duplex mode using ADSL2+ technology, since it is this technology that allows for maximum speed. The test results are listed in a table.
What is ADSL?
Asymmetric Digital Subscriber Line is a technology that allows for high-speed data transmission over a telephone line (ordinary copper telephone wire).
At the same time, from telephone modems tone frequency, the differences are significant - transmission is organized at a higher frequency than that of an analog telephone signal. Result: you have streams independent from each other and can carry on conversations while simultaneously being online. ADSL is currently capable of providing an incoming data stream speed of up to 24 Mbit/s and an outgoing one - up to 1 Mbit/s, but this is under ideal conditions, a short distance from the station and underground cable laying, etc. For the most part, the connection speed depends on the technical capabilities of the provider and on average reaches 6-8 Mbit/s.
In the case of ADSL, you simply connect to the telephone socket through a special device that performs frequency division - this is a splitter, a modem and telephone are connected to it, and the device receives output directly to your telephone line.
Which modem should I choose?
Currently available are ADSL modems with Ethernet and USB connection interfaces, and a wireless device with a Wi-Fi interface. The choice depends on your needs - the difference here is in the connection features and the capabilities provided: modems that connect as network devices offer more of them than their USB counterparts. For example, to connect one computer to the Internet, a simple model will be enough, but if there are two or more of them, then you need to pay attention to devices with a router function - the modem will in any case have an Ethernet or Wi-Fi interface.
You can find options from Ethernet models with and without a built-in switch (in this case, you will need to use an additional switch when connecting several computers). If you need increased security, then you can look at models with Firewall functions, anti-virus scanning, the ability to work with a virtual private network and other “useful things”.
If you use signaling on the same line in addition to telephony and Internet DSL, you will need a modem that supports the ANNEX B operating mode; this can be either a separate modem model or a built-in switching capability (ANNEX A - ANNEX B).
How to connect?
An important detail: if there are telephones connected in parallel in the apartment, and it is impossible to connect them through a splitter (they are located in other rooms), then each incoming call will cause a lot of headaches, which are associated with constant disconnections. To avoid this, you need to buy an additional device, a microfilter, for each of the devices, and connect them through it. One of the sides of the splitter has two inputs (Phone and DSL), the telephone cord from the modem is connected to the DSL input, and the telephone device is connected to the Phone. A splitter is a frequency divider, or, to put it more simply, it allows you to surf the Internet and communicate on the phone without any interference.
Now we need to turn on the modem itself and look at it. The lights should come on and blink. The main indicator is the DSL light, which should be on without blinking after the modem is restarted. If the light is on correctly, then you can connect the modem to your laptop or computer using a network connection. Ethernet cable RJ45. It happens that the light flashes quickly or simply does not light up. You can solve the problem by checking the quality of the cable connections and connection diagram or by calling your provider. Go ahead. The lights light up as they should. When you turn on the computer, connect the modem to the network card output, which is slightly wider than the telephone output.
When creating a new Internet connection, you simply need to configure the modem.
Depending on the configuration of your local network, the modem can be configured in the so-called bridge mode.
Selecting an ADSL modem and setting it up is a very simple task if an agreement has been concluded with the provider from whom you received a login and password, as well as a notification that the line has been connected to the Internet.
