Residential Gateway - Part 2

Since I am not much busy now a days, you may see multiple posts from me in a single day :). In the last part I talked about the Residential Gateway in general. In this post I will talk about the WAN side interface i.e. DSL.


DSL stands for Digital Subscriber Line. It is the technology that is used to transmit digital content over phone line (the very same line that is connected to your landline phone). Some of you must be wondering how is that possible? Will I be able to use my phone and Internet simultaneously?

The technology has the answer. The phone line that we have today is under utilized. It is used to carry only voice traffic which is transmitted over the frequency band 300 Hz to 3400 Hz. Whereas the cable is capable of carrying signals at very high frequency. The DSL technology makes use of the unused frequency bands to send/receive data.

What about simultaneous use? It is possible using a splitter/microfilter. A splitter is a small piece of hardware that is usually supplied by the broadband service provider. The phone line is connected to splitter. There are two output ports on a splitter. One port connects to DSL modem whereas the other port connects to phone. Splitter splits the signals based on the frequency. Signals with lower frequency

There are a number of variations/standards of DSL technology. They primarily differ in two parameters viz speed and distance they support. Ofcourse there are core technology differences. Note that with distance the signal quality deteriorates and it is not possible to install repeaters for data signals. Hence, distance plays important role.


The most popular DSL standards are ADSL and VDSL.

ADSL: Asymmetric Digital Subscriber Line
As the name suggests the download and upload speeds are different. In most of the home networks people download more than they upload. Hence, this results in very good user experience. The band from 25.875 kHz to 138 kHz is used for upstream communication, while 138 kHz to 1104 kHz is used for downstream communication.

ADSL supports download speed of upto 12 Mbps and upload speed of upto 1.5 Mbps. ADSL2+ extends the capability of ADSL by doubling the downstream bits which is done by extending the downstream frequency band from 1.1 MHz to 2.2 MHz. As a result ADSL2+ supports download speed of upto 24 Mbps. ADSL works for max distance upto 5000 meters from exchange. The close the exchange the better would be the signal quality and speed.

Following diagram shows the ADSL2+ router in the broadband network:
ADSL2+ Router in Network

VDSL: Very High Bitrate DSL
It is similar to ADSL2+ but uses high frequency band, in the order of 30Mhz. As a result it provides very high download and upload speed, approx 100 Mbps . The indicated max speeds are achievable for max distance of upto 300 meters from exchange.

The distance of 300 meters from exchange is not practical in most cases . Hence, Optical Network Unit is used to provide service even from a larger distance. The broadband service provider lays Optical cables from exchange to the locality (typically building or group of buildings) which are connected to ONU. The ONU then connects to routers at home over the phone line (DSL). Optical cables are capable of carrying data at speed of Gpbs.

Due to high speed VDSL is ideal for IPTV and HDTV services. Since it supports symmetric upload and download speeds as well it is suitable for video conferencing.

Following diagram shows the VDSL network diagram:
VDSL Network


Refer to following link for more technical differences between ADSL and VDSL:
http://www.pulsewan.com/data101/adsl_vdsl_basics.htm

Residential Gateway - Part 1

I worked for 2 years at a company that manufactures Residential Gateway. I primarily worked on the GUI and Configuration customization of these gateways. At times the work involved debugging functional issues that required understanding of the underlying protocols and standards. This provided me the opportunity to learn about various networking standards.

I referred Wikipedia and RFCs for most of the things I learned. I am going to share my learning through series of blogs. I don't intend to capture the internal details like multiplexing techniques or packet/frame formats as those can be obtained from standards documents. The information here will provide conceptual understanding and some useful facts about the technologies.

In this first installment let's understand what is Residential Gateway.

Residential Gateway is quite a popular term in USA but not a well known term in India. In India people refer it as Modem mostly because:
- It is just a Modem
- If it is not just a Modem then either people don't know about its features and/or they don't use other features.

The primary function of Residential Gateway is to enable broadband Internet connection for home users. It is a combination of modem and router. In addition, it provides other features like:
- Firewall
- NAT
- DHCP
- DNS
- VoIP

Following diagram shows the complete broadband ecosystem. The CPE in the diagram is the residential gateway:
Broadband Network

Following diagrams shows how a typical home network looks like:
Home Network 1
Home Network 2
Following diagram shows the common ports available on a residential gateway and sample devices that can be connected:
Residential Gateway Ports and Devices


If you look at the diagrams available at above links you will notice that the most common interface on the home or LAN side are:
- USB
- Ethernet (RJ-45)
- Wireless (Wifi or 802.11x)
- Coax (For TV or STB)

On the WAN side the most common interfaces are HPNA (DSL) and Cable (this is same as the one on which you get Cable TV service). Out of these the DSL interface is most commonly used, atleast in India. The primary reason for DSL popularity is that the basic infrastructure i.e. the phone line is already present. User just need to buy the modem cum router and he is ready to setup his/her home network.

I will talk about DSL and related technologies in the next part. Stay tuned...