/dev/null – Permission Denied

Strange enough I found a server today which had permissions set incorrectly for “/dev/null”

Here’s a fix:

ls -l /dev/null
You should see this if everything is correctly
crw-rw-rw- 1 root root 1, 3

If you get a different set of permissions like
this maybe:
-rw-r–r– 1 root root 1, 3

then you should (as root) delete the /dev/null with:
rm /dev/null

and recreate it (as root) again with:
mknod -m 0666 /dev/null c 1 3

Time Warner Hybrid-Fiber-Coaxial (HFC) Network

Time Warner Cable traditionally used a completely coaxial network to allow subscribers to view television channels. This cable system is also a type of network. A network is simply an interconnected system of things; in our case, televisions.

There are a wide variety of networks in use today. The most basic network, the bus, connects each node (e.g. workstation) to a common backbone. Time Warner’s old network was completely a bus network, now it is only cable to each neighborhood. Most bus networks are very unreliable because if the backbone is broken at any point the entire network fails.

Figure 1 – TWC Coaxial Bus network after leaving node

clip_image002 As channel availability increased and cable services broadened the cable system had to be redesigned. Time Warner chose a Hybrid Fiber Coax system (HFC) to provide the adequate bandwidth and scalability that they needed and will need in the future. The company chose high-speed optical fiber to connect each of their hubs and nodes. To connect the customers to the nodes and hubs they used regular coaxial cable.

Time Warner’s fiber network is a Synchronous Optical Fiber Network (SONET) ring. This network is fully redundant. It uses fiber above and below ground to provide diverse-route connections. This type of network is not only stable, but very fast. If someone were to break one of the fiber routes, the system automatically re-routes traffic to the diverse route.

For scalability, Time Warner has ninety-six count fiber in place. This allows them to run many networks within one fiber sheath. Each service requires 4 fibers, two to carry traffic clockwise around the ring, two to carry traffic counter-clockwise around the ring and a matching set of fibers to carry the diverse path (underground or overhead) traffic. They currently run services such as Video On Demand (VOD), cable TV and high-speed cable internet.

The signals are transported on single-mode fiber (for long-haul applications) from hub to hub. At the end of each multi-mode fiber (short-distance applications) run there is an optical to electrical converter that converts the fiber to coaxial cable. The coaxial lines have to be amplified when the signal is too weak to receive on the other end. Each time the signal is amplified, there is more noise introduced to the line. Time Warner keeps their cable network no deeper than six amplifiers to keep noise to a minumum. Fiber has the same issue but the amplifiers are regenerators which are called Iridium-Doped (the technology that the amplifiers use) Amplifiers which are very expensive.

The Time Warner network is a MAN (Metro Area Network) which is a network that connects an entire city. The entire Time Warner network consists of many different MAN’s. There are other locations such as Milwaukee and Tennessee that are linked together to form a WAN (Wide Area Network.)

Figure 2 – SDTV Channel

To fit all the data onto one single coaxial cable TWC uses multiple signaling methods and multiplexing. The coaxial cable network uses FDM (Frequency Division Multiplexing) to divide the available bandwidth into “chunks” of space for each television channel, data channel or high definition digital channel. The coaxial spectrum has a bandwidth of 750MHz. Each channel of SDTV (standard definition television) occupies 6MHz of bandwidth (see figure 2.)clip_image003 The coax contains a sub-band from 0-55MHz which contains return traffic and is divided into 1.5MHz “chunks”. Usable bandwidth on the cable spectrum starts at 55MHz. From 55MHz to 550MHz is where all the analog signaling exists. The space from 550MHz to 750MHz is all digital data using the digital to analog signaling method 256 QAM (256 bits per baud Quadrature Amplitude Modulation.) QAM is a combination of two simpler encoding methods called ASK (Amplitude Shift Keying) and PSK (Phase Shift Keying.) In ASK the amplitude of a signal is varied to transmit data. In PSK the signal phase is varied by each bit or group of bits.

Digital Channels can achieve ten channels in the same 6MHz space as one SDTV chunk. They use MPEG2 compression to compress the video and then 256 QAM signaling to deliver the information to the viewer.

Figure 3 – Capture of a video from paramount

clip_image005One main service that requires a very elaborate process to provide to customers is VOD (Video On Demand.) This service requires TWC to receive a signal from the “pitcher” or the satellite at a 3.7 to 4.2GHz C-Band transmission. The catcher receives the movie then sends the movie to the short stop which buffers the movie until it is received. The movie is then sent to the Asset Management System (AMS) which determines the validity of the movie, size and other details. Then, when the time is right, it sends the movie out to the propagation server which then sends the movie to each of the SeaChange servers located in nine of twenty hub sites. clip_image007

Figure 4 – Propagation of a movie

A SeaChange server is basically a large storage center for movies that allows the movie to be delivered to a VOD customer requesting a movie. The request is sent from your cable box through the return band on your coax back through a series of checks for account payment and permission to request the movie. The movie is then sent back to you on your own personal “chunk” of a 256 QAM signal and then appears on your television.

Time Warner Cable uses a wide variety of technology to deliver the best in entertainment and maximize productivity. Their extensive use of a HFC (Hybrid Fiber Coax) network is very efficient and robust. All of this is possible because of the fundamentals of Data Communication, signaling methods, multiplexing, and networks.

The entire Time Warner network (In Kansas City)


Video On Demand


The Cable Spectrum