Video transmission for broadcast or conferencing used multiple T-1s, or multiple T-3 circuits. With the deregulation of telephone companies and the rise in demand for very high capacity, a new type of service was developed — broadband. As explained, T-1's are formatted by the telephone carriers, but large corporate users, government entities, and even the home user want un-formatted bandwidth to be available for a large number of services.
T-1 and DS-1 services are fixed point-to-point systems, and dedicated to a single customer. These types of services are most often used to connect Traffic Operations Centers. They may also be used to bring the data and video images from a section of freeway back to a TOC.
DDS are digital voice channel equivalents as described previously and are used as a fixed point-to-point service. T-1 service is channelized to accommodate 24 DDS circuits. The terms T-1 and DS-1 are often used interchangeably, but each is a distinctly different service provided by telephone companies and carriers.
T-1 service is channelized with the carrier providing all multiplexing channel banks equipment. The customer is provided with 24 DS-0 interfaces. Each DS-0 interface has a maximum data capacity of 56 kbps or can accommodate one voice circuit.
The customer tells the carrier how to configure the local channel bank multiplexer. DS-1 service allows the customer to configure the high speed circuit.
The carrier provides and maintains the transmission path. The customers can channelize the DS-1 to their own specifications as long as the bandwidth required does not exceed 1. Customers may purchase fractional service to save money. In this case, they don't pay for a full T-1 or DS However, the economies for this type of service are only realized for longer distances. The local loop link for Fractional T-1 is still charged at the full service rate.
DS-3s are used for Distance Learning and broadcast quality video. They are also used in enterprise networks to connect major office centers. The primary difference between the services is that DS-1 is setup as a private line system with fixed communication points.
DSL service is typically used to provide broadband internet connectivity. Some carriers will create a "quasi" private circuit by linking two customer locations to a common Central Office. The primary advantage of DSL for the homeowner or small business is that it shares the existing telephone lines and it keeps the cost of installation at a much lower level. The SONET format allows different types of transmission signal formats to be carried on one line as a uniform payload with network management.
All of these signals will be unaffected by the fact that they are being transported as part of a SONET payload. This is referred to as an OC-1 Optical Carrier 1.
The number indicates the total of DS-3 channel equivalents in the payload. The routing of portions of the SONET payload to multiple points must be planned and built into a routing table.
These plans can be executed as part of a program to restore service in the event of an outage in a portion of the network. Some early adopters of SONET attempted to use this feature to provide for "time-of-day" routing changes. Often users were disappointed with the results.
However, it is possible to combine DS-3s into a single channel. An OC-3C concatenated is a group of DS-3s combined into a single payload to allow for the total use of the OC-3 as a single data stream. ATM is a data-link layer protocol that permits the integration of voice and data, and provides quality of service QoS capabilities.
This standards-based transport medium is widely used for access to a wide-area WAN data communications networks. ATM nodes are sometimes called "Edge Devices". These Edge Devices facilitate telecommunications systems to send data, video and voice at high speeds. ATM uses sophisticated network management features to allow carriers to guarantee quality of service.
Sometimes referred to as cell relay, ATM uses short, fixed-length packets called cells for transport. Information is divided among these cells, transmitted and then re-assembled at their final destination. Carriers also offer " Frame Relay " service for general data requirements that can accept a variable packet or frame size. Frame Relay systems use variable cell packets based on the amount of data to be transmitted. This allows for a more efficient use of a data communications network.
ATM services are offered by most carriers. A number of DOTs are using this type of service — especially in metropolitan areas — to connect CCTV cameras using compressed video , traffic signal systems, and dynamic message signs to Traffic Operations Centers.
The stable packet size is well suited for video transmission. ATM is generally not used by telephone companies for toll grade voice, although its stable packet size was developed to meet requirements for voice service. ATM devices typically have ports that allow for easy connectivity of legacy systems and the newer communications systems.
In a private or enterprise network, as shown in figure , ATM is effectively used for voice and video transport as well as data. ATM has fixed-length "cells" of 53 bytes in length in contrast to Frame Relay and Ethernet's variable-length "frames.
By catering to both forms of network traffic, ATM can be used to handle an end user's entire networking needs, removing the need for separate data and voice networks. The performance, however, can also be compromised, and the network may not be as efficient as dedicated networks for each service. ATM systems usually require DS-1 circuits, but can be made to work in a lower speed environment.
ATM does have a reputation for being difficult to interface to an existing network. However, competent network technicians can usually overcome most difficulties. Frequency Division Multiplexing FDM is used when large groups of analog voice or video channels are required. The available frequency bandwidth on an individual communications link is simply divided into a number of sub-channels, each carrying a different communication session.
A typical voice channel requires at least 3 kHz of bandwidth. If the basic communication link is capable of carrying 3 megahertz of bandwidth, approximately voice channels could be carried between two points. Frequency Division Multiplexing was used to carry several low speed less than bits per second data channels between two points, but was abandoned in favor of TDM which has an ability to carry more data channels with more capacity over greater distances with fewer engineering problems.
This type of system was used by Freeway Management Systems to carry video over coaxial cable. However, most coaxial systems have been replaced by fiber optic systems. Fiber has a greater bandwidth capability than coaxial cable, or twisted pair. The FDM scheme allows for multiple broadband video channels to be carried over a single strand of fiber. A beam of light is divided into segments called lambdas. These lambdas are actually different colors of light.
Light transmitted over a fiber is normally a group of frequencies that can be used to create a single communication channel, or multiple channels. The frequency group can be broken into several sub groups. These are nice examples of low and high pass filters. This diagram shows the audio, the upstream and the downstream spectra. All three look as if they have the same bandwidth but notice the logarithmic graph scale. In fact the downstream spectrum is about ten times the upstream bandwidth.
Both are many times the audio bandwidth. Here is a zoomed in view showing many overlapping channels each occupying about 4. The ADSL computer data is sent using many carriers. The higher frequency carriers suffer more from attenuation signal loss. The Connect America Funds incentivize broadband service providers to bring high-speed connectivity to rural areas. DSL is the preferred type of method in these types of sparsely populated areas due to low startup costs.
These units are ideal for smaller scale deployments. The following will demystify how the different methods transport information. The ATM protocol splits data into cells made up of 53 bytes. ATM networks can transport cells at rates of up to Mbps and Mbps. As broadband began to add more complex data traffic, ATMs began to incorporate a rudimentary ATM switching fabrics, switched virtual circuits SVCs , and a variety of other traffic management features.
Many service providers opt to build their networks using Ethernet for their backhaul uplinks. Ethernet, such as Metro Ethernet, can be used for both carrier backbone and access network segments. Unlike ATM cell relay, frame relay is a packet switching technology that transmits different sized frames. A frame carries more addressing and error handling identifier tags than ATM packets. However, the frame relay protocol can also be configured to use PVC to forward packets to their destination using permanent pathways as ATM cells do to achieve faster speeds.
Carrier Ethernet, such as Metro Ethernet, can be used for backbone and access network segments. Ethernet standards are constantly being expanded and improved. In fact, the Ethernet Alliance has recently announced new standards for the backhaul of networks:.
Be sure to visit the Ethernet Alliance website to learn more about these new standards. With constantly evolving Ethernet standards, Ethernet has become an integral component that maintains IP-Based networks cost-effective. The main differentiating features are subscriber capacity, throughput, packet loss, latency and jitter.
DSLAMs provide a range of subscriber capacity. There are three main metrics that dictate subscriber capacity: line density, subscriber and session capacity. Throughput measurements overview a variety of network environment factors that may influence the overall sustainable throughput of a unit including packet sizes, session volumes, and other network environment features such as IGMP snooping, QoS, AAA, and other related features depending on the capabilities of a DSLAM.
Throughput allows carriers to differentiate their service packages from their competitors and is one of the most important factors that carriers take into consideration when deciding which DSLAM to purchase. For example, a subscriber that is closer to a central office server of their ISP, will be able to experience faster rates using VDSL2 than a subscriber that lives farther away using the same equipment and xDSL technology.
Robust QoS features further improves the accuracy of throughput in real-world settings. These more complex types of traffic are more sensitive to delays or latency and requires more advanced traffic management features to reduce packet loss, latency and jitter.
ISPs and network installers can also set the prioritization of voice, video and data traffic to optimize performance. Since voice is more sensitive to delays, incoming and outgoing voice traffic can take priority over data traffic. As mentioned before, network installers will need to assess the amount of subscribers they are seeking to serve and the distance ranges they are seeking to cover. DSLAM units come in a variety of sizes with different subscriber capacities.
There are a myriad of DSLAM options built for large-scale deployments that can support several thousand subscribers. As broadband data has grown more complex, DSLAMs have had to account for value-added triple play services placing greater importance on traffic management features.
These units support varying subscriber capacity and DSL service types. As a network grows in size, additional units from different vendors can be added to a network. Devices such as the VXHDx provide longer distances and are designed for access networks.
These units are heftier in size measuring around 1. Knowing the approximate location of your nearest DSLAM will help you more accurately gauge the expected speed of your Internet service. They should be able to give you approximate speeds based on your location. Users will need to configure virtual local area networks VLANs , QoS, and reserve a set amount of bandwidth for voice—the traffic type most sensitive to latency—on their switches.
Great job to the doc author! Very brief yet very informative and useful. Thank you for the information. Great article!
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