Fractional T1 T 1
Fractional T1 is a digital private line service with capacity of 2, 4, 6, 8 or 12 two-point digital channels. This service is defined as providing bandwidth greater than 56 Kbps but less than 1.544 Mbps. Fractional T1 service provides the ability to combine several channels of digital or analog services. It is an Nx64 service packaged in two, four or six DS0 increments that allows transmitting and receiving a data payload of 364 kb/s, or (6 DS0s).
How many T1's in a DS3? How many DS3's are in an OC48?
There are so many different Telecommunications Terms and Acronyms, it's no wonder that even the experts can sometimes get mixed up. There are lots of articles and definitions available on the internet, but thought we would put together our own even it's just to help me understand the difference between an OC3 and a DS3!
The basic building blocks of Telco Circuits: DS0 - 64 kb circuit - a single voice line or POTS circuit (plain old telephone service).
24 DS0's = T1 1.544mb Circuit - 4-Wire Copper
28 T1's = DS3 45mb Circuit - Copper Coax circuit
3 DS3 = OC3 155mb Circuit - Fiber Optic Circuit
From there we move up to the OC's (Optical Carrier) These are usually considered backbone carrier circuits that connect large routers and provide service to ISP's (Internet Service Providers) or WISP's (Wireless Internet Service Providers). OC3 - 3 DS3's or 84 T1's or OC12 - 12 DS3's OC48 - 48 DS3's OC192 - 192 DS3's usually done by DWDM (dense wave division multiplexing) An OC192 would equal 5376 T1's or 129,024 voice calls!
T1 Cost and DS3 Cost
T1's and DS3's are evolving to commodity products where the underlying providers remain basically the same, but are resold to CLEC's (Competitive Local Exchange Providers) on a wholesale basis. The CLEC's then provide the service and add value by providing the circuit monitoring and customer service. Some CLEC's do this better than others and they do it for different prices too. It's usually not wise to shop for price only. The educated consumer understands that there is a bundle of equipment, service and guarantees with most products and your Telecommunication Services - T1, DS3, Dedicated T1, Point to Point Circuits are no exception to the rule. When you want T1 Pricing or DS3 Pricing a service like T1s-R-Us.Com can really make sense, so you can compare multiple T1 Service Providers and make an educated decision for the best value for you're particular situation when you are purchasing Telecom Services.
Choosing a T1 Service Provider
The t1 internet service has been around for over 40 years but never have there been so many "flavors" available. The number of T1 Service Provider companies has jumped since deregulation in 1996 from a few dozen to several thousand. As the telecom shakedown that began in 2001 continues many of these companies are trying to stay afloat by capturing as many customers as possible with inexpensive products they claim to be T1. Don't be fooled!
Many of the desperate T1 Service Provider companies are putting together inferior products they claim are T1 products. Be leery of products with the funny titles like "burstable" and "reach". These products are oversubscribed much like a DSL product. This means the provider puts a singled T1 connection in a CO (Central Office) and sells T1 connections to 3 or 4 customers hoping they don't all use the service at the same time. True T1 access means dedicated access to the internet and you always have access to 1.54Mbps.
Make sure your T1 product is a "clearchannel" product and is not shared with other users. Also be sure you have an SLA or Service Level Agreement from your T1 Service Provider. A SLA will specify the access you will receive and the penalty the service provider will pay if they do not provide such service. If you need help establishing the proper SLA seek the help of a professional if you don't have one in house. It's much better to pay a small fee up front than to enter a long term contract with an inferior carrier. You can also get professional assistance free of charge by simply contacting one of the many T1 brokers on the internet.
DSL Alternatives are Frame Relay and Fractional T1 - Want Broadband Service, but it's not available yet? There's broadband service available now to almost every corner of the United States. First check http://dslrus.com for Broadband Services available at your location. I'll bet there's at least Satellite DSL service available and if your lucky DSL and/or broadband cable. Satellite service isn't for everyone, the cost can be prohibitive and reliability is questionable. If you need reliable Brodband Service and DSL or Broadband Cable isn't available check out frame relay and fractional T1 service. You will pay more these carrier class services, but they can be deployed anywhere and the are typically priced less than a full T1.
This service provides a digital high capacity link on a two-point digital channel that allows simultaneous two-way transmission of serial, bipolar, return-to-zero synchronized digital signals at a transmission speed of 1.544 megabits per second (Mbps). It also allows simultaneous two-way transmission of serial, bipolar (B8ZS), return-to-zero, synchronized digital electrical signals at a transmission rate of 1.544 megabits per second (Mbps) + 20 pulses per minute (ppm).
Today, Internet access is provided over dial-up modems, ISDN lines, leased lines, and frame relay. Any Internet Service Provider (ISP) can easily use the existing telephone infrastructure to offer Internet access services. The availability of xDSL data modems allows the local exchange carrier (LEC) to offer high-speed access over the existing copper loop. To provide ADSL, HDSL, or SDSL services (collectively referred to as xDSL), the LEC can install modems at a serving location (Central Office (CO) or DLC site) and along with a compatible modem at the customer premise to provide high speed data transport for access to data networks such as the Internet or a Wide Area Network (WAN).
Asymmetrical Digital Subscriber Line (ADSL) is a technology that provides a high-speed downstream and a lower-speed upstream digital channel for data transport. This is done by frequency multiplexing the data signals on the same copper pair as the local telephone service (POTS) above the 0-4 KHz voice channel. The ADSL signal is superimposed on the copper pair by the use of low frequency/high frequency splitter networks at both the subscriber premise and at the serving location. Normal telephone service is not affected by the ADSL digital service. Most users of ADSL data services are predicted to use an ADSL modem that has a 10 Base T Ethernet interface. Some subscribers may chose to use one of the ATM interfaces that are being defined by the standards bodies. The transmission rate provided to the subscriber will depend on the length of the copper loop and the service rate selected. Rate adaptive modems can provide the subscriber with the highest transfer rate possible over a specific copper loop make-up.
Asymmetric Digital Subscriber Line (ADSL) is a modem technology which will allow copper pairs to be converted into paths for high speed data communications. The term "asymmetric" refers to the different upstream and downstream rates. Upstream refers to the direction from the customer to the network, and downstream refers to the direction from the network to the customer. ADSL modems divide the bandwidth of a telephone line into multiple channels, where one band (channel) is utilized for upstream traffic and another band for downstream traffic a third band is for POTS service. The three channels on a copper facility are created with the introduction of ADSL terminal units (modems) at either end of the loop. ADSL terminal units located in the Central Office and are called ATU-C's, and the ADSL terminal unit located in a home or business is called an ATU-R and is considered Customer Provided Equipment (CPE). These devices combine/separate the high speed and POTS signals carried on the copper facility.
Frame Relay Service
Historically, Frame Relay provided a simple, cost-effective alternative to leased lines. Because corporations used frame relay primarily for data transport and LAN-to-LAN connectivity, they were content to engineer their networks based upon our committed information rate (CIR) and access rate (AR) parameters. This is no longer the case. Enterprises now rely on frame relay for distributed access to mission-critical applications; applications that often require the ability handle voice, video, and data on the same network infrastructure. Traditional frame relay treats all traffic equally. Low-priority traffic bursts can interfere with the timely delivery of higher priority frames. Consequently, Verizon has no way to guarantee delivery within a specified amount of time and bandwidth.
With the introduction of Quality of Service (QoS), customers will have the choice of configuring their Frame Relay circuits with a high priority class of service in addition to the existing best effort class of service. Priority Frame Relay Permanent Virtual Circuits (PVCs) will provide Frame Relay customers with a high priority class of service for delay-sensitive, mission-critical applications. Over the past few years, market demand has increased for delay sensitive applications ranging from Systems Network Architecture (SNA) traffic to Voice over Frame Relay or VoIP. In addition, the establishment of a differentiated class of service hierarchy will greatly enhance the ability to meet the market demand for Service Level Agreements (SLAs) that clearly define performance characteristics, including throughput, delay, and frame loss.
Lastly, and perhaps most importantly, the development of Internet Protocol-Virtual Private Network (IP-VPN) offering is contingent upon the ability of Frame Relay to provide different classes of services. With the introduction of Priority Frame Relay Permanent Virtual Circuits, customers will have the choice of configuring their Frame Relay circuits with a high priority class of service and will also allow for customers to use their existing FRS investments to move toward a network based IP-VPN solution.
MMDS - Multi-Media Data Service
Multi-Media Data Service is an intraexchange transmission service that has an aggregate channel speed of 100 megabits per second. MMDS is be provisioned on a point to point or multi-point basis between customer locations using fiber optic cable between fiber optic multiplexers. MMDS service is usually only be available in areas that have existing fiber optic facilities.
MMDS service is designed for customers that require higher channel speeds than are presently offered in the current network. MMDS will bundle the equipment and circuits needed to interconnect certain local area networks, extend campus networks to offsight locations, provide computer to computer channel extension, connect high speed printers and provide other high speed data applications. MMDS service does not require routing through the central office and data security is normally not provided with this service offering.
Diverse Routed Facilities
Service provided over more than one distinct facility and/or from more than one piece of equipment. Examples include service provided from equipment housed in an alternate serving wire center ("ASWC") and service provided over cables that follow two different routes back to the serving central office. Diversity generally requires cable lengths greater than primary service to a customer location.
Facilities along a secondary path into a building or into a customer's premises using a different point of entry/departure and routing than that which is used to provide the primary service to that location. This is the cable from the serving manhole, pole or access point to the customer's demarcation point, and it may also include house or riser facilities to the Customer's premises.
TLS – Transparent LAN Service
LES – LAN Extension Service
TLS is a "switched" Ethernet service with data rates currently provided at 10M, 100M and 1000M (GigE). Transparent LAN Service (TLS) is a fiber-based access, switching and transport service that utilizes a shared backbone to provide customers with Ethernet LAN Interconnection among multiple sites within a LATA at native LAN speeds. Customers access the service via a dedicated single mode fiber (SMF) pair from their premises to the nearest deployed TLS switch. Many customers consider fiber access as secure (compared to copper). This service is offered to customers whose sites are within the acceptable range of the nearest deployed TLS switch. The local exchange carrier (LEC) will provision a Network Interface Device (NID) at each customer site to terminate the loop fiber and provide a standard interface for connecting into the customer's Local Area Network (LAN).
ATM - Asynchronous Transfer Mode
ATM stands for Asynchronous Transfer Mode. It is a communications networking technology that carries information (voice, data, and video) into fixed length cells. It is asynchronous in the sense that recurrence of the cells containing information from an individual user is not necessarily periodic. ATM's high speed and integration of traffic will enable the creation of new applications. It allows for the integration of networks improving efficiency and manageability. ATM specifications have been developed to be transported over twisted pair cable, coaxial cable and fiber optics.
It is very scalable and flexible in:
• Geographic distance
• Number of users
• Access and trunk bandwidths
ATM uses fixed length 53 byte cells transported and re-assembled at the destination. The cell makes ATM unique compared to other networking protocols:
• Allows the ability to mix different types of information (voice, data, and video)
• Allows the ability to switch the information very quickly in hardware providing dedicated bandwidth per connection, higher aggregate bandwidth, and flexible access speeds. The cell is broken in two sections, header (5 bytes) and payload (48 bytes). The header is the addressing mechanism and the payload carries the actual information.
The ATM layer is responsible for establishing logical connections across the network by sharing the resources of the physical layer. The 5-byte cell header contains an address identifier used to take the cell from source to destination. An address identifier is unique to each physical layer. The ATM switch assigns the needed identifier locally.
The address identifier in the cell header consists of two parts:
• Virtual channel identifier (VCI) – 12 bit field that identifies virtual channels carried by an ATM link.
• Virtual Path identifier (VPI) – 8 bit field that identifies virtual paths carried by an ATM link.
A VPI can contain multiple virtual channels. Each direction of the VP or VC requires an address. ATM connections relying on both VPI and VCI addresses for switching are known as virtual channel connections (VCC). Connections relying on the VPI address only are known as virtual path connections (VPC).
Two types of connections can be set up:
• Permanent virtual circuits (PVC) - Connections loaded into the switching tables of ATM switches
• Switched virtual circuits (SVC)– Connections set up on a call by call basis using signaling messages between the user and the network to exchange information about the type of connection required.
ATM supports six different classes of service or service categories, to support different applications with different levels of performance.
• Constant Bit Rate (CBR) – real time voice and video
- Low cell transfer delay required
- Low cell loss ratio required if voice/video are highly compressed
- End to end timing required
• Real Time Variable Bit Rate (rt-VBR) – packet voice and video
- Low cell transfer delay required
- Low cell loss ratio required if voice/video are highly compressed
- End to end timing required
• Non-real Time Variable Bit Rate (nrt-VBR) – data (connection oriented)
- Requires low cell loss ratio
• Variable Bit Rate Connectionless (VBR CL) – SMDS (connectionless)
- Requires low cell loss ratio
• Unspecified Bit Rate (UBR) – LAN traffic requiring low performance characteristics
• Available Bit Rate (ABR) – VBR traffic that can reduce its offered flow rate in response to network congestion
- Requires low cell loss ratio
ATM Adaptation Layers
In order for ATM to support different classes of services it must adapt or package different types of application traffic into the ATM layer. This function is performed by the ATM Adaptation Layer (AAL).
• AAL1 - Supports connection oriented services that have constant bit rates and specific timing and delay requirements. Sequence numbers are added to each cell so that the destination can detect whether a cell is missing.
• AAL2 - Consists of variable size packets encapsulated within the payload. It is used to carry VoIP (Voice Over IP).
• AAL3/4 - It is designed for connectionless oriented variable bit rate services. It is very heavy on overhead.
• AAL5 - Supports connection oriented variable bit rate data. Provides data transport for packets up to 65,536 bytes. Error control and padding are added to ensure a 48-byte payload.
Today, AALs are relatively independent of Service Class. Several alternative adaptation types are used for each class. AAL5 has been widely deployed for all popular applications.
Each virtual connection will be associated with a service category. The performance of each connection is measured by a quality of service (QoS). The seven QoS parameters defined by the ATM Forum are as follows:
• Sustained cell rate (SCR): Average number of cells per second that the connection can transfer into the network.
• Peak cell rate (PCR): Maximum number of cells per second the
connection can transfer into the network:
• Minimum cell rate (MCR): Smallest cell transfer rate that the
connection must always support.
• Burst Tolerance: Maximum length of time that the user can transfer at peak cell rate
• Cell loss ratio (CLR): Allowed percentage of cells that the connection can loose in the network as measured on an end-to-end-basis.
• Cell transfer delay (CTD): Time delay experienced by cells using the connection as measured on an end-to-end basis.
• Cell delay variation (CDV): Change in inter-arrival times of each cell.
When information needs to be communicated the user and the network exchange information about the needed quality of service through the set of QoS parameters outlined previously. The sender specifies the type, speed and other attributes of the call, which determine the end-to-end quality of service.
Assignment of QoS to each connection is critical for providing each application with the required service quality and for optimizing network usage. To make sure users follow connection rules ATM networks define various functions:
• Traffic shaping - Allows the source to modify the stream of cells sent over the virtual connection so that it conforms with bandwidth parameters (i.e. buffering)
• Traffic policing (Usage Parameter Control (UPC) at the UNI and Network Parameter Control at the NNI) - Ensures that connections conform to the established bandwidth parameters by either discarding or tagging cells.
Tagged cells are first dropped if switch buffers overflow.
• EPDPPD (Early Packet Discard and Partial Packet Discard)-
EPD and PPD algorithms are used to improve the throughput of higher layer protocols such as TCP/IP for ABR/UBR classes of service. When using Partial Packet Discard the switch begins to drop cells only when the buffer overflows. Once it drops a cell from a packet, it keeps dropping cells until the end of the packet. With Early Packet Discard the switch decides to drop the entire packet when the first cell of the packet arrives. If the switch does not have space available for one packet in its buffer, the entire packet is discarded.