DS1 Framing and Coding
Most T-1/DS1 circuits today are installed as B8ZS/ESF (Clear Channel) but you could come across a need to know more about the older format schemes.
Alternate Mark Inversion (AMI)
Older DS1 facilities are designed and provisioned with a superframe (D4) framing format and a line coding scheme known as Alternate Mark Inversion (AMI).
One of the rules of D4-AMI is that there can be no more than fifteen consecutive "zeros" transmitted in sequence. Violation of this rule causes line equipment such as repeaters and Channel Service Units (CSUs) to lose timing synchronization.
To prevent this loss of synchronization, D4-AMI equipment will insert a "one" rather than allow the sixteenth "zero" to be transmitted. While this is not a problem for voice transmission it can cause critical errors when transmitting data at speeds exceeding 56kbs.
D4 Versus B8ZS
In order to overcome this limitation caused by loss of synchronization. a different type of line coding scheme has been established which allows "clear channel" (64kbs or greater) transmission rates.
This scheme also utilizes the superframe (D4) framing format but the coding scheme is called Bipolar Eight Zero Substitution (B8ZS). Under the B8ZS coding scheme the originating CSUs will insert a code containing a predetermined pattern of "ones" when more than eight consecutive "zeros" are detected. On the receiving end the CSU will recognize the inserted code and convert it back to "zeros".
NOTE: For this line coding scheme to work all equipment in the DS, circuit must be B8ZS compatible.
Superframe (D4) Versus Extended Superframe
Both Superframe (D4) and Extended Superframe operate at the DS1 level (1.544.000 bits per second). The difference between the two is that with D4 there are 8,000 bits per second used for framing and with ESF there are only 4,000 bits per second used for framing. ESF framing creates 4,000 "extra" bits per second which may be used for some purpose other than framing. These extra 4,000 bits have been designated as the Facility Data Link (FDL).
The FDL is used by the customer to monitor the quality of the OS1 circuit on a real time. non-disruptive basis. This ability to monitor the DS1 circuit on a real time basis is a significant advantage over D4 framing. on which there is no ability to monitor the circuit performance.
Voice T-1/PRI
T1 PRI (ISDN) Voice Service is exactly the same as T1 “regular” service, with 1 exception: only 23 channels are dedicated to voice or data. The last DS-0 channel is taken off to handle other applications, such as caller ID, Call set-up information, and DID (direct inward Dialing) information. The PRI service is in wide usage today as many customers need incoming calls to ring at a specific station without the expense of a dedicated person (attendant) to handle the call. The PRI also allows all of the 23 “B”/Bearer channels to route any call to the customers voice hardware as compared to having a different hunt group for different business functions. Call Centers are frequent users of PRI Voice Service since they use on the 24th line to provide caller ID which informs the customers hardware the information to do customer look up (Screen POPS).
PRI Service is an IntraLATA offering supported by the Integrated Services Digital Network (ISDN) architecture.
PRI Service provides an ISDN-based, DS1 access to the telecommunications network and includes the flexibility of integration of multiple voice and/or data transmission channels on the same line. The service provides connectivity between ISDN compatible Customer Provided Equipment (CPE) and a Serving Central Office (SCO).
Calling Number Delivery (originating telephone number), Called Number Delivery (terminating telephone number) and Hunting functionality are inherent to PRI service. Called Number Delivery and Hunting functionality are inherent to this service. Calling Name Delivery (to the PRI CPE) is an optional feature that provides name information to a subscribing PRI customer.
Call delivery information is sent by the customer equipment that is originating the call. The following are three options can be sent to the terminating equipment:
Station Number
Appropriate Agency - calls marked private (Usually restricted to law enforcement)
Main Telephone Number
Depending on the equipment originating the call or the equipment to which a call is terminating, Calling / Called Number Delivery allows the following information to be delivered:
If customer equipment originates the call: (Customer A - originating call / customer B - terminating call)
Originating (Customer A) equipment sends message to PRI via D Channel, telling the PRI it has a call. PRI searches for an available B channel. Once the call is placed on an available B channel, it is then delivered to the CO.
CO sends call and call information (called or dialed number and calling line ID) across PRI to the Terminating equipment (Customer B).
In this scenario, Customer A (originating) can order Calling Name Delivery; however, this feature does not work successfully unless Customer B (terminating equipment) supports Calling Name Delivery.
If customer equipment terminates the call: (Customer B calls Customer A) Translation searches to identify Customer A capability.
If Customer A has Calling Name and Delivery, they can receive Calling / call originating name, Calling / call originating telephone number and the telephone number dialed to reach the CPE / point of termination. Because the call is terminating to Customer A PBX, Customer A is the one that can or cannot have Calling Name.
PRI (Primary Rate Integrated Services Digital Network) has become a very popular service because it provides the customer with the following benefits:
Scalability to grow their network as business needs dictate.
Flexibility to mix voice, data, and video over a single T1 (DS1) access line.
A variety of PRI serving arrangements to meet the needs of customers in different market segments.
PRI ISDN architecture
Increased productivity at a reduced network cost.
Multiple features at no additional cost that would be costly if added to services other than PRI. Examples include called/calling party number, DID, hunting, and call-by-call service.
Fax and modem lines can be carried over the PRI and only occupy a “B” channel when in use as compared to a typical line sitting idle waiting for a call
Separate hunt groups in a typical design are inefficient and all of the “B” (Bearer ) channels work in a pooled or shared environment.
Integrated T-1/PRI (Voice and Internet) Service
Integrated T1/PRI service is just what the name implies: voice, internet and data traffic on the same T1. Since you have 24 channels (23 with PRI) to work with, you can divide up the load according to your customers need. For example, a company may have 10 phone lines. You can allocate 10 DS-0 lines for Voice, and 14 DS-0 lines for Internet. In this example, the company could handle 10 simultaneous calls while surfing the web at a speed of 896 KBPS.
Many providers today are offering data in several types. The data service could be Internet, Frame Relay, MPLS or Point to Point connectivity depending on the customers need. This application is best when the voice needs are 12 calls (channels) or less. For example if a business needed 21 lines (channels) to meet his voice requirements only 2 or 3 channels are left to support any other data type needs. Usually if a business has 21 lines, it has a need for data channels at least as great as the voice need. Or simply stated a full T-1 for voice and a full T-1 for internet is needed.
Dynamic PRI
Broadband phone service, also known as VoIP, is the technology of tomorrow - today. This advancement in communication replaces the traditional Public Switched Telephone Network (PSTN). Unlike the PSTN, this telephony uses the public Internet to deliver packets of information to destination networks.
The only major difference between how you made your calls yesterday and how many cost-savvy companies are doing it today is the method of placing a call. PSTN uses multiple switches and a dedicated trunk circuit that stays locked throughout the duration of the call. The VoIP applications use switches as well, but they simply move data from one destination to another. The trunks, in the meantime, are now data links used to deliver the packets (circuit dedication is history).
With a PSTN, placing a call requires a circuit/dedicated path or channel. A technique called Time Division Multiplexing (TDM) allows the T1 circuit to be divided into 24 channels. Depending on a particular Customer, each of these channels is fixed, so a specific portion of the circuit is used for either voice or data. Because the locking of the circuit has hindered the flexibility of the T1, it is not efficient anymore. With the VoIP network, T1 circuits are set up to use the Internet Protocol (IP), thus using packet-switching to send/ receive voice and data. The bottom line is that the cost of the call is substantially less than in the old system, because the circuit is no longer locked up with one call. In the VoIP network, available circuits are shared; this, in turn, increases the functionality of your communication system and decreases the cost of the call.
This diagram is representative of how a Dynamic T1/ PRI would work.
Typical Service Feature:
Includes On-Net VoIP-to-VoIP Calling
Includes Location-to-location Dialing
Includes LATA-wide Local Calling
200 FREE domestic Long Distance minutes per line
Basic Local Calling Features
Dynamic allocation of bandwidth provides more efficient utilization of an access circuit than traditional TDM-based offerings. Supporting up to 24 voice lines / simul-taneous calls and utilizing fractional T1 Internet rates on a single access circuit, business will benefit from the most cost effective use of their Internet T1. Coupled with more Internet bandwidth than most SMB users currently have and great long distance rates, the growth and flexibility potential of this product make it a value for the right application. This service has few geographic limitations: almost anywhere you can get an Internet T1 - you can get Dynamic T1 service! In addition, the Customer can utilize the functionality of supplement features that are not currently available on existing telephone equipment. In a nutshell, Dynamic T1 combines the benefits of Voice over IP without the need to “forklift upgrade” existing phone systems, thus offering substantial savings and improving communication capabilities.
This product works well for certain business applications. It also has limitations at this time and may not be a benefit. Contact us for a discussion as to your application and this product.
For super high-speed Internet, the user can allocate ALL 24 channels to data, giving them a two-way connection to the Internet at 1.544 MBPS! (Typical DSL accounts run between 128KBPS and 512 KBPS. Up and 1.5 Meg to 6 Meg down) DS-1 Internet connections typically range from $300 to $1000, depending on the geographical distance between the customer and the providers POP (point of presence). In most metropolitan areas a loop for dedicated Internet customers is $150 to $300 but just know that the cost is a function of geography.
Customers who use T1 data connections are typically businesses who need very high speed internet connections to transact business, communicate electronically with other offices, and who either don’t qualify for DSL service or need more speed and increased reliability than DSL can offer. With more and more businesses using a VOIP or VPN application the dedicated T -1 is becoming more widely used. This is due to the introduction of latency (data packet delay) possible in cable or DSL connections.
Long Distance T-1
Customers with large amounts of inbound and/or outbound long distance expenses rely heavily on DS-1 (T-1) services. Since there are 24 DS-O circuits bundled together on one T-1 circuit, you can have 24 conversations connected to the long distance carrier at the same time. Any number of the total 24 channels (circuits) can be set up as:
Outbound only T1 (Long Distance)
800-inbound only T1 (Long Distance)
BI-directional T1 (Long Distance)
T1 L D, however, has two costs associated with it. 1. The Local Loop, and 2. Long Distance Usage. The Local Loop is the flat fee that it costs to connect the customer to the long distance switch. This circuit is usually provisioned by the local phone company. The Long Distance Usage (LDU) is a separate charge in addition to the local loop. LDU rates for dedicated accounts are usually 10-40% less than regular switched long distance rates. The minutes of usage dictates the rate per minute, minimum seconds usage and the decimal rounding on partial seconds that can be expected. Many companies that have extensive amounts of LDU have gone to VOIP. This can reduce the cost per minute even lower than traditional (TDM) calls. The call quality may suffer and decisions regarding the impact on the calling/called party have to be made.
