ATM Questions and Answers
What is ATM technology?
ATM is a high speed networking technology based on two very simple concepts-network conformity and QoS. Information (data, video, Internet and voice) transferred across an ATM network is carried in 53-byte chunks known as cells. The fixed length cell enables ATM to transfer information faster than Frame Relay or Internet Protocol (IP) with the highest degree of data integrity and the lowest degree of latency of any existing networking technology. A 53-byte cell is comprised of 48 bytes of payload (user information) and a 5-byte header. The 5-byte header enables all addressing and QoS information to be tagged at the front of a cell and allows the payload (user information) to be switched, instead of routed, without delay and according to priority. Using a uniform cell structure allows all information to travel the network at a known interval. By putting information from different business applications into cells, the cells can share the same network and still be treated in the same manner without latency or loss of integrity.
What is the difference between Frame Relay and ATM?
Frame Relay has a variable length packet (frame) whose size depends upon the amount of information transmitted and ATM uses a fixed length packet (a 53-byte cell). The switching of cells enables information to be switched very quickly in hardware and has the ability to intermingle data, video, Internet and voice without compromising the unique need of each application.
What are PPD and EPD?
PPD stands for Partial Packet Discard and EPD stands for Early Packet Discard. PPD and EPD are the commonly used industry terminology for intelligent frame discard capability as an optional congestion recovery procedure. EPD is a packet discard mechanism congestion control technique that discards every cell from an ATM Adaptation Layer 5 (AAL5) Packet Data Unit (PDU) when an ATM device buffer overflows. It prevents cells from entering the buffer, reserving remaining buffer capacity for cells from packets already admitted. PPD occurs when a device drops one or more cells of the same AAL5 PDU. It discards all remaining cells of an AAL5 PDU except the last one. This mechanism avoids transmitting cells across the network the AA5 receiver would fail to reassemble.
What Quality of Service (QoS) are provided?
All five ATM service classes are available, with the exception of Variable Bit Rate-real time (VBR-rt) and Available Bit Rate (ABR) are not available on DS1 and IMA ports. Those customers connecting interLATA service to or from an in region state, or connecting in region to out of region via Intermedia, will have their ATM Service limited to the combined strengths of the three networks: Qwest Corporation (QC), Intermedia and Qwest Communications Corporation (QCC). The most notable difference is the service class, which is limited to CBR and VBR-nrt.
What speeds and ATM service categories are available for ATM to DIA?
ATM to DIA PVCs are available only on DS3 and above ports using CBR service category.
What is the difference between a VPC and VCC?
A Virtual Path Connection (VPC) can carry multiple Virtual Channel Connections (VCCs). It is our customer's responsibility to provision VCCs within a VPC. The sum of all the VCCs needs to conform to the traffic parameters that the customer signed up for on a VPC. Think of it as a phone number. If another carrier comes to Denver and buys 303-222 phone numbers from us (VPC), then we would route all the phone calls btw/ 303-222-0000 and 303-222-9999 to that carrier. The carrier will assign the individual numbers within that range themselves (VCCs). Qwest will just treat them as a "block of numbers" that is carried from one end-point to the other.
A customer bought a VPC and asked us to manage the VCCs on their ATM network. Is that possible?
When you set up a VPC across the ATM network, the switch does not have the ability to look into the individual VCCs within that VPC. The customer buys a VPC with the intent of managing the VCCs within that VPC. The customer also selects the bandwidth and the Class of Service for the aggregate of all the VCCs in the VPC. The customer is responsible for managing to the aforementioned parameters. That is the whole reason for the VPCs existence-flexibility and customer control. If the customer does not want that, they need to convert to VCCs (PVCs) and Qwest will manage it for them.
Can a customer oversubscribe an ATM HOST? For example, I have a DS-3 ATM with 45 Megs of Internet access at a host location in AZ, I have two other remote locations in AZ with 9 Megs of ATM pointing back to the host location in AZ with the 45 Megs of Internet. Please clarify Qwest's over subscription policy.
The answer depends on the QoS, the amount of traffic and the number of users. You can't get more than 45Mbps at one time through a 45Mbps port. This is not a case of oversubscription, but rather bursting. Please refer to the Qwest ATM Technical Publication 77378 regarding the bursting policy.
Does a customer need fiber-optic cabling to their business to subscribe to Qwest ATM Service?
No, ATM Service supports both optical (OCn) and electrical interfaces (T1, DS3)
What Type of Applications is ATM Used For?
Applications suitable for ATM Service include:
- Voice and video
- Packetized voice and video
- Systems Network Architecture (SNA)
- WAN/VPN connectivity
- Web hosting
- E-commerce
- Client-server (terminal-host) data
- LAN interconnection
- LAN emulation
- Remote access
- File transfer
- Internet/intranet/extranet access
- E-mail messaging
- Text imaging
- Forms processing
Global Communications Group Q&A
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