By. Blanca J. Polo, Apr 11, 2002.
- AAL, Why do we need them?
- QoS, Quality of Service
- Traffic Descriptors
- Traffic shaping
- SONET, Synchronous Optical Network
- HDLC
- ATM is a connection oriented data link layer protocol.
- Information is transmitted in cells, that are 53 bytes long.
- ATM runs at 155.52 Mbps this makes it compatible with SONET
The goal of AAL is to provide useful services to application programs and to shield them from mechanisms of chopping data up into cells at the source and reassembling them at the destination
A generic AAL operation is described when the information to be send is handled by the CS and the SAR. Both of them add their headers and trailers (if needed). The message is later on chopped and fit into cells.
AAL 1
AAL 2
Used for compressed audio or video. The rate can vary strongly
AAL 3/4
Works for stream or message. Used for traffic that is sensitive for errors and loss but NOT time dependent.
AAL 5
Designed for the telecommunications industry . Also supports message or stream. It can be used for guaranteed delivery with flow control, as well as for unreliable service.
QoS uses admission and bandwidth allocation control mechanisms in order to guarantee the to the user a specified measure of the following:
- PCR, peak cell rate
- SCR, sustained cell rate
- MCR, minimum cell rate
- CVDT, cell variation delay tolerance
- Leaky bucket
- Token bucket
- A mechanism by which bursty traffic can be shaped to present a steady stream of traffic to the network.
- The leaky bucket algorithm was designed to control the rate at which ATM cell traffic is transmitted within an ATM network.
- The size of the bucket and the transmit rate are generally user-configurable and measured in bytes.
- It uses a FIFO flow.
- It is possible for the bucket to fill up and subsequent flows to be discarded.
- Provides shaping and ingress rate control.
- This mechanism dictates when traffic can be transmitted based on the precence of the tokens in the bucket.
- Make it possible for different carriers to interwork
- Unify the US and Japanese systems.
- Multiplex multiple digital channels together
- Provide support for operation, administration and maintenance.
- Section: Fiber going directly from any device to any other device, with nothing in between.
- Line: A run between two multiplexers possibly with one or more repeaters in the middle
- Path: Connection between the source and the destination.
A block of 810 bytes, a rectangle 9-rows high and 90-columns wide is sent every 125 m sec
- 24 voice channels multiplexed together using TDM
- Each channel inserts 8 bits into the output stream (7 bits for data, 1 for control)
- Total: 56,000 bps of data and 8000 bps control.
- 24 x 8 = 192 bits every 125 m sec gives a gross data rate of 1.544 Mbps.
- SONET sends one frame out every 125 m sec.
- This is the same as 8000 frames/sec, which matches POTS.
- 8 x 810 = 6840 bits transmitted 8000 times per second gives a rate of 51.84 Mbps this is the basic SONET channel.
- All SONET trunks are multiple of STS-1
There are 87 columns per SONET frame to send data. The user data is called Synchronous Payload Envelope (SPE) and it can begin anywhere within the frame.
Multiplexing of multiple data streams are called tributaries
ATM runs at 155 Mbps, these cells will perfectly fit in SONET OC-3c trunks.
- Bit-oriented link layer protocol for the transmission of data over synchronous networks.
- Transmissions consist of binary data without any special control codes.
- Information in the frame contains control and response commands.
- HDLC supports full duplex transmission.
- The normal mode is unbalanced
- The asynchronous mode is unbalanced
- The asynchronous balanced mode is designated for point to point connections over a duplex line.
