BBN Notes 3/24/97 Lecture 13
BBN Notes for Lecture 13 (3/24/97)
ATM Adaption Layer (AAL)
Some basic underlying motivations for ATM:
- One size fits all (with payload length = 48 bytes).
- Fewer layers makes for lower overhead (as compared to the OSI model).
With this in mind, two basic functions have to be performed by ATM:
- Segmentation: Break the data to fit into cells, and conversely
do re-assembly at the receiver.
(To be efficient, do segmentation at as low a layer as possible to reduce
excessive overhead due to headers added at each layer.)
- Interface with the user and the user PDU's (Protocol Data Units):
SPDU's (Service Protocol Data Units).
(ATM considers four generic network traffic types:
DATA, VOICE, VIDEO and OTHER)
With these motivations, B-ISDN put the AAL right over ATM layer and divided
it into 2 sublayers, as shown:
Purpose: Provide the end to end connection appropriate for the
end users.
"Adapt ATM to the service." Functions include
- Timing & Recovery of CS-PDU's (blocks of data as viewed by CS).
- Provision of SAP (Service Access Point).
- More under specific AAL's.
Purpose: Make the connection work over ATM.
"Adapt the traffic stream to ATM." Functions include
- Take blocks of bits (CS-PDU's) from CS.
- Break into 48-byte SAR-PDU's (including SAR header, if any).
- Reassemble at the destination.
- Recovery/Error Control for the SAR-PDU's.
CCITT (now ITU-T) decided that there are 3 characteristics that distinguish
services:
- Timing (between source & destination)
- Realtime (e.g. interactive voice, video, etc)
vs.
- Non-realtime (e.g. data)
[Aside: Is this enough?
- No distinction made between hard realtime (everything must arrive
within the time limit, e.g. missile control) and soft realtime (some may
arrive late and is simply thrown away, e.g. voice communication).
- No distinction made between interactive non-realtime (WWW) and
non-interactive non-realtime.
]End Aside
- Bit rate
- Constant (telephone call)
versus
- Variable (compressed video)
- Connection
- Connection-oriented
versus
- Connectionless
The ITU-T defined four service classes : A, B, C, D
|
Class
|
Timing
|
Bit Rate |
Connection oriented? |
Corresponding AAL |
| A |
RT |
Constant |
YES |
AAL1 (CBR) |
| B |
RT |
Variable |
YES |
AAL2 (VBR) |
| C |
Non-RT |
Variable |
YES |
AAL3 (Connection oriented data) |
| D |
Non-RT |
Variable |
NO |
AAL4 (Connectionless data) |
Notes:
- AAL 3 and AAL 4 have since been merged into AAL3/4.
- New : AAL5 created for real connectionless-type services
(e.g. IP over ATM).
- Still waiting for AAL2.
BBN Notes for Lecture 13 (3/24/97)
Notes taken by Wai-Keung Albert Wong, wongw2@rpi.edu,
from a lecture by Prof. K.S. Vastola, vastola@ecse.rpi.edu,
on March 24, 1997.