ATM Adaptation Layer (AAL) 1, 2, 3 / 4 and 5
AAL 1 – CBR constant bit rate traffic for voice trunks and video.
AAL 2 – VBR variable bite rate with a timing relationship between sender and receiver aka voice over ATM VBR Codec.
AAL 3 / 4 – VBR traffic with no timing relationship between sender and receiver allows both connection oriented and connectionless traffic types.
AAL 5 – is a thinner interface for those bearer services that do not require the services of AAL 3 / 4 it also has Available Bit Rate to take up any excess capacity in the circuit.
From an engineering and implementation perspective, which vendor’s equipment do you want to use and how do they implement AAL services.
In general on the ATM edge devices and switches I have worked on the propagation delay was 15 to 20 micro seconds per switch or edge device. Assuming your data will be traversing one to two switches at each end and several in the WAN then you can expect reasonably fixed delays of 150 to 200 micro seconds with jitter parameters of less than 5 -10 micro seconds not including speed of light issues.
With routers you will need to turn buffering off and you will still have propagation in the double to triple milli-seconds range with jitter in the multi milli-seconds range.
With ATM you setup your VPI/VCI combinations and with current ATM equipment look at QOS for both the overall VPI and different VCCs. For widely disparagent QoS parameter sets, setup a separate VPI for each parameter set.
For an ATM circuit you can setup either a uni-directional channel or a bidirectional pair of channels with either symmetric or asymmetric bandwidth. You can setup either a point to point or point to multipoint configuration. With current ATM switches you can setup one VPI/VCI channel for a single channel distribution until you get to a common switch closest to the customer where that switch can replicate the ATM cell stream to as many output ports as required to minimize duplicate streams of data in the network.
Having been one of the MPLS “experts” as the vendors were building the equipment trying to do ATM style of QoS at layers three and four does not work. IP needs to enhance the PPP (point to point protocol) that runs on serial links such as POS to Muti-channel PPP. Multi-link PPP was developed to bond smaller channels together to have a larger one. MCPPP needs to implement SAR for different size packets to be able to run VoIP and large 2k, 4k, 8k and 16k packets on the same serial link. Several major router vendors have implemented their routers as large cell switches with packet SARing at input and exit. They have also implemented proprietary inter router links that keep the packets in cell form until they leave the companies routing domain. These cell switching routers use larger cell sizes than ATM but the switching is extremely fast with most of the qos and routing table functions occurring on the interface cards in either standard or custom network processors.
Being an “Enganeer” I want to apply the right tool to the right job. Tools come and go.
First there was point to point and then circuit switched. Then packet route, point to point rings, broadband circuit switched, broadband packet route and switch, fiber based point to point with WDM and DWDM.
ATM gave us zero hop routing of IP packets from ingress to egress Ethernet.(who cares about the overhead since typical IP packets have more overhead in them than ATM cells) Fiber DWDM with ROADM and the like will give us zero hop optical routing of IP packet streams from ingress IP aggregation point to egress point. This will probably use GMPLS with the L being a lambda not a label but who knows.
I plan to design a hub and spoke WAN using ATM. The data traversing the WAN is US equities market data. Market data can be in two flavors multicast and TCP client/server. Another facet of market data is it is bursty in nature and is very sensitive to packet loss and latency (like voice). What type of ATM AAL format would be best for this topology? Is there any other concerns I should be aware of. Thx, Philip __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com