O ye of little packets (or cells), if you are going to
complain about the question at least answer it.
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.
Time to
get off my White hobby horse and go back to
work.
If you have any questions e-mail me directly.
John (ISDN) Lee
I Still Do not kNow
Its Suites Dennis's Needs
Philip Lavine wrote:
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
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