Bucky,
OK... I've got a minute here.... To refresh, I think our discussion is on the merits of ATM versus IP. I think we generally agree that ATM is an excellent transport protocol and that IP is an excellent edge protocol. Your arguments however suggest edge ATM devices that support IP and I've taken the approach that the edge devices should be "faster" IP devices since this matches existing deployed technologies in ISP's, and is much more flexible, yet can deliver on QoS. So, ball's in my court.
ATM is a connection-oriented protocol. Yes, it CAN guarantee delay in congestion scenarios.
No..sorry. "Congestion in an ATM network is defined as a state in which the components of the network, be it the switches, physical links, or hosts, are not able to meet the negotiated network performance objectives which include (for Qos) peak-peak CDV - cell delay variation, maxCTD cell transfer delay, and CLR cell loss ratio. So, when congestion occurs all bets are off and an ATM network suffers as would an IP network.
There is a connection admission control (CAC) procedure every time a new virtual connection is initiated, and if the connection is granted, then specific performance parameters must be honored for the life of that connection.
You are correct about CAC in that a new circuit (VC) will not be added if no resources exist or if it's addition will affect QoS on other connections, however CAC is re-negotiated by the end-points periodically throughout the connection...and you would want this capability since traffic types would vary thoughout the life of the connection. In IP this is all dynamic...no periodic renegotiation is required.
Furthermore, ATM has dedicated signaling channels, which further isolates an ATM network's performance and management from congestion. With RSVP, signaling is done with UDP or ICMP packets (I forget which), which in the event of congestion are vulnerable to being delayed or discarded.
You discuss this as if RSVP and UNI/PNNI are mutually exclusive...these day's they are not. Both RSVP and UNI/PNNI (ATM signalling) are used to transport traffic specifications and QoS parameters into the network. Both employ admission control before accepting a reservation. But the differences are great. As you continue to point out IP is connectionless and ATM is connection-oriented. IP is a robust and dynamic networking architecture where any changes in topology or resource availability will be addressed by simply rerouting the packets to another path and reestablishing the reservation state. On the other hand, ATM takes the approach that reservations are retained for the duration of the connection (even for SVC's). If, however, the topology changes, then ATM must rebuild the connection from scratch! Some other points:
Mulicast. IP supports bidirectional many to many transmissions. ATM supports unidirectional point to multipoint. This means that there will be a lot of additional ATM VC's required to emuliate the IP mulicast model.
Heterogeneity. Receivers in the ISA model can requirest and receive different levels of QoS. ATM VC's are a single QoS. ISA receivers can also renegotiate QoS levels during a connection. ATM currently requires that a new VC with a different QoS be established.
Routing. RSVP and IP routing are separate. ATM signaling and router (via UNI 3.1/4.0 and PNNI Phase 1) are performed concurrently.
Result...ATM is very rigid and therefore not as well adaptable to network edges where flexbility is required. It however is an excellent transport architecture.
Furthermore ATM is not able to offer multiple prioritization for multiple types of IP traffic.
See above.....requires multiple VC's
.....ISP's have told me....they specifically said RSVP would cause their networks to collapse. RSVP is currently available in the Cisco IOS, and can be turned on for a small licensing fee perhaps. But no one wants it.
Well, I'm not at liberty to comment on this. I'd say simply watch this space.... sorry I can't provide anymore detail right now.
The ATM VC will honor the contract under which it was set up. If a high priority is assigned to the VC (CBR or VBR-RT), then that contract will be honored and there will be no packet loss by the ATM router/switch, congestion or otherwise.
Not true however the CLR (cell loss ratio) can be defined for static network environments. But as switches or network links fail cells will be lost. This will occur more in open loop networks or with equipment that has small ingress VC buffers or with equipment that allocates buffers on a per port rather than a per VC basis. Key point..... cells can and will be discarded at the network entry point if the portend congestion by exceeding a negotiated cell rate. It is left to the network endpoint (read IP devices) to retransmit the packet.
Gary, thanks again for your comments. I don't know if Ascend will knock of Cisco, but they're one of the few threats I see at this point. The other is NT, who are working real hard on some of the same technologies that Ascend is working on.
Well, perhaps ASND has a lead right now...and I expect this will be a great year for good ol' ASND, but I think too many folks on this thread are counting mighty CSCO out of the backbone race. Too soon? Perhaps.... I expect we'll revisit this proposition sometime in the not too distant future.
Cisco is definitely king of networking now, but it's healthy to have a paranoia of your competitors. If I was Chambers, I'd be REAL worried about Ascend now.
He does and he is.... He always says that he's worried and paranoid and I believe him...
They are winning and stealing lots of contracts that he should have won. One day he may wake up and fine his 7500's & 12000's obsolete, replaced by a new generation of ATM router/switches.
Well, you could be right, but I suspect that his plan is to make those devices 8500's <ggg> or perhaps another cisco device that we have yet to see. Remember once again, ASND is definitely ahead with there latest ATM products but CSCO has a great opportunity still to counter. They have lost some contracts, but all is far from lost my friend. And, if I were ASND I'd be concerned about CSCO coming after their last business niche after being batted out of the RAS market.
***gary lunges to the baseline and nails a crossing shot down the line***** :)
OG |
