If you weren't aware, Chesapeake continues to lead the way in advanced technical services. In addition to our other technical skill areas, we have a great staff of Asynchronous Transfer Mode(ATM) experts to help our clients, be it consulting or training.
In this issue I'm going to do an overview of ATM to give everyone a common understanding of what ATM does for us. This overview is exactly that, an overview. Each of these subjects has entire standards documents written about it, so if there are questions, please email me tomv@ ccci.com. I intend to cover the different signaling specifications required for Switched Virtual Circuits (SVCs), discuss the public and private addressing structure as it relates to SVCs, and highlight the dependence that ATM has on internationally accepted standards.
What ATM can do for us?
The promise of ATM has always been to do everything for everyone.
Realistically, ATM is a transport method over which users can send a
variety of traffic types. For example, a business could have up to three
separated types of data circuits installed. There would be a data circuit
for Internet connectivity. Then another
circuit for all the voice traffic. And yet another circuit for those weekly
video teleconference sessions with Headquarters. Utilizing the ATM model,
the business can now replace all three existing circuits with a single ATM circuit.
Today, the business would probably send the traffic over a Permanent Virtual Circuit (PVC), due to its more widespread deployment than SVCs. However, in the future, carriers will offer SVCs to maximize both service offerings and profit. The user establishes SVCs only when required. The carrier then sells the same bandwidth repeatedly (called oversubscription) because, statistically, not everyone will want to use their bandwidth at the same time, which should lower the price of the service offering. The phone service does this today. Statistically, not everyone wants to call at the same time. On Mother's Day however, everyone wants to call and lots of people get "All Circuits Are Busy" messages. The cost of the service is cheaper because the phone network is engineered to handle a smaller traffic load, rather than the traffic load it could expect on Mother's Day. A SVC is created by a process similar to a phone call. There is the call setup phase (dialing), the call usage phase (talking), and the call teardown phase (hanging up).
Implementing SVCs requires agreement on a variety of common standards. =46igure 1 highlights the signaling standards that vendors implement for interoperable ATM traffic. The standards fall into two basic categories: User-Network Interface (UNI) and Network-Network-Interface or Network-Node-Interface (NNI). Both UNI and NNI standards must be supported for SVC establishment.
Among other things, UNI defines the required signaling specification between an end user device, like a SPARCstation with an ATM adapter or a router with an ATM uplink, and the directly attached ATM switch. When it wants to establish an SVC, the end user device sends a connection request message to the distant end with which it wants to communicate. This signaling request starts at the user device, and is forwarded through the network of switches until it reaches the final user device. The final user can ideally satisfy the signaling request and sends back a connection acknowledgment message. The circuit is actually established as the connection message is sent back to the requester.
The currently implemented UNI standards are UNI 3.0 and UNI 3.1. Both ends of the individual link must support the same UNI specification. By the way, UNI 3.1 is NOT BACKWARD COMPATIBLE with UNI 3.0. (It has to do with underlying ITU signaling specifications, but it really doesn't matter - they don't speak to each other). As an interesting side note, the UNI spec got so big after 3.1, that they had to break it into three component pieces, Traffic Management 4.0, ILMI 4.0, and Signaling 4.0. So remember that, when you go looking for the latest specifications, there is no UNI 4.0.
Once the signaling connection request message reaches the switch, the NNI forwards the signaling connection request towards the final destination. The NNI, generically, has two functions: signaling and routing. The signaling portion is similar to UNI signaling the NNI passes the signaling request to the next-hop switch that leads to the ultimate destination. The routing portion of the NNI actually determines what port to go out to reach that next-hop switch. The approved standards for NNI today are Interim Inter-Switch Protocol (IISP) and Private Network-Network Interface (PNNI). IISP was first. It uses static routing and UNI signaling. The dynamic routing capabilities for an ATM environment were very difficult to iron out, so the ATM forum approved an interim inter-switch protocol for starters. Currently, businesses that desire to connect switches from different vendors together for SVCs must use IISP. PNNI, which implements dynamic routing in the ATM cloud, is currently only implemented in proprietary solutions. However, interoperable implementations of PNNI are just around the corner.
Figure 2 highlights private versus public ATM clouds.
Private networks have addresses assigned based on organizations, similar to IP addresses. Private
networks use Network Service Access Point (NSAP) addresses. They come in
two flavors, based upon who assigns the address. These are Data Country
Code and International Code Designator. In the United States ANSI assigns
the first addresses while GAO (under the auspices of NIST) assigns the
second address type. All the ATM equipment within a private cloud is
usually administered by the same organization.
Public networks have addresses based on purely geography, like phone numbers. Wide area carriers that provide a public data service (ATT, Sprint , MCI, etc.) use Public ATM addresses. These Public ATM addresses include an E.164 ISDN phone number as part of the address. Public networks have their own NNI standards. The ATM Forum's public NNI is called B-ICI (Broadband Intercarrier Interchange). As far as the public UNI is concerned, the UNI just has to be able to recognize E.164 addresses. UNI 3.0 and UNI 3.1 both meet this requirement. This means that an end-user could connect to a public ATM network using UNI 3.0 or UNI 3.1. To muddy the waters, a private switch (using mostly NSAP addresses) that connects to a public ATM cloud is considered a public UNI connection, because it is a user of public services -even though it is a switch to switch link! The distinguishing factor is that the connection point to the ATM cloud must be E.164 addressable.
This short explanation provides the tools necessary to support ATM SVC circuits. Other issues like widespread circuit availability and pricing must be resolved before ATM SVCs become widespread in the marketplace.
Besides understanding what's required for SVCs, the business must also understand that it has to have the end user equipment that takes advantage of ATM. Things like ATM video boxes, ATM circuit emulation cards, and ATM cell multiplexors. They will cost money, but, ideally, the economies of scale from having only a single circuit will save money in the long run.
FORE Systems and Cisco Systems are the number one and number two LAN ATM market vendors, respectively. Both vendors implement all the interoperability specifications and provide their own added value. Between the two they offer complete ATM product lines. They actually work together just fine; I've had PVC and SVC networks composed of the two vendors' equipment working together without any hassles.
One further thing that helps ATM interoperability is that ATM is based whenever possible on international standards. For example, ATM can operate over 155Mbps links. The only thing required to make European and American ATM switches speak to each other over the same 155Mbps fiber link is agreement on which software framing format they will use. The framing formats in this case are SDH versus SONET, either of which is very easy to configure. The UNI signaling standards themselves are based on ITU signaling specifications. For example, UNI 3.1 uses a subset of the ITU Q.2931 specification. Since the signaling standards are based on ITU specifications, ATM is generally accepted overseas. Compare this with the international acceptance between T1/E1 and T3/E3.
In summary, ATM acceptance is based upon common standards. The major ATM standards body today is the ATM Forum, which bases its standards whenever possible on other established international specifications. There are currently enough standards approved and implemented that allow total SVC interoperability.
Additional Resources The ATM Forum has an ftp site,
ftp://ftp.atmforum.com
that allows you to download all the approved ATM Forum standards. Look in /pub/approved-specs. In addition to the ATM Forum's ftp site, they have a Web site,
http://www.atmforum.com http://www.atmforum.com
that keeps people up to date on what's going on. They also have a specwatch hotlink that gives a summary of the current status of ATM Forum specifications. Both Fore and Cisco have lots of ATM information on their Web sites too:
http://www.fore.com/atmedu/whitep/index.html http://www.cisco.com/warp/customer/769/ls1010.html http://www.cisco.com/warp/customer/729/c5000/c5000_tc.htm http://www.cisco.com/warp/customer/614/12.html http://www.cisco.com/warp/customer/730/LS1010/lsatm_wp.htm
Terms and Acronyms
ANSI - American National Standards Institute
ATM - Asynchronous Transfer Mode
B-ICI - Broadband Intercarrier Interchange
Data Country Code - NSAP address format
E1 - E-carrier system 2.048 Mbps
E.164 - International addresses format
E3 - E-carrier system 34 Mbps
GSA - Government Services Administration
ITU - International Telecommunications Union
International Code Designator - NSAP address format
NIST - National Institute of Standards and Technology
NNI-Network - Network Interface or Network-Node Interface
NSAP - Network Service Access Point
PVC - Permanent Virtual Circuits
Q.2931 - International signaling standard
SDH - Synchronous Digital Hierarchy
SONET - Synchronous Optical Network
SVC - Switched Virtual CircuitsT1=F3T-carrier system 1.544 Mbps
T3 - T-carrier system 45 Mbps
UNI - User-Network Interface