Advertisment

NETWORKING

author-image
VoicenData Bureau
New Update

Frame Relay, ATM, IP, …which one is for you?

Don’t ask that outdated question. Especially, when they can together do wonders for

you. Doubts…? This paper from networking industry leader Cisco, tells you why.

Advertisment

Every few years the networking

vendors’ marketing people revive a concept that has been around since the beginning

of time, or at least the beginning of networks: they lock onto a promising new technology

as the multiservice networking solution.

Ideally, a multiservice network is able to

run applications of differing performance characteristics with a minimum of compromises.

For example, voice requires minimal bandwidth, but is highly delay-sensitive. File

transfers require large amounts of bandwidth, but are relatively delay-insensitive. Said

another way, a multiservice network provides the ability to integrate all of your

applications into a single network, with minimal performance compromises. The idea is that

a single network is cheaper, easier, and more efficient to operate than multiple networks.

What began a number of years ago with Frame

Relay, continued with Asynchronous Transfer Mode (ATM), and now focuses on IP, is the

concept that there is one truly multiservice network technology that will efficiently

operate over low-speed and high-speed lines, transporting voice, video, e-mail, and SNA.

You really can’t blame the marketers; after all, sales people are always looking for

that one "golden bullet" that will seal the deal, and it is the job of marketing

to provide it. But in this case, the golden bullet is more accurately described as the

"golden Swiss army knife". But we’ll get to that.

Advertisment

The problem is that we get stuck on the

"golden bullet" idea. People get caught up in a particular technology and rely

on that favourite to the exclusion of nearly everything else. UNIX programmers are nearly

religious about their particular flavour of operating system (and the same could be said

for the Microsoft NT backers). Apple users have almost a siege mentality against Windows

users. In networking, Ethernet versus Token Ring arguments continue even today, and IP

bigots regularly battle ATM bigots over which technology provides the best integration

hopes. So the idea that there should be a single multiservice network technology is not

surprising, nor is it new.

Let’s take a look at three

technologies that have carried the multiservice title: Frame Relay, ATM, and IP. Each has

its strengths and weaknesses, but each also plays an important role in the multiservice

network.

History of Multiservice Solutions



About seven years ago, Frame Relay came into the market and with it the hype as

the newest data, voice, and video integration technology. Frame Relay was developed in the

early nineties to take advantage of the newer digital networks that were being installed.

These networks had much lower error rates than the analog networks they replaced, and the

thinking was that all of the correction features of X.25 were no longer needed. Most

networking vendors support Frame Relay and nearly every service provider does as well.

Advertisment

As a multiservice networking technology,

Frame Relay has not lived up to the original hype. While a number of vendors have been

successful selling Frame Relay Access Devices (FRADs, which combine nearly any data

application into Frame Relay) and voice-over-Frame Relay solutions, standardization

efforts on critical applications such as voice and fax lagged behind market requirements.

Priority standardization also remains elusive, as does the ability to easily transit

multiple Frame Relay networks seamlessly.

The largest contributor to Frame

Relay’s ability to merge data and voice applications is the recent development of

Service Level Management (SLM) and the next generation of SLM, which includes traffic

shaping. (Traffic shaping is the metering of traffic from the LAN onto the WAN based on

policies or traffic profiles that have been defined.) When SLM systems support voice, we

can expect to see greater gains in Frame Relay multiservice networking. Had these tools

been available five years ago, we might have found the "golden bullet" in Frame

Relay.Although the market has definitely shifted

its focus to IP, Frame Relay continues to grow at an impressive rate, between 20 - 40

percent a year by most estimates. It’s a low- to high-speed, mature technology that

is simple to deploy and supports basic Quality of Service (QoS).

bgcolor="#E9E9E9"> box.gif (345 bytes) hspace="4" vspace="4" align="left">Put together IP, ATM, Frame

Relay, combine their strengths and minimize their weaknesses to create a scalable,

interoperable, high-performance network that suits most enterprise networking needs
Advertisment

ATM was the next technology to be

given the mantle of world’s best integration technology. The main benefit of ATM is

that it promises five different classes of service to meet the needs of multiple

application types, including the ability to mimic circuit switching (something Frame Relay

does not do). Each service class defines individual performance characteristics, ranging

from best effort (Unspecified Bit Rate or UBR) to highly controlled, full-time bandwidth

(Constant Bit Rate or CBR).

alt="The multiservice network can be seeen as overlaying layers, with users connecting to the layer that offers them the most appropriate service"

align="right" hspace="4" vspace="4">It is this ability to

discriminate between applications and their respective performance requirements that makes

ATM such a useful multiservice networking technology. In effect, ATM gives us a mini

"golden Swiss army knife" at the core of our network.

ATM benefits from extensive service-level

definitions that are required by a variety of applications. Currently ATM is the only

common transport technology capable of imitating other services such as a full-time Time

Division Multiplexing (TDM) circuit, best-effort Variable Bit Rate (VBR) transport, and

controlled delay/loss services.

Advertisment

But ATM is not the "golden

bullet". A drawback to this technology is the overhead imposed by ATM framing and the

bandwidth that it consumes on lower-speed lines (less than T1/E1). Five bytes of overhead

are used for every 48 bytes of payload (commonly referred to as the "cell tax").

ATM also pads a cell with blank bytes if there is not enough data to fill all 48 bytes.

Very few voice or data applications segment the bit stream into neat 48 byte segments, a

situation that potentially creates additional overhead in the form of the cell padding.

IP is the latest technology to bear the

multiservice title. It is getting harder to avoid all of the hype surrounding IP, even if

you are not associated with the telecom industry. Service providers are touting their

voice-over-IP calling card services, and nearly every vendor has some sort of IP

positioning statement for voice, video, and SNA. Users are inundated with vendors claiming

to have ideal, IP technology-based solutions to the integrated communications problem. In

fact, they may be right. IP is the only technology that is popularly deployed all the way

from the desktop to the WAN core. But an awful lot depends on other choices the user

makes.

The reality of generic IP is that real-time

applications often do not work well because of variable queuing delays, congestion losses,

and the inability to share bandwidth on a particular link between applications with

different performance requirements. IP networks, as originally conceived, offer

"best-effort" QoS. Mission-critical applications, including voice, need much

more.The largest hole in IP QoS implementations

is the ability of the transport network (probably Frame Relay or ATM) to recognize what

service was required from the applications it is carrying. The usual scheme involved

mapping particular applications manually into specific service levels (DLCIs or a

particular ATM QoS). Current standards efforts rely on the use of IP’s socket

information to identify applications automatically. Policy-based networks should use the

Type-of-Service (ToS) bits (Diff-Serv) to signal desired class of service. An additional

emerging standard, called Multiprotocol Label Switching (MPLS), promises to take IP QoS to

the next step by using labels on each packet to indicate service attributes such as

different service classes that utilize specific queuing schemes.

Advertisment

Solution: The Golden Swiss Army Knife



SNA managers, who frequently carry the weight of their corporation on their

shoulders, have reinforced the idea that there is not one all-encompassing standalone

technology. They are willing to pay the rates for leased-lines (by some estimates, more

than 60 percent of SNA networks are still on leased-lines), as well as operate parallel

networks, in exchange for guaranteed QoS.

In today’s multiservice network,

technology agnosticism provides "the golden Swiss army knife". Sometimes you

need ATM, sometimes Frame Relay, and sometimes, even IP. Each network provides its own

service benefits, as well as cost, feature, and performance compromises. The best solution

today for an integrated network that supports data, voice, and video is, well, an

integrated one. The most successful multiservice networks use a combination of

technologies that have been around for years: ATM at the core, Frame Relay and IP as

low-speed access, and IP performing application integration functions.

size="2" color="#FFFFFF">Application Areas for ATM Service Categories
Advertisment
color="#FFFFFF">Application Area color="#FFFFFF">CBR color="#FFFFFF">rt-VBR color="#FFFFFF">nrt-VBR color="#FFFFFF">ABR color="#FFFFFF">UBR Critical Data *** * *** * NS LAN

Interconnect/LAN Emulation
* * ** *** ** Data

Transport/Interworking



(IP-Frame Relay-SMDS)
* * ** *** ** Circuit

Emulation-PABX
***   ** NS NS NS



PSTN/ISDN

Videoconferencing
*** NQ NQ NS NS Compressed Audio

* *** ** ** * Video

Distribution
*** ** * NS NS Interactive

Multimedia
*** *** ** ** * Scoring:

***Optimum   **Good   *Fair   NS: Not Suitable

  NQ: Not Quoted–Are felt presently not applicable with advantage (might be

applicable in the future)

Source: The ATM Forum

Don’t be mistaken:

"combination" does not imply "compromise," and settling is not part of

the bargain. These technologies are maturing into valuable role players in the

multiservice network. And while some standards work remains to be done, we can see the

light at the end of the tunnel.

Policy-based networks should use the ToS

bits (Diff-Serv) to signal the ATM or Frame Relay network of the desired class of service.

This linkage between application type and network service is critical to the operation of

multiservice networks. Having this linkage for both Frame Relay and ATM enables network

users to get the most out of multiservice networks, without having to compromise

efficiency or performance.

Using other technologies such as IP or

Frame Relay to aggregate traffic can help to alleviate the partially filled cell problem

and also create greater efficiencies at lower speeds. The strengths of ATM, as well as its

weaknesses, help its position in the multitechnology, multiservice network. The use of

Frame Relay and IP as access to ATM at the core of IP networks supports the

multitechnology, multiservice network proposition.

Put together, these technologies combine

their strengths and minimize their weaknesses to create a scalable, interoperable,

high-performance network that suits most enterprise network needs—and in this

industry, most is not a bad thing. Just as in politics, a majority usually amounts to a

winner.

IP’s Dual Personality



We seem to use confusing terminology when we talk about multiservice networks.

Vendors and service providers are actually doing us a favour by referring to these

networks as IP networks. IP is the technology most visible to the end user, and is really

the technology that gets the ball rolling. IP supports voice, video, SNA, SAP, e-mail,

Web, and most anything else you care to put over it. The rest is just transport—which

may sound simple, but it’s the details that will get you.

Because IP is doing such a terrific job of

getting everything together, it is now up to the network to get that data from one side of

the network to the other—in such a way that the data is still valuable or usable when

it arrives. This is where Frame Relay and ATM come in. By our estimation, nearly 60

percent of IP access lines actually run Frame Relay at the network level. And even IP

networks take advantage of ATM at the network core. This is not heresy, but smart

application of technology.

Where all this takes us is pretty simple.

The idea that we can integrate our data, voice, and video applications into a common

infrastructure just makes sense. The technologies that we will use to accomplish this are

not new, and the word technologies—plural—is key. There is not one "golden

bullet" that will do what we want. It has been tried with Frame Relay, ATM, and even

IP. We can, however, take advantage of the best of these technologies without compromising

on important scalability, efficiency, and performance requirements.

Advertisment