Frame Relay: Trends to Watch. High-speed frame relay provides a viable alternative to end users who are not ready to commit to ATM services
Mark Kaplan [NN], May 1998
telecoms-mag.com
One of the truisms of data communications is that the need for speed is
ever-increasing. People want things to happen faster than they did before
and that demand expands to the limit--and then beyond--the capabilities
of the available mechanism.
From its initial public service offering in 1991, frame relay has become
the data technology of choice for organizations around the world that
need to implement networks at speeds of T1/E1 and below. According
to the 1997 Frame Relay Market Study by Distributed Networking
Associates in Greensboro, N.C., there are now approximately 25,000
frame relay users worldwide with a total of about 270,000 ports in
production. Close to 70 percent of these ports are at the rate of 64 kbps
or below (DS0).
Bandwidth Drivers
Demand for increased bandwidth is emerging from both the residential
and business communities. New-generation applications such as
audio/video clips and interactive multimedia shopping and gaming sites
appearing on the Internet are gaining rapid acceptance with consumers.
We are in the midst of growth in both the density and volume of
applications, as well as geometric growth in the sheer numbers of people
accessing these sites. This will continue to grow as the new generation of
high-performance, low-cost PCs are snapped up--each with a 33k or a
56k modem--and these new users get on-line.
Inside the Internet, service providers are deploying high-speed
backbones to reduce congestion. Increasingly, users are demanding
56-kbps, T1, and even T3 connectivity for their Web servers. To keep
pace with these pressures, Internet service providers are turning to frame
relay to provide high-performance, cost-effective solutions to their
customers.
At the enterprise, the growing demand on corporate information systems
is creating the need for yet more bandwidth, especially in high-traffic
areas of the network. The richness of the content within the enterprise
network continues to consume all available bandwidth. As corporations
become geographically dispersed, users are increasingly less content with
WAN-speed bottlenecks and are looking for near-LAN speeds to all
information servers within the enterprise network. Additionally, the sheer
volume of data exchanged daily between major computer centers within
the enterprise is driving many corporations to look at trunk speeds in
excess of 2 Mbps.
In recognition of this market-driven need for greater capacities and
speeds, the Frame Relay Forum recently amended the User-to-Network
Interface (UNI) FRF1.1 and Network-to-Network Interface (NNI)
FRF 2.1 implementation agreements (IAs) to meet users' demands for
access speeds up to 45 Mbps (DS3). The Frame Relay Forum is
currently developing an implementation agreement for multilink frame
relay (MFR) to address needs for capacities between T1/E1 and T3/E3.
MFR is a software-defined means of inverse multiplexing several
low-speed links to act as a single higher speed link. For example, a site
requiring a 6-Mbps access link could tie together four T1 links (or three
E1s) to create a logical 6-Mbps line at a cost below that of installing a T3
access line, depending upon local tariff structures.
The major frame relay switch vendors currently support frame relay
access and trunk speeds up to and including DS3 (45 Mbps). 1997 saw
some vendors announce support for OC-3 frame relay, with higher
speed support expected to come to market in the near term. Multilink
support will rapidly follow the adoption of the standard.
ATM and Frame Relay
Most carriers are offering ATM (asynchronous transfer mode) services in
parallel with their frame relay services with connection points between the
two. In fact, some frame services are offered with a higher speed ATM
service as the core transport mechanism between the frame relay switch
points. It is important to understand the two types of interworking
available. The first is network interworking (NIW), which can be thought
of as encapsulation or tunneling frame relay frames through the ATM
network to interconnect two (or more) frame relay attached devices.
With NIW, the variable length frames are segmented and packaged into
the payload of the ATM cells without disturbing the frame header
information. The increase in overhead is offset by the higher switching
speeds and the larger trunks interconnecting the ATM switches.
If NIW can be thought of as tunneling, service interworking (SIW) can
be described as a translation service between frame relay and ATM.
Here, by mapping the frame header information into the ATM header, a
frame relay device can establish communications with an ATM device.
Together, NIW and SIW foster the coexistence of frame relay and ATM
and allow users to choose the technology that best meets the traffic
requirements and budget allowance of each site.
ATM vs. Frame Relay
Frame relay was initially designed to provide transport for the delay of
insensitive data sent by higher level applications capable of recovering
lost or dropped frames. Frame relay has no inherent frame correction
mechanism; errored frames are simply discarded and higher layer
protocols determine what frames need to be re-sent. Since the frames
vary in length and traverse the buffers and matrices of the switches, delay
across the network is of a non-determinative nature. While prioritization
mechanisms are sometimes employed to provide differential services, to
date there are no true quality of service (QoS) levels available from frame
relay networks based on standard metrics. The Frame Relay Forum and
ITU-T (International Telecommunications Union-Telecommunications
Standardization Sector) translation are attempting to address this issue
during the coming year.
In contrast, ATM was built from the ground up to provide differential
QoS levels (e.g., constant bit rate, variable bit rate, available bit rate,
unspecified bit rate) with consistent characteristics (at least for the
constant bit rate and real-time variable bit rate service levels). Utilizing a
fixed-length payload of 48 bytes and a 5-byte header, ATM switches are
able to provide highly determinative service classes each optimized for
specific types of data and applications.
The question then is which technology to use for a given network?
Frame relay is historically very good at transporting data which is not
highly dependent upon precise delivery intervals. Examples of this are
typical client-server database queries, e-mail and file transfers, and
broadcast video applications. ATM has typically been tagged as the
transport of choice for delay-sensitive information such as interactive
video and voice. However, recent enhancements to the basic frame relay
service have tended to blur some of the distinctions between the two.
FRF.11 has defined standards for carrying voice over frame relay.
FRF.12 has defined fragmentation issues to create a more determinative
delay pattern to the frames by chopping up the large data frames into
smaller pieces to better match the size of the voice frames; thus it is able
to minimize frame delay variation through the network.
So, the choice of which technology to use where depends on the
character of the predominant traffic. If your network will be transporting
a high degree of voice traffic and/or near-broadcast quality video, ATM
is most likely the better choice due to its ability to support constant bit
rate (CBR) traffic as well as inherent broader bandwidth (currently up to
OC-12, or 622 Mbps, with plans to extend to OC-48).
If the dominant traffic type is non-delay-sensitive data (which could be
voice, video, fax, imaging, multimedia, file transfer, or e-mail), frame relay
is a better choice even if bandwidth in the range of 45 Mbps is required.
Frame has less overhead than ATM, is more readily understood by most
networking professionals, and is easier to install, and frame equipment
and services are generally less expensive than ATM devices and
services.
Topology Drives Speed
Many frame relay adopters have deployed their networks in a star
topology similar to previous leased line networks. This has allowed users
to realize an immediate benefit of frame relay--primarily lower cost. In
this configuration, one location (typically the corporate computer site)
receives traffic from numerous branches or remote locations. Since a
large number of relatively lower speed locations are concentrated for
delivery to this site, the bandwidth required at the central site is set at
some percentage of the total bandwidth of all remote sites.
Initially, customers ordered 64-kbps access (DS0) for remote office
locations and 1.55 Mbps (DS1) for the headquarters site. When traffic
or the number of locations increased and signs of congestion became
apparent at the headquarters site, customers were relegated to ordering
another DS1 facility (either as a separate DS0 or as part of an NX64
service) from their service providers, which increased access cost and
required incremental hardware (ports) and customer premises equipment
(CPE). Now these same customers can obtain service at speeds up to
45 Mbps without requiring incremental hardware. The individual CPE
may need replacing or upgrading but additional devices are typically not
required.
The need to integrate or replace with ATM may not be required in this
type of network based on the mix of applications and services offered.
No gain would be realized by moving traditional or legacy applications to
an ATM network despite the promise of greater bandwidth than
available via high-speed frame relay.
High-speed frame relay provides a viable alternative to end users who
are not ready to commit to ATM services. Standards-based solutions
exist today for access at DS0, DS1, and DS3 rates. The Frame Relay
Forum is close to agreement for an IA for MFR for rates between T1
and DS3 in increments of either 1.544 Mbps (T1) or 2 Mbps (E1). With
support on the horizon for SONET rates beyond 155 Mbps, frame relay
insures that your investment in equipment and services can be maximized
into the future.
Mark Kaplan is the senior marketing manager for frame relay
products at Newbridge Networks Inc. He is also the chairman of the
Market Development and Education Committee of the Frame Relay
Forum. He can be reached at (703) 736-5792.
[TOP]
|
