Frame Relay and IP
An Easy-to-Follow Guide to Saving Money with Satellite Technology
March 2005
(courtesy of ND SatCom)

Introduction

The tremendous growth of the Internet is changing the telecommunication world. IP Protocol has become very popular and is more and more used as transport protocol supporting all services: data, voice and video. When considering the use of IP protocol for satellite communication, especially for real time applications, two aspects have to be taken into consideration:

• While in terrestrial networks the bandwidth in backbone systems is with hardly any limitations, the bandwidth in satellite communication is and most likely will remain, a rare resource which has to be used as efficiently as possible.
• There are still some shortcomings especially regarding Quality of Service when using IP technology in WAN or even public environments.

On the other hand Frame Relay Networks via Satellite have been built and brought into service very successfully since many years. These networks are able to support data, voice and video with high quality and reliability. The following discusses the characteristics of Frame Relay and IP and how they may interact with each other.

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Introduction

Since 1998, the satellite industry has suffered from a series of major systemic problems involving a number of different satellite buses and manufacturers, most notably the Boeing 601 and 702, which suffered from systemic problems with solar arrays (702), on-board processors (601), and ion propulsion subsystem (601). These failures have created an operational and financial crisis in the satellite industry. Operationally, the repeated loss of satellite capacity (in shortened life spans and the partial or total loss of on-orbit transponder assets) has caused immediate problems for satellite operators as in the loss of Galaxy 4 in May 1998, which affected millions of pagers across the United States. In broader view, problems such as those on XM Satellite Radio’s Boeing 702 satellites have damaged business plans and threatened some operator’s financial survival. Operators have not only lost income in the immediate aftermath of such incidents, but have also seen expenses, in the form of satellite insurance rates, rise precipitously, noticeably increasing the cost of replacement satellites. In this environment, satellite reliability has become an issue of paramount importance. In this study, Frost & Sullivan assesses the reliability of the various commercially available communications satellite buses and considers the nature of the recent failures and their affects on the space insurance industry.

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Frame Relay

Frame Relay is a layer 2 transport protocol. It was established in the early 90's as the successor of X.25 networks. Stripping off layer 3 functionality has dramatically increased the throughput and provides high bandwidth up to 2 Mbit/s or even 34/45 Mbit/s for WAN data services. In the meantime FR has evolved to an efficient and reliable protocol supporting multiple services including voice and video Frame Relay technology relies on the availability of underlying high quality transmission systems with error rates < 10-6 and the availability of end to end protocols in the end user equipment.

The Frame Relay packet consists of a 2 byte header for address and control information. Permanent Virtual Circuits are established between the sites. Using the DLCI, see figure below, the packets are switched to the destination.

There are two types of mechanism to minimise, detect and recover from congestion situations:

• Discard Eligibility
• Explicit Congestion Notification

A CIR (Committed Information Rate) is allocated to every connection guaranteeing a minimum throughput. If the CIR rate is exceeded the DE (Discard Eligibility) bit is set. In case of congestion it is allowed to discard those packets. However it is also possible to set up SVCs (Switched Virtual Circuits) for the duration of a connection but this functionality was only implemented in very few networks.

Explicit Congestion Notification is performed by setting the FECN (Forward Explicit Congestion Notification) and BECN (Backward Explicit Congestion Notification) bits. In case of congestion the concerned nodes and CPEs will be informed to reduce the data rates sent towards the network.

Figure 1: FR Protocol

Since Frame Relay technology resides at OSI layer 2 it is transparent for any layer 3 protocol. One of the most conveyed protocols by Frame Relay is IP. However other protocols are also supported such as SNA, DECNET, IPX and Binary Synchronous Communications (BSC).

Real time services can be supported very economically and with a high quality via Frame Relay networks. Frame Relay Access Devices offer high quality voice coding with a bandwidth of 8 kit/s or even only 5,6 kbit/s.

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IP Protocol

IP is a network layer protocol (layer 3) with an end to end address. The TCP/UDP/IP protocol family was invented in the early 80's to provide a very reliable and robust protocol suite since it was primarily intended for military purposes. Since then it has become the network layer protocol No. 1 in the LAN and corporate environment. Due to the enormous success of the internet it is also used for WAN and public services. Together with protocols such as RIP, OSPF and BGP routing is performed throughout the networks.

The IP protocol is connectionless, no connection is set up, no bandwidth is reserved. There is no guarantee that a packet will arrive. The packet is sent with a unique address. It is routed through the network based on its IP address to its destination node. If one link fails the packet can use an alternative link. The TCP (Transmission Control Protocol) consists of a Flow Control Mechanism and checks if the packets are in the correct order and have all arrived. If not, the packets have to be retransmitted. The TCP protocol is used for data applications only, for real time service: voice and video UDP (User Datagram Protocol) is used, see figure below. In UDP there is no flow control and retransmission since this would take to much time and would lead to unacceptable performance of the voice and video transmissions.

Figure 2: Protocol Stack IP Applications

To cope with the requirements of real time traffic with a guaranteed QoS via wide area networks a number of additional real time protocols have to be introduced. Two different approaches are currently under investigation by the Internet Engineering Task Force (IETF):

• INTSERV (Integrated Services) and RVSP (Resource Reservation Protocol)
• DIFF (Differentiated Services)

INTSERV explicitly reserves resources i.e. bandwidth. RVSP is used as reservation protocol between sender and receiver for per-flow signalling. It has to be supported throughout the network by any Router. Since every flow has to be managed and traced by a Router the scalability to larger networks is quite limited.

DIFFSERV offers several quality classes. Traffic entering the network domain at the edge router is aggregated and classified according to it's service class for consistent treatment at each transit router inside the network. The eight-bit Type of Service (TOS) field is used to allocate the service classes, see figure below. The DIFFSERV group of the IETF has renamed this field to DS (Differentiated Service).

Figure 3: IPV4 Protocol Header

Due to the lower complexity DIFFSERV has been chosen today in most network installations. However this approach is not able to ensure high quality calls in all cases. Within a DIFFSERV network there are no reservations for dedicated connections. Therefore the assignment of IP packets to certain service classes can not be guaranteed throughout the network in case of increased traffic or even congestion.

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Voice over IP

The IP protocol has become the defatco standard for the telecommunication and IT network infrastructure. IP networks are supporting multimedia applications as IP Video streaming and Voice over IP is becoming more and more wide spread. However there are still some issues which need to be addressed:

• Due to the massive rollout of fibre networks there are no bandwidth limitations in terrestrial backbone networks. This facilitates the support of voice since under these conditions congestion and jitter is very unlikely. However these conditions do not always apply. Available Bandwidth and the quality of networks may vary tremendously from the backbone network to the access network and the Local Area Network within the customer premises. In terrestrial networks service quality suffer from the fact that networks with different quality will be interconnected to provide a service for wide area network. This chain is only as strong as the weakest link. Satellite can offer one stop shopping but suffer from the fact that bandwidth is rare and expensive. Means are required to ensure that the required bandwidth is available for the duration of the call. The introduction of DIFFSERV is a first step in that direction.
• Since voice is very sensitive to delay variations (Jitter) and packet loss additional measure need to be taken to offer high quality voice via an IP based network. The reservation of dedicated bandwidth for each call is required to preserve the quality of the connection in case of competing applications with the same rare resource e.g. parallel calls, file transfers etc.. Today there is no unique standardised signalling procedure available. Various signalling protocols are still competing: H.323 / SIP / MGCP. It is up to the users choice which kind of equipment he will use. It has to be mentioned that the use of a dedicated signalling protocol will introduce a call set-up time of 3 - 5 seconds since a phone has to contact a central gatekeeper via the satellite link.
• As outlined below the overhead for voice is quite significant. At least 40 bytes are required per voice packet. Using the G.729-A speech code the basic frame size is a 10 bytes packet with a 10 ms sample period. Typically one combines two voice samples within a packet which results in a bandwidth consumption of 39,2 Kbit/s via Ethernet. This ratio can be improved by increasing the frame size of 60 ms which reduces the bandwidth consumption to 16,8 kbit/s. However there is a trade off since this approach introduces 40 ms additional delay. Considering the end to end propagation delay of 260 ms via satellite this additional delay might be acceptable. An alternative method could be Header Compression which may reduce the complete header to few bytes.

Figure 4: Encapsulation of voice via IP

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FR versus IP

Frame Relay technology offers distinct advantages for wide area networks.

• Frame Relay is a more efficient WAN protocol than IP, using only five bytes overhead versus 20 for IP. In addition if offers easy and efficient switching capabilities with high reliability.
• It offers a QoS guaranteeing Committed Information Rates (CIR) per user and connection. This enables the operator of a network to dimension the network properly. BECN (Backward Explicit Congestion Notification) and FECN (Forward Explicit Congestion Notifications) are implemented to inform the senders in case of congestion to reduce their rates accordingly. If not traffic will be discarded when exceeding the CIR. These procedures allow full use of bandwidth capacity.
• It has been proven that Voice over FR achieves toll quality in a very cost effective way. The current standard for voice compression is ITU G.729, it delivers an exceptionally high level of voice quality and uses only 8 kit/s of bandwidth.
• Since Frame Relay supports reside at layer 2 it support all network layer protocols e.g. DECNET, IPX, SNA and of course IP itself. IP is the most used protocol across Frame Relay.
• Furthermore Frame Relay has been designed to optimise the performance of several applications which include

• Enterprise Resource Planning (EPR) applications e.g. SAP, Oracle and Baan.
• SNA applications
• Binary Synchronous Communications (BSC)

On the other hand IP traffic has become the protocol of choice for all end users applications such as Web access, E-commerce, Mailing (E-mail, Video Mail, Voice Mail), Video Conferencing, Voice over IP, Teletraining, Telemedicine and Video Streaming. Corporate IT infrastructures are based mainly on IP technology. This makes the support of IP mandatory.

The figure below shows a comparison between IP and Frame Relay.

Figure 5: FR vs. IP

As the comparison shows each technology has it's strength. The best approach might be to jointly use FR and IP in order to offer best of both technologies to fulfil customer requirements.

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Solutions via SkyWAN®

SkyWAN®, a MF-TDMA VSAT system, is aimed at building innovative and cost effective satellite networks to address the many requirements corporations have in today's environment where traditional terrestrial solutions do not suffice.

The system allows high speed, hubless communication between geographically scattered site all over a country, a continent, or beyond. Through the fully dynamic bandwidth allocation scheme, space segment capacity is automatically assigned to a station requiring transmission capacity.

Since SkyWAN® provides Frame Relay data transport and switching functionality as well as IP Routing functionality its up to the end user choice which type of service he prefers. Most legacy applications such as Radar, SCADA, Bit transparent and HDLC protocols are transferred best using Frame Relay. Typical IP applications such as File Transfer, Mailing, Web access, IP Video or Voice over IP will supported via the Ethernet interface with Routing and TCP Acceleration capabilities.

SkyWAN® is able to offer various service and priority levels for real time and non real time services. In particular SkyWAN® provides toll service quality for voice either via Frame Relay or via IP. This is accomplished with a dedicated real time queue and the support of dynamic call set-up, bandwidth allocation and preservation for each call for both technologies in single platform: SkyWAN®.

Figure 6: SkyWAN® Network Architecture

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Conclusion

In order to fulfil customer requirements the question is not whether or not to use FR or IP; but in a given situation which technology suits best. For legacy protocols and traditional analogue or digital voice installations Frame Relay has still some advantages in terms of efficiency and the observance of quality requirements. Business customers expect the fulfilment of Service Level Agreements (SLAs), the guarantee of certain quality of service parameters. Although IP standards and protocols have become mature there is still some work to be done in the IP community to be able to offer services at the same quality level. So it is up to the user choice which technology he finally uses. SkyWAN® supports both in an yet unmatched way.

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ND SatCom AG
P.O.Box
88039 Friedrichshafen
Germany

tel: +49 (0)7545 939 0
fax: +49 (0)7545 939 8701
email: info@ndsatcom.com

http://www.ndsatcom.com

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