Home >> October 2008 Edition >> FOCUS - SatLink Mesh Networking
FOCUS - SatLink Mesh Networking
by Richard Forberg

A promising new generation of satellite mesh networking with greater efficiency and reliability has emerged as a means of achieving low latency connections between two VSATs. Only a single “hop” to the satellite is needed, which reduces transit delays and bandwidth consumption in roughly half the time that was previously required. This certainly moves mesh networking to the top of any implementation list for real time traffic applications such as local telephony in rural areas (e.g. village-to-village).

While mesh networking among ground stations has been available for many years using Demand Assigned Multiple Access (DAMA) methods, which assign capacity for voice or data on a circuit switched basis, this new generation is optimized for packet switching and the Internet Protocol (IP). In these new networks, capacity can be assigned according to rapidly varying data packet loads (e.g. as associated with web applications), as well as continuous packet loads (e.g. as associated with voice or video conferencing over IP), as is used today for star-topology VSAT networks using MF-TDMA techniques.

The dynamic assignment of satellite bandwidth according to the instantaneous IP packet loads greatly reduces total bandwidth consumption in comparison with the static assignment of circuit switching. In addition, packet headers are also compressed by the VSATs, creating further bandwidth savings.

Extending these dynamic, packet-oriented MF-TDMA techniques in star topologies to mesh networks is far from trivial — many challenges must be overcome. STM has worked through these challenges with its new SatLink mesh networking technology. This is an extension of STM’s current DVB-RCS compliant SatLink product family. Consequently, all core technology elements of SatLink mesh networks also leverage the DVB-RCS standards for the maximum benefit.

Mesh Network Implementation
Today’s certified DVB-RCS SatLink 1910 VSAT for star topologies is easily upgraded to also support mesh networks through the addition of a small plug-on card. This card, plus a software upgrade, affords the SatLink 1910 the ability to receive TDMA bursts directly from other VSATs. The SatLink 1910 VSAT, of course, also continues to receive user traffic, network clock, and burst plan information from the SatLink hub over its DVB-S2 compliant TDM forward link and communicates back to the SatLink hub using DVB-RCS compliant MF-TDMA return links. What’s new is the ability to be in direct communication with many other VSATs over what are called “mesh links,” which also follow the DVB-RCS standards.

Each VSAT can have a large number of concurrently active mesh links. Furthermore, mesh links are established and removed dynamically under control of the hub, as needed, according to the supported inter-VSAT traffic patterns. Mesh links in this packet environment, however, differ from circuit-oriented DAMA mesh links because if there are no packets for a VSAT to send over an active mesh link for period of time, the mesh link consumes essentially zero bandwidth, even though it remains “active” as a table entry.

A key feature of the SatLink 1910 Mesh VSAT is that it can receive multiple concurrent TDMA bursts from other VSATs in the same mesh network. This greatly reduces blocking probabilities, which are common problems in mesh networks where ground stations can receive only one burst at a time. Low blocking probability for burst reception assures good voice quality, lower error rates, and increased bandwidth efficiency for data traffic, as the loss of even a single fragment of a TCP packet results in the re-transmission of many TCP packets.

The SatLink Carrier-Class Hub is a high-availability system optimized for IP networking with ACM (Adaptive Coding and Modulation), providing great versatility, reliability, and scalability. In its baseline configuration, it delivers capacity for 2,500 VSATs, fast-failover redundancy in all core components, and growth potential as many as ten forward links and tens of thousands of VSATs. Such makes the system ideal for carrier networks, large ISPs, and large private IP networks, including large carrier VoIP networks.

The SatLink hubs support scalable combined hybrid star and mesh networking and allows the mesh network to be partitioned into smaller mesh subnets only interconnected through the hub. The hub can be used as a shared WAN gateway for these mesh subnets, but they may as well use any of the VSATs for access to the WAN.

In a mesh network, many challenges arise due to a potentially huge number of mesh links. To illustrate, consider a mesh network with only 100 VSATs. Because of the multiple links per VSAT, there are potentially 10,000 half-duplex TDMA links (vs. only 100 return links in a star topology).

The core technical challenge in these new mesh networks is assigning and scheduling traffic onto mesh links dynamically.

The capacity assignment and scheduling process — which is performed by the hub based on requests from the VSAT — is much more difficult with the huge multitude of mesh links. STM has partitioned this problem without losing the bandwidth efficiency gains obtained from dynamic bandwidth assignment. The bandwidth distribution is adjusted every 100 msecs based on the immediate transmission needs for the mesh and return links of each VSAT. The distribution respects different QoS requirements.

Another issue is traffic routing. IP routing is more complex in a mesh network, as each IP packet addressed to any computer on any VSAT in the mesh must be mapped to, and transmitted directly to, the appropriate egress VSAT by the ingress VSAT. In order to handle this problem, each VSAT is required to have its own route table for all possible IP hosts or subnets. With this table, the VSAT will be able to determine where to send an IP packet most efficiently, whether it is to the hub, or to another VSAT.

The table is updated constantly to accommodate for hosts or subnets being added to or deleted from the overall extended network. Unfortunately, the standard routing protocols used to update route tables in terrestrial IP networks introduce a lot of extra traffic. In a satellite network, this consumes large amounts of valuable bandwidth, making standard routing protocols unsuitable for large meshed VSAT networks. STM has solved this dilemma by implementing a customized route table update process that’s been optimized for mesh satellite networks, with very low overhead.
Tracking the performance of mesh links is a related issue also resolved in SatLink mesh networking. Unlike star topology return links, the hub cannot directly measure the link quality and error rates on the mesh links between two VSATs. Measurements have to be taken by each VSAT and reported to the hub systematically — this information is then fed into the route table update process. This allows each VSAT to countermeasure when a mesh link is down due to heavy rain at a distant VSAT by adjusting routes accordingly.

Due to the high number of links to monitor, regular tracking of the performance of all possible mesh links could be quite expensive in terms of satellite bandwidth in a large mesh network. However, STM has developed techniques to make tracking and collecting network traffic statistics much more efficient. This effort to reduce bandwidth consumption results in a network that operates smoothly at a minimized cost.

Some good applications of mesh networks include voice and video conferencing. Any private network, government or corporate, with distributed data applications to many sites is also an ideal candidate when cost-effective terrestrial connections are unavailable or unreliable. A SatLink mesh network operated on a shared-use basis from a service operator’s hub can also efficiently serve the market for private lines with dynamic capacity between two or more sites for any type of data application.

With the cost-effective nature of today’s VSAT technology for IP networking, this new generation of mesh networks will, over time, replace the traditional and more expensive circuit-oriented DAMA technologies. Just as IP networking is replacing circuit-oriented terrestrial networks, this evolution is inevitable also for VSAT networking.

About the author
Richard Forberg is responsible for STM’s marketing, product planning and strategic business initiatives. He has 25 years of management experience in network systems, services and software companies. He has held senior marketing, product management and business strategy roles at networking equipment vendor 3Com Corporation, distributed computing systems supplier Digital Equipment Corp., plus various high technology start-ups. Mr. Forberg holds a B.S. in Physics from Beloit College, an MS in Physics from M.I.T and a M.S. in Management Sciences & Engineering from Stanford University.