In any distribution system the last leg is always the slowest and most expensive, and so it has proved with data networking. In the case of data, the last mile is sandwiched between two faster parts of the network, that within the enterprise and within the core. Therefore, the last mile well and truly has been the bottleneck, inhibiting broadband applications and making wide area network services much more expensive than they would otherwise be.
Although the last mile will always be a bottleneck to some extent, the problem is at last being eased by a whole raft of technologies and delivery mechanisms. The problem lies in deciding which is appropriate for a given situation. Cost, availability, and need for bandwidth are three obvious decision criteria, but there are also some fundamental distinctions between different service types that make some more appropriate for given situations.
Satellite is a good example, being well suited for broadcast but hopeless for peer-to-peer communications. Once the service is up and running, the only additional cost for each extra user within the transmission range is the receiving equipment. Satellite is therefore ideally suited for multicast and broadcast applications where virtually all the traffic is one way out to the subscriber.
DSL and cable modems services, based respectively on the installed copper telephone wires and the cable TV network, were designed to solve the residential last mile problem, or more literally the last few miles. This remains largely true of cable modems, given that relatively few business offices are passed by the network.
But according to Gartner's analyst Jouni Forsman, DSL is being taken up significantly by businesses in Europe and is likely to be so in the UK as the belated roll out of the necessary exchange equipment by BT proceeds. "Prior to DSL, the main broadband access options for enterprises were leased circuits (either direct fibre or copper), which is expensive. The appeal of DSL to sites that have outgrown lower speed dial up is that it is comparable in performance to leased lines, but much cheaper," said Forsman.
The current generation of DSL services operate at a variety of speeds between 512Kbps and 8Mbps, and are suitable for SMEs and smaller branch sites of larger organisations as well as teleworkers.
At present the only flavour widely available is ADSL which is faster downstream than upstream and best suited for applications such as business TV and high speed Internet access that do not require fast upstream circuits, rather than point to point file transfer or e-mail. But coming soon will be SDSL (symmetric digital subscriber line), which will offer equal speeds of perhaps 1Mbps or 2Mbps in both directions, and so have greater appeal for the business sector, as it does already in the US.
According to Gartner there will be 750,000 DSL lines serving business customers in Europe by the end of 2002, doubling to 1.5 million a year later. But for larger sites, for applications such as disaster recovery, remote storage area networking and server consolidation, DSL is nowhere near fast enough. Here the main interest is focused on the continuing Ethernet saga and the ascent to 10 gigabit speeds.
Although the impending 10 gigabit Ethernet standard will initially be of greater interest to service providers within their exchange points and co-location facilities, it is hugely relevant for enterprises by illuminating the course of networking for the foreseeable future, according to Andy Palmer, EMEA vice president of gigabit Ethernet switch and equipment vendor Foundry Networks. "There is now a clear roadmap forward, making it easier for the wait and see guys to join the Ethernet camp," said Palmer.
As well as operating 10 fold faster, the new 10 gigabit Ethernet standard finally ditches some old Ethernet baggage, such as the collision detection mechanism used to allow two way communication within a single circuit, which was a major constraint on performance and scalability.
Ten gigabit Ethernet is truly full duplex with no collisions, and as such is potentially capable of competing with any other transmission technology for broadband communications. Also, being all fibre as well as no longer relying on collision detection, the old distance limitations are rapidly being eroded. This is allowing the huge economies of scale of the Ethernet community to be brought to bear on the Wan, threatening A TM and SDH/SONET, which have held sway here.
Of greater interest for most enterprises though is the debate between the relative merits of 10 gigabit Ethernet (in conjunction with IP), and another broadband protocol, fibre channel, for future storage area networks (Sans). At this point a little confusion needs to be cleared up. Fibre channel and the current single gigabit Ethernet standard share the same physical layer standard for transmission over fibre, which means that hardware for both has identical electronic components for connecting to the medium itself.
The difference lies in the protocol stack for transmitting data frames between points. Ethernet transmits frames one at a time onto the wire without requiring direct acknowledgement, while fibre channel transmits data in blocks, waiting until the destination switch confirms it is ready to receive. This synchronisation in fibre channel is needed to provide reliable high-speed transmission of data between storage systems and servers.
According to Edward Hopkins, EMEA product marketing manager for gigabit Ethernet system vendor Extreme Networks, 10 gigabit Ethernet in particular is primed to take over the storage mantle from fibre channel, because the essential point-to-point transmission capability is now there.
But according to Paul Trowbridge, marketing director of fibre channel vendor Brocade and also a director of the Fibre Channel Industry Association, such IP/Ethernet storage solutions will not be available until late 2002 at least. And when they are, the additional buffer memory and on board processors of the network adapters needed to support the block transfer will bring the price up to similar levels. So when the Ethernet community point to the lower cost of current gigabit Ethernet adapters compared with fibre channel adapters, they are not comparing like with like.
Broadband: The options
Definitions of broadband networks vary. The weakest definition is any transmission service faster than a modem, i.e. in excess of 56Kbps, which would include even basic rate ISDN. But much more bandwidth, at least 750Kbps, is needed to transmit say VHS quality video, so some people take this as a minimum requirement. The most common definition is somewhere in between, above 256Kbps, which is the upstream capacity of some ADSL services. On this basis, options for metropolitan and access services include:
- ADSL - Asynchronous service with downstream speeds of 2 - 8Mbps although closely to the latter, suitable for video on demand and other multimedia multicast type services.
- SDSL - Synchronous version of DSL better suited to site-to-site applications within enterprises.
- VDSL - Future high speed version of DSL capable of delivering multi-channel broadband services and very high resolution video.
- ATM - Once challenging for end-to-end network supremacy, ATM never really caught on in the LAN and is now threatened with extinction by 10 gigabit Ethernet, being stuck at 2.5Gbps.
- SDH/SONET - The physical layer optical transmission protocol used within core carrier networks, also threatened by 10 gigabit Ethernet in the longer term.
- Gigabit Ethernet - The 1Gbps version of Ethernet, with versions for both fibre and copper within the enterprise, but also for metropolitan area services at distances up to about 15 Kms.
- 10 gigabit Ethernet - 10 gigabit version of Ethernet, for fibre only, initially aimed primarily at metropolitan area networks with a range of up to 40 Kms. Future versions will sustain greater distances and also reach down into the enterprise.
- Fibre channel - The block data protocol used for transmission among storage systems, and between them and servers. Initial speed 1Gbps, but extended to 2Gbps. Future versions likely to share the 10Gbps Ethernet physical transmission layer, just as happened at the single gigabit level.
- iSCSI - Emerging protocol for high speed transmission of storage data over IP and Ethernet.
- FC IP - Emerging protocol for carrying fibre channel protocol over an IP backbone for long distance storage traffic.
- Infiniband - Super gigabit protocol (can scale up to about 32Gbps with current specifications) for inter-server communication. Connection between Infiniband and storage protocols such as fibre channel will be needed.