HYBRID SWITCHING

Hybrid, as the name implies, is a combination of techniques. Various combinations of hybrid switching have appeared in data communications over the years, but have been relegated for the most part to CPE, outside the classical telephone world. In the broader context, IEEE 802.xx LLC (logical link control) and a combination of higher layer Ethernet packets mapped to Internet packets could be labeled a hybrid. However, continued innovation and development emerging after the 1984 deregulation of the long distance telephone business led to newer approaches to transmission and switching such as SONET/SDH, ATM, and other packet- or cell-based technologies. The impact of the Internet Society, in particular the Internet Engineering Task Force (IETF), on global communications standards has been significant to say the least. An example of the kind of innovation that is likely to survive and grow well in to the future is embodied in a technique called multi-protocol label switching (MPLS) sometimes truncated to simply label switching. MPLS is a combination of techniques derived from IP and ATM switching and protocols.

Other Emerging Switching and Routing Technologies

ASON (Automatic Switched Optical Network), GMPLS (Generalized Multi-Protocol Label Switching), and RPR (Resilient Packet Ring) are in active standards development and various stages of maturity at the time of this writing. A full description of each is far beyond the scope of this book. However, a brief description of each is noteworthy. ASON and GMPLS are often seen as a source of contention and competition between ITU and IETF standards development groups, but this is more an impression gained from reading press reports. ASON is a new architecture that will leverage existing SONET/SDH architecture by adding optical switching and control protocols. The motivation for ITU members and their suppliers lies mainly in reduction of capital investment at the core and operations cost associated with provisioning, while at the same time enabling paying customers to use the network in new and innovative ways. Recognize that this work is heavily biased by ITU due process in which the focus is working from well-defined detail requirements for architecture. The basic approach is to agree on the architecture and then work out a set of underlying control protocols, and don’t forget that the background and experience of the developers is heavily influenced by years of operating TDM networks controlled by CCS7/SS7 and DTMF signaling.

GMPLS is an initiative that grew out of effort directed at extending MPLS traffic-engineering techniques. Recall that MPLS as a concept is something of a hybrid or a combination of circuit and packet switching. Motivation for this work lies in the simple desire to more effectively and efficiently control the transmission or transport facilities—circuits in the network. How to do this? Simply invent a new protocol that tells the network to ‘‘do this’’ or ‘‘do that’’ based on a new bright idea, almost without regard to practical business considerations.

The main interest driving the work of both groups appears to be some form of standardized control of the multiplexing and transmission layers in the network. Significant work remains to be accomplished before stability and maturity, sufficient to entice suppliers to invest in design and development of real products capable of attracting service provider investment is achieved. Overall, the serial cycles of standards stabilization, product design, and, finally, deployment by service providers may take 2, 3, 5, or maybe 7 or 8 years. For now, the most appropriate action for the user community is simply monitoring events as they occur.

RPR (Resilient Packet Ring) is the most mature of the three. This work is currently under the moniker IEEE 802.17. Essentially, this is an extension of BLSR (bi-directional line switched ring) technology whereby TDM/PDH, or the more common TDM/SONET/SDH, is modified to replace TDM with Ethernet, or something akin to Ethernet packets. Such an architecture would permit more efficient use of the bandwidth on a given transmission facility because it would allow multiple classes of service on a single facility. Another way of looking at it is to consider that typical BLSR configuration is such that only half of the facility is carrying live traffic, while the other is idle waiting to pickup the traffic when the load-bearing side fails. If the TDM is replaced with Ethernet or similar packet switching techniques capable of carrying classed traffic and priority based packet switching, both sides of the BLSR become useful. If one side fails, lower priority traffic gets delayed or dropped while higher priority traffic moves.

More information on these emerging standards and technologies is as close as your favorite Internet search engine.

PACKET SWITCHING

One only need examine history over the past couple of hundred years to see that older communications models and methods bear resemblance to some of the current crop of fast growing methods. For example it’s not difficult to see the similarity between smoke signals and telegraph messages. If one takes the smoke signal model and imagines a sender sending a message to a receiver and the receiver repeating the message to another smoke sender and so on, it’s easy to see the resemblance to packet forwarding characteristics of the Internet protocol or email.

If you have any interest at all in the Internet and have done any reading on the subject, you’re aware that it’s based on packet switching. Packet switching depends on some basic functional elements including transmission links, and a switching engine called a router.

There are a lot of similarities between cell-based switching and packet-based switching, and there are some differences. Packet switching is simply making decisions about where to send the packet at hand. Packets are like cells in the sense that they must be opened, intelligence found about where they are headed and where they have been, and then switched and/or routed. The instructions are just inside the packet with a few other tidbits of information.

One of the fundamental differences between packet switching and cell switching is at the heart of most of the ambiguity and hand-wringing that occurs when considering the routing-switching—layer 2-layer 3 solution. It’s really quite simple. Cells, PDH streams, and PPP (HDLC) are layer 2 functions. What separates these techniques from packet techniques is a time base. Packets, at least IP datagrams that make up user datagram protocol and Telecommunications protocol over Internet packets, have no reference or relationship to any clocking, timing, or basic synchronizing intelligence. They are just out there somewhere in the Ether. T1/E1, PPP/HDLC, ATM, and Ethernet all have clocking and synchronizing information embedded in the stream.

Packet switching is nothing more than switching and/or routing at packet borders, or between packets after the details in the packet header have been opened and read. Only after the entity has been opened and read can it be routed or switched to a second port. Many times the mail system is used as a metaphor for packet switching. It’s a pretty good metaphor, but with some subtle differences. First, the packet entity must be opened and read. It does not have an outside and an inside unless the payload has been encrypted, or otherwise sealed and secured in some way. One of the fundamental flaws in the Internet everyone knows about and experiences every day is simple courtesy and security. In addition to the payload and addressing information, there are other significant details inside the packet entity exposed for any and everyone to see and do with as they please. These other details have to do with all kinds of fun things that can muck up the overall machinery such as administrative control of the routing machines.

A view that says the Internet has evolved from prior well-known methods and technology wouldn’t be difficult to contend, but would likely be more difficult to defend. Many modern IP network designers seem blissfully unaware that the Internet is critically dependent on an underlying transmission infrastructure they simply refer to as the network layer. Very few of them have a clue about the importance of network clocking and timing. Many think packet over SONET/SDH isn’t a big deal because it’s done all the time (over PDH), which for the most part goes over SONET/SDH any way. Very few realize that HDLC, or PPP framing, is as rigid and fixed as T1, E1, T3, or E3. A few understand the details of packet-over SONET/SDH. The ones that do understand this fundamental know that Internet architecture includes layer 1 and layer 2 and is not, as the rest of their esteemed colleagues contend, self-healing.

Throw in all the mumbo jumbo about connection-oriented and connectionless protocols and mumble solution in between every fifth use of the word router or whatever else can be thought of, but the basics remain the same. That is, something on the premises, or at the network access point, contains or establishes intelligence that tells the network how to set up a connection between two or more points, and thereby transmit and receive information through the network. Anyone can play around with semantics all day about dumb terminals and smart networks, or at the other extreme, they call intelligent terminals and dumb networks. At the end of the day, what’s important is effective and efficient use of limited resources.

If confusion reigns, stop and ask a couple basic questions: ‘‘Is it circuit, cell, or packet?’’ ‘‘What is being shared?’’ Is it time, bandwidth, or both? What are the interface, bitrate, and active protocols on the facility? What is supposed to be done with it? What did the customer ask for? What is being delivered? Is it broke? With a little patience and perseverance, confusion will soon stop raining, the clouds will pass, and matters will clear up as you climb up or down the stack of bits and bytes.