COMMUNICATIONS NETWORK SWITCHING AND TRANSMISSION ARCHITECTURE

Telecom network architecture in 2004 is very much a child of many years of growth and technical evolution driven by telephone calls. Telephone calls require 64 Kbs and are point-to-point connections used for short periods. Telephone service economics where the longer the distance between two points of a call costs more money influenced most of the traffic into local and regional calling patterns. These regional calling patterns led to hierarchical network architecture with high capacity switch sites in fewer numbers piecing together circuits to enable regional and national telephone calls.

Figure 1 is an illustration of the hierarchical nature of the telephone network.

 

Figure 1: Classical Telecom Hierarchical Architecture

The access tandems and certain designated transmission facilities are generally referred to as the access network. These switches formed the technical and economic basis for a philosophy the FCC dubbed equal access. Next time you pay a telephone bill, look at the term FCC access charge. Nope, the money doesn’t go to the FCC. It goes into the owner of the access facilities. And according to FCC regulations, the owner of the access facilities in each LATA must afford equal access to any inter-exchange carrier (IXC) willing to arrange for a connection between their facilities and the local exchange carrier’s (LEC) facilities.

On the other side of that equation, equal access means the LEC must offer their subscribers the ability to select their long distance, or IXC.

The higher levels in the hierarchy vary, depending on the size and extent of the individual long distance carriers. AT&T operates much as it did before divestiture. Others, such as MCI and Sprint, have flatter or fewer levels in their switching hierarchy. All the long distance carriers have gateways to international networks and their own shares in international transmission facilities. Piecing together two or more SONET links end to end creates an extension of the physical boundaries of the network with the addition of more links (Figure 2).

 

Figure 2: SONET Transmission Segment

All the layers are extended across the interface. Each layer becomes an extension of itself each time a SONET transmission facility is connected to another. Linking two or more in series along a path or route, extends the boundary or domain as defined in the facility specification. Figure 2 shows a block diagram of SONET transmission infrastructure linked to provide continuous bandwidth that could be used to provision an E-1/T-1/J1 or E3/DS3/J3 private line. Such a facility could be used as a link in a data communications network or to connect two routers in an inter-network or intra-network link.

Another method of linking facilities is the mid-span meet, commonly used by carriers to link or interface between networks. This is depicted in Figure 3 and shows links between LECs and IXCs.

 

Figure 3: Linked SONET Transmission Facility

A variation on the light wave terminal equipment (LTE) is the add-drop multiplexer (ADM). SONET transmission capacity is built around the 51 Mbs STS. An LTE breaks out all the STS in the optical carrier (OC) and makes them available. If the LTE is used at a location where all STS are not terminated in local routing/switching equipment, then any transit streams must be connected directly to other LTE facing other routes. The ADM breaks out and terminates a limited quantity of STS from the optical carrier. For example, the office in which the terminal is located might only require 1 or 2 STS from an OC-3, OC-12, or higher rate transmission facility. Using an ADM would be less costly in terms of capital investment and operations expense.

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.

High Speed Packet Switching Technology

High-speed backbone networks are networks that provide rapid variable data rate (dynamic bandwidth) transport between switching centers. Traditional switching systems have been limited to low-speed fixed bandwidth connections.

High-speed switching systems are telecommunications infrastructures composed of circuits and equipment capable of near-instantaneous connection of end points at near-perfect efficiency and required data transmission throughput.

High-speed packet switching technology allows multiple communication channels to share the resources of a data communication network. This allows the same network to integrate voice, data, and video signals. In addition to the rapid switching of packets, packet switches are designed to handle different types of packets in different ways. Packet switches receive incoming packets, update the address information of the packets with their new destination address, temporarily store the packets until the next path and channel becomes available, and then transfers the packet to the appropriate communication line (Path) and channel (time slot or portion of a time slot).

Figure 1 shows a high-speed data packet switching system. This diagram shows that several high-speed data transmission lines are providing packet to the packet switch. The packet switch uses a routing table to search for the incoming address and then it replaces the address with the new packet destination address (the next switch or end point). The data packet is then stored in buffer memory where it waits for availability of its destination path and channel. This diagram also shows how the packet switch manages excessive network switch activity. As the packet switch gets busy (receives more data than it can process), the buffer memory begins to fill. As the buffer memory nears exhaustion, packets within the memory will be reviewed for priority and low-level priority data packets will be discarded.


Figure 1: High-Speed Packet Switching