Case Study: Asynchronous Transfer M ode

CHAPT ER 9 COMMUNICAT ION 393 span of the network, which is a normal but unwanted condition, the collision signal is not propagated to the other network links, and by limiting certain types of traffic from being sent to all other interfaces. A router connects one network to another see Figure 9-18b, and makes deci- sions with respect to forwarding packets across its boundaries. A router by defini- tion has more than one network interface and forwards packets between interfaces. T he network protocols used on either side of a router can differ. A router forwards packets based on the protocol, whereas a switch forwards packets based only on the destination address. A switch is a high speed hub with no shared bandwidth, as illustrated in Figure 9-18c. A switch eliminates media access conflicts because there is no contention for the media. An example of a switch is discussed in Section 10.9.2, in which an external con- troller sets up source-to-destination paths. An enhancement is a self-routing network , that sets up source-to-destination connections on-the-fly, based on the destination addresses in the headers of packets traversing the network. As an example, consider designing a 4-input, 4-output self-routing switch. We can accomplish this using the bubblesort algorithm, in which packets with the smallest addresses are bubbled to the top, by making pairwise exchanges starting from the top and working toward the bottom, dropping the packet with the larg- est address to the bottom on each pass. For n channels, there are nn-12 com- parisons that need to be made. For this case, n=4, and so 44-12 = 6 comparisons need to be made, which means that the switch needs 6 comparison boxes. T he corresponding 4-input, 4-output self-routing switch is shown in Figure 9-19. Unsorted packets enter at the left, and emerge in sorted order by destina- tion address on the right. See problem A-28 for the design of one of the compar- ison boxes.

9.6 Case Study: Asynchronous Transfer M ode

Historically, there have been different networks that carry different types of information: • T elex old style teletypewriters, used for news feeds, stock quotes, etc. 394 CHAPT ER 9 COMMUNICAT ION • “Plain old telephone service” POT S via the public switched telephone network PST N; • Data, via the packet switched data network PSDN; • T elevision via: 1 ground based antennae; 2 community antennae T V CAT V known as cable T V; and 3 satellites; • Local data via local area networks LANs. Each network is separately planned, dimensioned, developed, and operated. T his specialization results in independent worldwide networks. Bandwidth is not shared among the networks, yet when video traffic peaks at prime time, tele- phone traffic lulls. Although there may always be video programs available on a particular channel at all hours of the day, less-expensive local programming may be used in the off-peak hours, reducing the need for video traffic via satellites or via long distance wireline networks. Each type of service needs to deal with peak traffic and bursty traffic on its own, although it would be more economical to share networks as long as the peaks and bursts do not coincide. An attempt at sharing is Integrated Services Digital Network ISDN, which comes in two forms. Narrowband-ISDN NISDN is designed for 64 Kbs telephone switching. NISDN allows voice and data traffic to be carried over the same network. Broadband-ISDN BISDN supports video traffic as well as voice and data traffic. T he ISDN Basic Rate Interface BRI is used for NISDN. T he BRI offers two Bearer B channels at 64 Kbs per channel, for user data, and one Data D channel at 16 Kbs which is used for control and signaling. T he total bit rate is 192 Kbs when higher level framing overhead is taken into account, but the max- 4 1 2 3 1 2 3 4 1 4 1 2 2 4 3 4 2 3 Figure 9-19 A 4 × 4 self-routing switch based on the bubblesort algorithm. CHAPT ER 9 COMMUNICAT ION 395 imum rate available to the user is 128 Kbs when the two B channels are “bonded” into a single channel. T he ISDN Primary Rate Interface PRI offers 23 B channels and one 64 Kbs D channel, and is commonly known as a “T 1 line.” ISDN lines can be leased from telecommunication companies and can be config- ured to set up private networks. As a general rule of thumb, the monthly rate for leasing 10 64 Kbs lines equals the rate for leasing a T 1 line that supports 23 64 Kbs lines. A problem with the overall economics with ISDN is that the sup- ported bit rates are service dependent NISDN is an example of a service as opposed to traffic dependent, which is needed by modern networks. Services must fall into even multiples of B and D channels in the offered ratios, or else they will not make efficient use of the available bandwidth. We may not know what bit rates future services will need, and so we need a service independent bit transport for a network to be extensible. T he goal is to have a single network that can serve everyone, which is where asynchronous transfer mode AT M comes in. 9.6.1 SYNCHRONOUS VS. ASYNCHRONOUS TRANSFER MODE With synchronous transfer mode ST M, also known as time division multi- plexing T DM, a data stream is made up of time slots that are assigned to chan- nels in a round robin fashion. Figure 9-20a illustrates T DM for four channels. A station can only send during a preassigned time slot. Other unused time slots may go by while a station waits for its preassigned time slot. In the public switched telephone network, a time slot is 125 µs, which allows for 8000 samples per second at 8 bits per sample, resulting in 64 Kbs for a voice grade line. Figure 9-20b shows asynchronous transfer mode, which may still use 125 µs framing, but now, any station can use any slot, and the network is used more effi- ciently. On the down side, operating an AT M network is much more complex than operating a simpler T DM network. 9.6.2 WHAT IS ATM? AT M is a combination of hardware and a set of protocols that delivers a guaran- teed bandwidth with a bounded low latency. Two enabling progress areas that make AT M possible are: 396 CHAPT ER 9 COMMUNICAT ION 1 T echnology – T he speed and density of VLSI allows network function- ality to be pushed into the end-systems rather than throughout the in- termediary components. Error detection is only performed at the endpoints, for example, instead of at every intermediate router as is common for the Internet. T he development of high bandwidth, low bit-error rate optical fiber is a supporting technology that makes this ap- proach practical. 2 Systems – Fast packet switches that introduce a low latency into end-to-end communication enable the delivery of real-time services. Typical speeds for AT M are in the range of 1 Gbs – 2.5 Gbs, with an average delay of only 450 µs for each switch that handles an AT M packet. 9.6.3 ATM NETWORK ARCHITECTURE Before an AT M transfer takes place, a connection must be established by con- tacting a signaling control point SCP which is a network device that has the authority to configure AT M switches for the transfer. All packets are constrained to follow the path set up by the SCP, and all packets arrive in order. T he job of the switch is very simple: it simply looks at the destination address in 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 Station time slots = Station data = Nothing to send 1 Cells, shown by source = Station data = Nothing to send 1 3 1 2 4 1 3 1 2 a b Figure 9-20 a Time division multiplexing vs. b asynchronous transfer mode. CHAPT ER 9 COMMUNICAT ION 397 the header, and then it sends the packet on the path indicated in the header. A new destination address is placed in the header, as described further below. All AT M packets have the same fixed size of 53 bytes, as shown in Figure 9-21. T he packet created by an end system for injection into the network, which is known as the user-to-network interface UNI format is shown in Figure 9-21a, and the packet format used from that point onward, which is known as the network-to-network interface NNI format is shown in Figure 9-21b. T he generic flow control GFC field is used at the network boundary to police how packets are allowed to enter the network. Once a packet is in the network, the GFC field of the UNI format is no longer needed, and is then combined with the 8-bit virtual path identifier VPI field to form a 12-bit VPI field in the NNI format. T he VPI field can be thought of as identifying one particular home for a cable that carries cable T V channels, whereas the virtual channel identifier VCI field identifies the specific channel. T he payload type identifier PT I identifies whether the data field carries user data or network data, and other information. T he cell loss priority CLP bit determines whether this cell can be dropped during times of congestion. T he header error control HEC is a CRC over the header. T he 48-byte payload field follows. GFC VPI VPI VCI PTI C L P HEC 48-byte information field 8 7 6 5 4 3 2 1 1 2 3 4 5 6 53 Bits Bytes a VPI VCI PTI C L P HEC 48-byte information field 8 7 6 5 4 3 2 1 b Figure 9-21 Format of an ATM packet. a User-to-network interface UNI format; and b net- work-to-network interface NNI format. 398 CHAPT ER 9 COMMUNICAT ION Figure 9-22 shows a simple AT M network that consists of three switches. Each switch has a symmetric routing table that can be driven in either direction. For example, at AT M Switch 1, an incoming packet on the left that has a VPI field of 7 is sent to the right after changing the VPI field to 5. Likewise, a packet that comes into AT M Switch 1 from the right with a VPI field of 5 is sent to the left after changing the VPI field to 7. T his is referred to as “virtual path switch- ing” because only the VPI field is used in routing the packet. T he VCI field is unchanged. T here can also be “virtual channel switching” in which routing is done based on the VCI field, and the VPI field is left unchanged. 9.6.4 OUTLOOK ON ATM Although AT M holds a great deal of promise for an extensible network that serves many user needs, economies of scale favor legacy networks such as Ether- net for end-user systems. AT M appears mostly in backbone networks, with mul- tiplexors and concentrators used for connecting legacy networks at the boundary of the backbone to the backbone itself. AT M continues its penetration into back- bone networks and also to the desktop, which is currently the exception rather than the rule. Whether AT M makes it to the desktop pervasively, time will tell. ■ SUMMARY Communication involves the transfer of information between systems. As a rule, ATM Switch 1 ATM Switch 2 ATM Switch 3 VPI OUT VPI IN 7 9 5 7 VPI OUT VPI IN 5 7 VPI OUT VPI IN 7 3 VPI = 9 VCI = 3, 4 VPI = 3 VCI = 3, 4 VPI = 7 VCI = 3, 4 VPI = 7 VCI = 1, 2, 3 VPI = 5 VCI 1, 2, 3 VPI = 7 VCI = 1, 2, 3 Figure 9-22 VPI switching in an ATM network. Adapted from [dePrycker, 1993]. CHAPT ER 9 COMMUNICAT ION 399 data is transferred in bit-serial fashion because the time-of-flight delays dominate the transfer time for high speed networks. However, modulation schemes allow many bits to be encoded in a single sample, such as for a dibit. The choice of mod- ulation scheme impacts the introduction of errors into the transmission of infor- mation. Error detection and correction are made possible through redundancy, in which there are more bit patterns possible than the number of valid bit patterns. If the error bit patterns do not have a single closest valid codeword, then error detection is possible but error correction is not possible. If every error bit pattern is reachable from only one valid bit pattern, then error correction is also possible. Local area networks LANs manage complexity by using layering that is based on the OSI model. These days, LANs are typically connected to wide area networks WANs, most notably, the Internet. The Internet is based on the TCPIP protocol suite. User data is encapsulated at the Application, Transport, Network, and Link layers, and is sent through the Internet and de-encapsulated de-muxed on the receiving system. As it is passed through the Internet, the data traverses various transmission media, that vary in bandwidth and distance capabilities. ■ FURT HER READING Needleman, 1990 and Schnaidt, 1990 give a thorough treatment of local area networks according to the OSI model, and Tanenbaum, 1996 is a good refer- ence on network communication in general. Halsall, 1996 gives a thorough and readable treatment of network media types. dePrycker, 1993 gives an in-depth account of AT M and its characteristics. Tanenbaum, 1999 and Stallings, 1996 give readable explanations of Ham- ming encoding. Hamming, 1986 and Peterson and Weldon, 1972 give more detailed treatments of error-correcting codes. Halsall, F., Data Communications, Computer Networks, and Open Systems, 4e, Addison-Wesley, 1996. Hamming, R. W., Coding and Information Theory, 2e, Prentice-Hall, 1986. Needleman, R., Understanding Networks, Simon and Schuster, New York, 1990. Peterson, W. Wesley and E. J. Weldon, Jr., Error-Correcting Codes, 2e, T he MIT Press, 1972. 400 CHAPT ER 9 COMMUNICAT ION de Prycker, Martin, Asynchronous Transfer Mode: Solution for Broadband ISDN, 2e, Ellis Horwood, 1993. Schnaidt, P., LAN Tutorial, Miller Freeman Publications, California, 1990. Stallings, W., Computer Organization and Architecture: Designing for Performance, 4e, Prentice Hall, Upper Saddle River, 1996. Tanenbaum, A., Computer Networks, 3e, Prentice Hall, Upper Saddle River, 1996. Tanenbaum, A., Structured Computer Organization, 4e, Prentice Hall, Engle- wood Cliffs, 1999. ■ PROBLEMS 9.1 What is the Hamming distance for the ASCII SEC code discussed in Sec- tion 9.4.2? 9.2 Construct the SEC code for the ASCII character ‘Q’ using even parity. 9.3 For parts a through d below, use a SEC code with even parity. a How many check bits should be added to encode a six-bit word? b Construct the SEC code for the six-bit word: 1 0 1 1 0 0. When construct- ing the code, number the bits from right to left starting with 1 as for the method described in Section 9.4.2. c A receiver sees a two-bit SEC encoded word that looks like this: 1 1 1 0 0. What is the initial two-bit pattern? d T he 12-bit word: 1 0 1 1 1 0 0 1 1 0 0 1 complete with an SEC code even parity is received. What 12-bit word was actually sent? 9.4 How many check bits are needed for a SEC code for an initial word size of 1024? 9.5 Construct a checksum word for EBCDIC characters ‘V’ through ‘Z’ using CHAPT ER 9 COMMUNICAT ION 401 vertical redundancy checking with even parity. DO NOT use longitudinal redundancy checking. 9.6 Compare the number of bits used for parity in the SEC code with the simple parity VRC code, for 1024 eight-bit characters: a Compute the number of check bits generated using SEC only horizon- tally. b Compute the number of checksum bits using VRC only. 9.7 T he SEC code discussed in Section 9.4.2 can be turned into a double error detectingSEC DEDSEC code by adding one more bit that creates even parity over the SEC code which includes the parity bit being added. Explain how double error detection works while also maintaining single error correction with this approach. 9.8 Compute the CRC for a message to be transmitted Mx = 101100110 and a generator polynomial Gx = x 3 + x 2 + 1. 9.9 What is the longest burst error that CRC-32 is sure to catch? 9.10 To which IPv4 class does address 165.230.140.67 belong? 9.11 How many networks not hosts can the IPv4 class A, B, and C addresses support? T hat is, how many distinct class A, B, and C network addresses can there be? Do not consider reserved addresses. 9.12 Network media always carry data in bit-serial fashion, and virtually never in parallel. T hat is not to say that data could not be carried over a network in byte-parallel or word-parallel fashion; there simply is no advantage to doing it this way. To see why this is the case, calculate the time required to transmit a 32-bit word between two computers over a 32-foot network. T he network speed is 1 Gbps per channel. T he time-of-flight delay imposed by the distance is 1 ns per foot. Calculate the time to transmit the 32-bit word using a single channel bit-serial fashion and using 32 channels word-parallel fashion. 9.13 An AT M network has two switches A and B that switch on the virtual path identifier only. T he network topology and the routing tables for the 402 CHAPT ER 9 COMMUNICAT ION switches are shown below: A cell comes into AT M switch A from Host 1 with the following UNI header first 4 bytes: Show the first four bytes of the outgoing NNI header. ATM Switch A VPI OUT VPI IN 7 3 2 ATM Switch B VPI OUT VPI IN 6 7 1 Host 1 8 7 6 5 4 3 2 1 1 2 3 4 Bits Bytes 1 1 1 1 1 1 CHAPT ER 10 T RENDS IN COMPUT ER ARCHIT ECT URE 403 In the earlier chapters, the fetch-execute cycle is described in the form: “fetch an instruction, execute that instruction, fetch the next instruction, etc. ” T his gives the impression of a straight-line linear progression of program execution. In fact, the processor architectures of today have many advanced features that go beyond this simple paradigm. T hese features include pipelining , in which several instructions sharing the same hardware can simultaneously be in various phases of execution, superscalar execution , in which several instructions are executed simultaneously using different portions of the hardware, with possibly only some of the results contributing to the overall computation, very long instruction word VLIW architectures, in which each instruction word specifies multiple instructions of smaller widths that are executed simultaneously, and parallel processing , in which multiple processors are coordinated to work on a single problem. In this chapter we cover issues that relate to these features. T he discussion begins with issues that led to the emergence of the reduced instruction set computer RISC and examples of RISC features and characteristics. Following that, we cover an advanced feature used specifically in SPARC architectures: overlapping register windows . We then cover two important architectural features: supersca- lar execution and VLIW architectures. We then move into the topic of parallel processing, touching both on parallel architectures and program decomposition. T he chapter includes with case studies covering Intel’s Merced architecture, the PowerPC 601, and an example of a pervasive parallel architecture that can be found in a home videogame system.

10.1 Quantitative Analyses of Program Execution