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Lectures 9-10: DATA NETWORK SUPPORT by Prof. Michael Tse In this lecture, we will take a look at the available technology infrastructure that connects people to the Internet. We will review a few common forms of network technology and study their operations and limitations. Connection/access technologies 1. xDSL (x Digital Subscriber Line) 2. ISDN (Integrated Services Digital Network) Switching technologies 1. Frame relay 2. SMDS (Switched Multi-megabit Data Service) 3. ATM (Asynchronous Transfer Mode) Reading materials A very useful source of reference is http://www.gare.co.uk/technology_watch/index.htm About ADSL, see http://www.cs.tut.fi/tlt/stuff/adsl/pt_adsl.html About ISDN, see http://www.ralphb.net/ISDN/ and http://www.isdnzone.com/ About ATM, see http://www.atmforum.com/ About frame relay, see http://www.frforum.com/basicguide/toc.html xDSL (Digital Subscriber Line) DSL is a technology to employ existing telephone network (copper wires) to transmit high-speed data for the Internet users. In fact, POTS (plain-old- telephone service) uses just a little bit of what the copper wires can do. The fact is that POTS uses 4kHz bandwidth while the copper twisted pair has up to 1.5MHz over a distance of 3km. A few basic forms have emerged for different applications/performance. So, the "x" can be "A", "S", "H" and "V" as defined below: 1. ADSL Asymmetric DSL: 1.5 Mbps-384kbps/384-128kbps 2. HDSL High-bit-rate DSL: 1.5 Mbps/1.5 Mbps (4Wire) 3. SDSL Single-line DSL: 1.5 Mbps/1.5 Mbps (2Wire) 4. VDSL Very high DSL: 13 Mbps-52 Mbps/1.5 Mbps- 2.3 Mbps. 5. IDSL ISDN DSL: 128 Kbps/128 Kbps. 6. RADSL Rate Adaptive DSL: 384kbps/128kbps Note: T1 line is an earlier technology that uses the same old copper wires. Typically T1 is leased to a company (usually provided by the phone company) for high-speed data transmission. No voice can be used simultaneously. But, the latest ADSL can put voice and data together, and at a more affordable price. It is now penetrating our homes! You will see how and why later. Overview of frequency spectrum of DSL Spectrum of the copper wires (conventional twisted pair in existing telephone networks) The copper wire bandwidth can be up to 1.5MHz for a distance of 3 km. For shorter distance, it can be even wider. Spectrum of DSL Basically the DSL spectrum extends from 20kHz up to 1.1MHz. Thus, DSL can be handled by traditional telephone copper wires for a few kilometers. One important observation is that the attenuation increases as frequency goes higher. So, DSL will experience poor performance for the high-frequency part. For example, ADSL uses a discrete multi-tone (DMT) technique (to be discussed later) which allocates the high-frequency part mainly for downstream transmission. Hence, for longer distance transmission where the high-frequency attenuation is severe, the downstream data rate can be much affected. [Note: Downstream -- towards the user. Upstream -- towards the ISP.] Asymmetric DSL (ADSL) ADSL is widely used for Internet connection via existing telephone lines. The "A" stands for asymmetric, which means the upstream and downstream bandwidths are unequal. For most Internet applications, like poping mail messages, web browsing, downloading files, the downstream data rate is more demanding. Also, for video-on-demand, the traffic is mainly downstream. So, it makes sense to partition a much larger bandwidth for downstream traffic than for upstream. Advantages --- up to 140 times faster than VF modems; simultaneous voice and data transmission; using same existing copper wires Installation --- a special ADSL modem is installed at the user's premises. Discrete Multi-Tone (DMT) DSL The ADSL technology uses a discrete multi-tone (DMT) approach to send data through the copper wire. Simply put, there are 256 modems turning on at the same time and transmitting data at different frequency slots. The modulation technique for each tone partition is QAM (Quadrature Amplitude Modulation). Upstream -- 20 - 138 kHz Downstream -- 140 - 1100 kHz For long distance, the downstream data rate is drastically reduced because of the high attenuation at the high-frequency region. Other line coding schemes Other forms of DSL may use other coding schemes. The different coding schemes have different impact on the frequency spectrum. For example, some HDSL use a 1B2Q (1- binary-2-quaternary) scheme, and the T1 line uses a AMI (alternate mark inversion) scheme. Obviously, as shown, the AMI needs a much wider spectrum (more high frequency components) than the 1B2Q scheme. Both schemes require no additional filters. For ADSL, additional hardware (ADSL modem) is needed for the DMT scheme, and special filtering is needed to compensate the high-frequency attenuation. Question: What else does ADSL offer over the T1 or HDSL? Look at the spectrum in an earlier slide!! Integrated Services Digital Network (ISDN) ISDN is another technology that uses existing copper-wire telephone lines for combined digital telephone, data, video plus other services. The most common type of ISDN that serves homes and most private companies is the Basic Rate Interface (BRI). For larger business, Primary Rate Interface (PBI) is used. ISDN divides the telephone line into a number of digital "B" channels and one "D" channel, each of which can be used simultaneously. (BTW, "B" stands for bearer, and "D" stands for data.) Rate Channel Function Number (kbps) 2 for BRI carrying B 64 23 for data PRI 16 for BRI D control 1 64 for PRI To access BRI service, it is necessary to subscribe to an ISDN phone line. Customer must be within 18000 feet (about 3.4 miles or 5.5 km) of the telephone company central office for BRI service; beyond that, expensive repeater devices are required, or ISDN service may not be available at all. Customers will also need special equipment to communicate with the phone company switch and with other ISDN devices. These devices include ISDN Terminal Adapters (sometimes called, incorrectly, "ISDN Modems") and ISDN Routers. ISDN installation From the telephone company to NT-1 The phone company usually provides a U-interface, which is a two-wire interface directly out of the phone switch. This U-interface is connected to Network Termination- 1 (NT-1), which then gives an S/T-interface. From NT-1 to NT-2 inside the home NT-1 simply changes the 2- wire interface to a 4-wire S/T- interface. This S/T-interface can connect up to 7 devices/equipme nt. (Note: Inside an ISDN equipment, there is an NT-2 which handles the lower-layer ISDN protocols.) ISDN terminal equipment (TE) Some terminal equipment can interface with S/T-interface directly, but some don't. Terminal equipment 1 (TE-1) refers to those that can be connected to S/T-interface (the ISDN capable), and terminal equipment 2 (TE-2) refers to those that cannot be connected directly to S/T-interface but have a POTS phone jet. For the latter, we need a terminal adaptor to connect it to the S/T-interface. For details of protocols, see http://www.ralphb.net/ISDN. Frame relay What is frame relay? Basically frame relay is a high- speed communications technology that is used to connect LAN and Internet. A frame relay network consists of endpoints (e.g., PCs, servers, host computers), frame relay access equipment (e.g., bridges, routers, hosts, frame relay access devices) and network devices (e.g., switches, network routers, T1/E1 multiplexers). Why frame relay? The main purpose is to provide high-speed data transfer for packet-switched systems such as ones that are based on the old X.25 and TCP/IP. The existing switching method in telephone network (PSTN) is based on circuit-switched technology; the frame relay technology is developed to bridge this gap. Brief operation of frame relay Frame relay technology is based on the concept of using virtual circuits (VCs) --- 2-way software-defined transmission paths that replace private lines in the network. It is a way of sending information over a wide area network (WAN) that divides the information into frames or packets. The frames travel through a series of switches within the frame relay network and arrive at their destination. NEXT LAST Frame relay -- virtual circuits Virtual circuits Two types of virtual circuits are 1. Permanent virtual circuits (PVCs) -- original standard; more commonly used 2. Switched virtual circuits (SVCs) -- getting popular now PVC PVCs are set up by a network operator (e.g., a private [PolyU] network or a service provider). A PVC is defined as a connection between two sites or endpoints; fixed path, not to be set up on a call-by-call basis; pre-configured by the provider or network manager with given bandwidth allocated packet-by-packet (i.e., no bandwidth is allocated to the path until actual data is transmitted.) Note that actual path taken through the network may change from time to time, such as when automatic rerouting takes place after a new site is added. Also, since the beginning and end of the circuit will not change, the PVC is like a dedicated point-to- point circuit. SVC SVCs are more complex than PVCs. They are available on a call-by-call basis. When setting up a SVC, the user specifies a destination address similar to a phone number. The network dynamically establishes connections based on requests by many users (as opposed to PVCs where a central network operator configures the network). The network then quickly establishes the connection and allocates bandwidth based on the user's requests. Frame relay -- operation overview 1. Virtual point-to-point circuit is established first. 2. The frame's destination is specified in a data link connection identifier (DLCI) header, which is added to the packet. 3. If there is an error handling a frame (because of congestion usually), it just discards the frame. 4. The frame relay network has no error correction; it relies on the higher layer protocols in the user devices to recover the lost frames by retransmitting them. 5. Clearly, without error correction, frame relay requires lines with low error rates to achieve good performance. 6. Usually, congestion is the main cause of discards; thus, the network's ability to avoid and react to congestion is extremely important in ensuring satisfactory performance. SMDS (Switched Multi-megabit Data Service) SMDS is another high-speed communication technology, and its function is similar to frame relay network. So, SMDS is a competitor of frame relay. SMDS is considerably faster (up to 34Mbps now; 155Mbps will be available), and operates in a different way. 1. It is a connectionless service, i.e., there is no traditional call set-up/tear-down process such as encountered with X.25. Like the LAN on customer's premises, every SMDS cell contains the FULL address of the destination in the header. Local exchange cell relay switches route each packet to the destination LAN and then onto the destination machine. The shared bandwidth ensures high speed routing. 2. When a local PC or workstation wants to send data to a remote machine, the LAN creates data packets and forwards these to the SMDS compatible router. The router, which is equipped with an SMDS interface card, converts the variable length frames generated by the LAN into fixed length SMDS cells, which are then sent to the local public exchange via a bearer link. On receipt, the destination SMDS router converts the cells back into the LAN packet format and forwards them to the destination PC of workstation. 3. It consists of routers which embrace IP packets in SMDS packets (this is done by SMDS Interface Protocol (SIP)). Other features: 1. It has additional provision for restricted access to certain paths; it has added security. 2. It has multicast capability (e.g., one-to-many broadcast of video). 3. It can create virtual private network (specially arranged and charged). 4. It has a mechanism to keep track of users' activity and network usage for accounting. 5. Access speed is from 4 Mbps up to 32 Mbps. Hence, it can directly link to 10Mbps Ethernet LANs and 16Mbps token ring LANs. Reference: http://www.gare.co.uk/technology_watch/smds.htm Hardware Switch Technology -- ATM (Asynchronous Transfer Mode) The hardware switching technology that supports very fast switching network is the Asynchronous Transfer Mode (ATM). Why ATM is fast? Using hardware --- It achieves very high speed by using static hardware (not software) to route packets of uniform length (cells). Each cell is 53 octets long (1 octet = 8 bits) which includes 48 octets of data and 5 octets of addressing and control information. Because of uniform length, hardware switching is possible. ATM switches are designed to switch data streams in parallel so that where cells belonging to one call are within the switch this does not delay cells passing through the switch belonging to another call. Being asynchronous --- The other crucial feature is it being asynchronous. Unlike traditional synchronous switching which uses Time Division Multiplexing (where the data position within a group of data is synchronized to the channel it belongs to), ATM does not synchronize with the time slots, but uses an identifier in each cell to indicate which channel it belongs to. For details of switcher design, see http://www.atmforum.com/atmforum/library/notes1.html. NEXT LAST Comparison Switching technologies The aforediscussed network technologies differ in operation, cost and speed. From what we see at the present time, frame relay and SMDS seem to transient technologies for switching networks which will probably still be there for some time. In future, when speed is crucial, ATM will be the ultimate way to go, unless a new powerful technology emerges in the near future. Access technologies ADSL will still the main technology for accessing Internet from homes and small businesses. This will obviously sustain a rather long transient since the traditional copper wires are there and it will take some time before optical fibe can penetrate and totally replace the existing telephone network. Summary In this lecture, we have reviewed some current technologies for supporting network access from homes, businesses and corporations. We have seen how the conventional telephone hardware can be used to provide relatively wideband transmission. In addition we have reviewed some network connection technologies such as frame relay, SMDS and ATM.
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