3: Other LANs

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In the wired era, one could get along quite well with nothing but Ethernet and the occasional long-haul point-to-point link joining different sites. However, there are important alternatives out there. Some, like token ring, are mostly of historical importance; others, like virtual circuits, are of great conceptual importance but – so far – of only modest day-to-day significance. And then there is wireless. It would be difficult to imagine contemporary laptop networking, let alone mobile devices, without it. In both homes and offices, Wi-Fi connectivity is the norm. Mobile networking is ubiquitous. A return to being tethered by wires is almost unthinkable.

3.1: Virtual Private Networks
Suppose you want to connect to your workplace network from home, but your workplace has a security policy that does not allow “outside” IP addresses to access essential internal resources. How do you proceed, without leasing a dedicated telecommunications line? A virtual private network (VPN) provides a solution; it supports creation of virtual links that join far-flung nodes via the Internet. Your home computer creates an ordinary Internet connection (TCP or UDP) to a workplace VPN server.
3.2: Carrier Ethernet
Carrier Ethernet is a leased-line point-to-point link between two sites, where the subscriber interface at each end of the line looks like Ethernet (in some flavor). The physical path in between sites, however, need not have anything to do with Ethernet; it may be implemented however the carrier wishes. In particular, it will be (or at least appear to be) full-duplex, it will be collision-free, and its length may far exceed the maximum permitted by any IEEE Ethernet standard.
3.3: Token Ring
A significant part of the previous chapter was devoted to classic Ethernet’s collision mechanism for supporting shared media access. After that, it may come as a surprise that there is a simple multiple-access mechanism that is not only collision-free, but which supports fairness in the sense that if N stations wish to send then each will receive 1/N of the opportunities. That method is Token Ring.
3.4: Virtual Circuits
Virtual-circuit switching (or routing) is an alternative to datagram switching, which was introduced in Chapter 1. In datagram switching, routers know the next_hop to each destination, and packets are addressed by destination. In virtual-circuit switching, routers know about end-to-end connections, and packets are “addressed” by a connection ID.
3.5: Asynchronous Transfer Mode - ATM
ATM is a network mechanism intended to accommodate real-time traffic as well as bulk data transfer. We present ATM here as a LAN layer, for which it is still sometimes used, but it was originally proposed as a replacement for the IP layer as well, and, to an extent, the Transport layer. These broader plans were not greeted with universal enthusiasm within the IETF.
3.6: Adventures in Radioland
For the remainder of this chapter we leave wires (and fiber) behind, and contemplate the transmission of packets via radio, freeing nodes from their cable tethers. Wi-fi (3.7 Wi-Fi) and mobile wireless (3.8 WiMAX and LTE) are now ubiquitous. But radio is not quite like wire, and wireless transmission of packets brings several changes.
3.7: Wi-Fi
Wi-Fi is a trademark of the Wi-Fi Alliance denoting any of several IEEE wireless-networking protocols in the 802.11 family. (Strictly speaking, these are all amendments to the original 802.11 standard, but they are also de facto standards in their own right.) Like classic Ethernet, Wi-Fi must deal with collisions; unlike Ethernet, however, Wi-Fi is unable to detect collisions in progress, complicating the backoff and retransmission algorithms.
3.8: WiMAX and LTE
WiMAX and LTE are both wireless network technologies suitable for data connections to mobile (and sometimes stationary) devices. WiMAX is an IEEE standard, 802.16; its original name is WirelessMAN (for Metropolitan Area Network), and this name appears intermittently in the IEEE standards. LTE (the acronym itself stands for Long Term Evolution) is a product of the mobile telecom world; it was designed for mobile subscribers from the beginning.
3.9: Fixed Wireless
This category includes all wireless-service-provider systems where the subscriber’s location does not change. Often, but not always, the subscriber will have an outdoor antenna for improved reception and range. Fixed-wireless systems can involve relay through satellites, or can be terrestrial.
3.10: Epilog and Exercises
Along with a few niche protocols, we have focused primarily here on wireless and on virtual circuits. Wireless, of course, is enormously important: it is the enabler for mobile devices, and has largely replaced traditional Ethernet for home and office workstations.