Difference between Broadband Networking Versus Local Area Networking - Linux

We use the terms broadband and WAN as synonyms.

Although not precisely correct, this is the terminology in common use at the time of publication.WAN network interfaces faster than analog modems are called “broadband” connections. As discussed in earlier sections, the simpler broadband or WAN protocols with some exceptions establish point-to-point connections. They rely on higher layer protocols to provide multicast or broadcast capability. In contrast, LANs are democratic. All the machines on a LAN share the same chunk of bandwidth. They can all talk at once or at least try to talk at once. They can all see each other’s transmissions. Therefore, LANs provide a mechanism at the physical layer to negotiate access to the physical network to make sure that there is only one speaker at a time. In general, LANs have a fairly complicated physical layer but very simple data link layers. LANs will checksum packets to make sure that bit-level communications are okay. Although packet integrity is usually guaranteed, errors will result in dropped packets.

WANs usually have relatively simple direct serial interfaces at the physical layer. Higher layers,usually the data link layer, have to do all the work of arbitrating bandwidth and negotiating the connections. Therefore, when compared to LANs, WANs have simpler physical layers but more complicated data link layers. The data link layers will compensate for transmission problems at the physical layer provided sufficient bandwidth is available.

From the viewpoint of TCP/IP, if the data link and physical layers do their jobs, the details of data transmission are mostly transparent to Layer 3, the network layer, and the layers above Layer 3. If the lower layers don’t do their job, the result will be dropped packets. If the data transmission is using the TCP transport protocol, effort will be made to retransmit dropped packets at the lower layers. However, as we will see in later chapters, TCP can’t entirely compensate for low-quality links.

Local Area Networks

LANs are newer than WANs. The first and still the most popular LAN is Ethernet. The link layer is far simpler in Ethernet than it is in broadband protocols. A basic Ethernet Layer 2 interface consists only of a framing layer and the Media Access Control (MAC) header. In its simplest form, the MAC header contains the source address, the destination address, and the protocol number of the payload. The complexity of the LANs is hidden in the physical layer (PHY). LANs have the characteristic where each machine can see the transmissions of all the other machines that are directly reachable. There is no allocated bandwidth or channels. Instead, LAN protocols provide a way to negotiate access to the network by allowing only one machine to transmit at a time. There have been various schemes and we don’t intend to cover them all here.However, the most popular LAN, Ethernet provides a method called Carrier Sense Multiple Access with Collision Detect (CSMA/CD) where each machine waits for a clear carrier before starting to transmit a packet. In addition, each machine generates a Frame Check Sequence (FCS) code, and the recipient machine will drop the packet if the FCS is bad.

Generally, all the transmission details, including collision detection and error detection, are done in hardware or low-level firmware. This hardware is the physical layer and it is hidden from the data link layer and all other layers. With most Ethernet interfaces, the data link layer is responsible for receiving the incoming packets, which are placed directly in a circular buffer by the Direct Memory Access (DMA) capability of the Ethernet interface. Incidentally, many Ethernet interfaces can place the packets arriving sequentially in noncontiguous locations and this is known as scatter-gather capability. When available on an Ethernet interface, Linux makes use of scatter-gather by minimizing expensive copying of packets received from Ethernet interfaces. Similarly, outgoing packets are queued at the Ethernet chip for transmission, and if the interface has scatter-gather capability, these packets are transmitted directly from a linked list so they don’t have to be copied separately by software into a separate buffer.

Most LAN protocols, although they might be far more complex than Ethernet, present a fairly simple Ethernet-compatible interface at the data link layer. This type of interface is specified by the ISO 8802 series. Wireless protocols also provide an Ethernet-like interface to the data link layer. They do have Authentication and Authorization (AA) capability, but this function is invisible to the upper layers. Essentially, once a user is authenticated, from the TCP/IP point of view, the wireless LAN provides a simple Ethernet type interface. Each machine on the immediate LAN can see all the other machines that are directly reachable.

Wide Area Networks

Broadband interfaces generally involve public carrier networks, including everything from DSLand cable modems to high-speed optical interfaces such as OC192. Essentially, from the standpoint of this book, there are two types ofWAN interfaces. The first is a long-range, reliable, point-to-point link through a dedicated or private line. An example would be a dedicated physical twisted pair or a leased T1 line, and although of less interest, this type of interface will be discussed briefly in order to build an understanding of basic data transmission methods. The other more complex type of WAN interface attaches to a public network where data traffic is merged with other rate payers’ data or where IP packets are sub modulated on a shared carrier along with other data or voice traffic. This second type of interface includes IP over Frame Relay, (IPOA), or DSL, and any other interface where IP traffic is carried on another network type.

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