Multiple Access Protocols and LANs
In the introduction to this chapter, we noted that there are two types of network links: point-to-point links, and broadcast links. A point-to-point link consists of a single sender on one end of the link, and a single receiver at the other end of the link. Many link-layer protocols have been designed for point-to-point links; PPP (the point-to-point protocol) and HDLC are two such protocols that we’ll cover later in this chapter. The second type of link, a broadcast link, can have multiple sending and receiving nodes all connected to the same, single, shared broadcast channel. The term “broadcast” is used here because when any one node transmits a frame, the channel broadcasts the frame and each of the other nodes receives a copy. Ethernet is probably the most widely deployed broadcast link technology; we’ll cover Ethernet in detail in section 5.5. In this section we’ll take step back from specific link layer protocols and first examine a problem of central importance to the data link layer: how to coordinate the access of multiple sending and receiving nodes to a shared broadcast channel – the so-called multiple access problem. Broadcast channels are often used in local area networks (LANs), networks that are geographically concentrated in a single building (or on a corporate or university campus). Thus, we’ll also look at how multiple access channels are used in LANs at the end of this section.
We are all familiar with the notion of broadcasting, as television has been using it since its invention. But traditional television is a one-way broadcast (i.e., one fixed node transmitting to many receiving nodes), while nodes on a computer network broadcast channel can both send and receive. Perhaps a more apt human analogy for a broadcast channel is a cocktail party, where many people gather together in a large room (the air providing the broadcast medium) to talk and listen. A second good analogy is something many readers will be familiar with – a classroom – where teacher(s) and student(s) similarly share the same, single, broadcast medium. A central problem in both scenarios is that of determining who gets to talk (i.e., transmit into the channel), and when. As humans, we’ve evolved an elaborate set of protocols for sharing the broadcast channel (“Give everyone a chance to speak.” “Don’t speak until you are spoken to.” “Don’t monopolize the conversation.” “Raise your hand if you have question.” “Don’t interrupt when someone is speaking.” “Don’t fall asleep when someone else is talking.”).
Computer networks similarly have protocols – so-called multiple access protocols – by which nodes regulate their transmission onto the shared broadcast channel. As shown in Figure 5.3-1, multiple access protocols are needed in a wide variety of network settings, including both wired and wireless local area networks, and satellite networks. Figure 5.3-2 takes a more abstract view of the broadcast channel and of the nodes sharing that channel. Although technically each node accesses the broadcast channel through its adapter, in this section we will refer to the node as the sending and receiving device. In practice, hundreds or even thousands of nodes can directly communicate over a broadcast channel.
A broadcast channel interconnecting four nodes.
Because all nodes are capable of transmitting frames, more than two nodes can transmit frames at the same time. When this happens, all of the nodes receive multiple frames at the same time, that is, the transmitted frames collide at all of the receivers. Typically, when there is a collision, none of the receiving nodes can make any sense of any of the frames that were transmitted; in a sense, the signals of the colliding frame become inextricably tangled together. Thus, all the frames involved in the collision are lost, and the broadcast channel is wasted during the collision interval. Clearly, if many nodes want to frequently transmit frames, many transmissions will result in collisions, and much of the bandwidth of the broadcast channel will be wasted.
In order to ensure that the broadcast channel performs useful work when multiple nodes are active, it is necessary to somehow coordinate the transmissions of the active nodes. This coordination job is the responsibility of the multiple access protocol. Over the past thirty years, thousands of papers and hundreds of Ph.D. dissertations have been written on multiple access protocols; a comprehensive survey of this body of work is [Rom 1980] Furthermore, dozens of different protocols have been implemented in a variety of link-layer technologies. Nevertheless, we can classify just about any multiple access protocol as belonging to one of three categories: channel partitioning protocols, random access protocols, and taking-turns protocols. We’ll cover these categories of multiple access protocols in the following three subsections. Let us conclude this overview by noting that ideally, a multiple access protocol for a broadcast channel of rate R bits per second should have the following desirable characteristics:
When only one node has data to send, that node has a throughput of R bps.
When M nodes have data to send, each of these nodes has a throughput of R/M bps. This need not necessarily imply that each of the M nodes always have an instantaneous rate of R/M , but rather that each node should have an average transmission rate of R/M over some suitably-defined interval of time.
The protocol is decentralized, i.e., there are no master nodes that can fail and bring down the entire system.
The protocol is simple, so that it is inexpensive to implement.
Channel Partitioning Protocols
Recall from our early discussion back in section 1.4, that Time Division Multiplexing (TDM) and Frequency Division Multiplexing (FDM) are two techniques that can be used to partition a broadcast channel’s bandwidth among all nodes sharing that channel. As an example, suppose the channel supports N nodes and that the transmission rate of the channel is R bps. TDM divides time into time frames (not to be confused the unit of data, the frame, at the data link layer) and further divides each time frame into N time slots. Each slot time is then assigned to one of the N nodes. Whenever a node has a frame to send, it transmits the frame’s bits during its assigned time slot in the revolving TDM frame. Typically, frame sizes are chosen so that a single frame can be transmitting during a slot time. Figure 5.3-3 shows a simple four-node TDM example. Returning to our cocktail party analogy, a TDM-regulated cocktail party would allow one partygoer to speak for a fixed period of time, and then allow another partygoer to speak for the same amount of time, and so on. Once everyone has had their chance to talk, the pattern repeats.
TDM is appealing as it eliminates collisions and is perfectly fair: each node gets a dedicated transmission rate of R/N bps during each slot time. However, it has two major drawbacks. First, a node is limited to this rate of R/N bps over a slot’s time even when it is the only node with frames to send. A second drawback is that a node must always wait for its turn in the transmission sequence – again, even when it is the only node with a frame to send. Imagine the partygoer who is the only one with anything to say (and imagine that this is the even rarer circumstance where everyone at the party wants to hear what that one person has to say). Clearly, TDM would be a poor choice for a multiple access protocol for this particular party.
While TDM shares the broadcast channel in time, FDM divides the R bps channel into different frequencies (each with a bandwidth of R/N) and assigns each frequency to one of the N nodes. FDM thus creates N “smaller” channels of R/N bps out of the single, “larger” R bps channel. FDM shares both the advantages and drawbacks of TDM. It avoids collisions and divides the bandwidth fairly among the N nodes. However, FDM also shares a principal disadvantage with TDM – a node is limited to a bandwidth of R/N, even when it is the only node with frames to send.
Random Access Protocols
The second broad class of multiple access protocols are so-called random access protocols. In a random access protocol, a transmitting node always transmits at the full rate of the channel, namely, R bps. When there is a collision, each node involved in the collision repeatedly retransmit its frame until the frame gets through without a collision. But when a node experiences a collision, it doesn’t necessarily retransmit the frame right away. Instead it waits a random delay before retransmitting the frame. Each node involved in a collision chooses independent random delays. Because after a collision the random delays are independently chosen, it is possible that one of the nodes will pick a delay that is sufficiently less than the delays of the other colliding nodes, and will therefore be able to “sneak” its frame into the channel without a collision.
There are dozens if not hundreds of random access protocols described in the literature [Rom 1990, Bertsekas 1992]. In this section we’ll describe a few of the most commonly used random access protocols – the ALOHA protocols [Abramson 1970, Abramson 1985] and the Carrier Sense Multiple Access (CSMA) protocols [Kleinrock 1975]. Later, in section 5.5, we’ll cover the details of Ethernet [Metcalfe 1976], a popular and widely deployed CSMA protocol.
CSMA – Carrier Sense Multiple Access
In both slotted and pure ALOHA, a node’s decision to transmit is made independently of the activity of the other nodes attached to the broadcast channel. In particular, a node neither pays attention to whether another node happens to be transmitting when it begins to transmit, nor stops transmitting if another node begins to interfere with its transmission. In our cocktail party analogy, ALOHA protocols are quite like a boorish partygoer who continues to chatter away regardless of whether other people are talking. As humans, we have human protocols that allow allows us to not only behave with more civility, but also to decrease the amount of time spent “colliding” with each other in conversation and consequently increasing the amount of amount of data we exchange in our conversations. Specifically, there are two important rules for polite human conversation:
Listen before speaking. If someone else is speaking, wait until they are done. In the networking world, this is termed carrier sensing – a node listens to the channel before transmitting. If a frame from another node is currently being transmitted into the channel, a node then waits (“backs off”) a random amount of time and then again senses the channel. If the channel is sensed to be idle, the node then begins frame transmission. Otherwise, the node waits another random amount of time and repeats this process.
If someone else begins talking at the same time, stop talking. In the networking world, this is termed collision detection – a transmitting node listens to the channel while it is transmitting. If it detects that another node is transmitting an interfering frame, it stops transmitting and uses some protocol to determine when it should next attempt to transmit.
These two rules are embodied in the family of CSMA (Carrier Sense Multiple Access) and CSMA/CD (CSMA with Collision Detection) protocols [Kleinrock 1975, Metcalfe 1976, Lam 1980, Rom 1990] . Many variations on CSMA and CSMA/CD have been proposed, with the differences being primarily in the manner in which nodes perform backoff. The reader can consult these references for the details of these protocols. We’ll study the CSMA/CD scheme used in Ethernet in detail in Section 5.5. Here, we’ll consider a few of the most important, and fundamental, characteristics of CSMA and CSMA/CD.
Recall that two desirable properties of a multiple access protocol are (i) when only one node is active, the active node has a throughput of R bps, and (ii) when M nodes are active, then each active node has a throughput of nearly R/M bps. The ALOHA and CSMA protocols have this first property but not the second. This has motivated researchers to create another class of protocols — the taking-turns protocols. As with random-access protocols, there are dozens of taking-turns protocols, and each one of these protocols has many variations. We’ll discuss two of the more important protocols here. The first one is the polling protocol. The polling protocol requires one of the nodes to be designated as a “master node” (or requires the introduction of a new node serving as the master). The master node polls each of the nodes in a round-robin fashion. In particular, the master node first sends a message to node 1, saying that it can transmit up to some maximum number of frames. After node 1 transmits some frames (from zero up to the maximum number), the master node tells node 2 it can transmit up to the maximum number of frames. (The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel.) The procedure continues in this manner, with the master node polling each of the nodes in a cyclic manner.
The polling protocol eliminates the collisions and the empty slots that plague the random access protocols. This allows it to have a much higher efficiency. But it also has a few drawbacks. The first drawback is that the protocol introduces a polling delay, the amount of time required to notify a node that it can transmit. If, for example, only one node is active, then the node will transmit at a rate less than R bps, as the master node must poll each of the inactive nodes in turn, each time the active node sends its maximum number of frames. The second drawback, which is potentially more serious, is that if the master node fails, the entire channel becomes inoperative.
The second taking-turn protocol is the token-passing protocol. In this protocol there is no master node. A small, special-purpose frame known as a token is exchanged among the nodes in some fixed order. For example, node 1 might always send the token to node 2, node 2 might always send the token to node 3, node N might always send the token to node 1. When a node receives a token, it holds onto the token only if it has some frames to transmit; otherwise, it immediately forwards the token to the next node. If a node does have frames to transmit when it receives the token, it sends up to a maximum number of frames and then forwards the token to the next node. Token passing is decentralized and has a high efficiency. But it has its problems as well. For example, the failure of one node can crash the entire channel. Or if a node accidentally neglects to release the token, then some recovery procedure must be invoked to get the token back in circulation? Over the years many token-passing products have been developed, and each one had to address these as well as other sticky issues.
Local Area Networks
Multiple access protocols are used in conjunction with many different types of broadcast channels. They have been used for satellite and wireless channels, whose nodes transmit over a common frequency spectrum. They are currently used in the upstream channel for cable access to the Internet (see Section 1.5). And they are extensively used in local area networks (LANs).
Recall that a LAN is a computer network that is concentrated in a geographical area, such as in a building or on a university campus. When a user accesses the Internet from a university or corporate campus, the access is almost always by way of a LAN. For this type of Internet access, the user’s host is a node on the LAN, and the LAN provides access to the Internet through a router, as shown in Figure 5.3-9. The LAN is a single “link” between each user host and the router; it therefore uses a link-layer protocol, which incorporates a multiple access protocol. The transmission rate, R, of most LANs is very high. Even in the early 1980s, 10 Mbps LANs were common; today, 100 Mbps LANs are common, and 1 Gbps LANs are available.
User hosts access an Internet Web server through a LAN. The broadcast channel between a user host and the router consists of one “link”.
In the 1980s and the early 1990s, two classes of LAN technologies were popular in the workplace. The first class consists of the Ethernet LANs (also known as 802.3 LANs [IEEE 1998b, Spurgeon 1999]), which are random-access based. The second class of LAN technologies are token-passing technologies, including token ring (also known as IEEE 802.5 [IEEE 1998]) and FDDI (also known as Fiber Distributed Data Interface [Jain 1994]). Because we shall explore the Ethernet technologies in some detail in Section 5.4, we focus our discussion here on the token-passing LANs. Our discussion on token-passing technologies is intentionally brief, since these technologies have become relatively minor players in the face of relentless Ethernet competition. Nevertheless, in order to provide examples about token-passing technology and to give a little historical perspective, it is useful to say a few words about token rings.
In a token ring LAN, the N nodes of the LAN (hosts and routers) are connected in a ring by direct links. The topology of the token ring defines the token-passing order. When a node obtains the token and sends a frame, the frame propagates around the entire ring, thereby creating a virtual broadcast channel. The node that sends the frame has the responsibility of removing the frame from the ring. FDDI was designed for geographically larger LANs (so called MANs, that is, metropolitan area networks). For geographically large LANs (spread out over several kilometers) it is inefficient to let a frame propagate back to the sending node once the frame has passed the destination node. FDDI has the destination node remove the frame from the ring. (Strictly speaking, FDDI is not a pure broadcast channel, as every node does not receive every transmitted frame.) You can learn more about token ring and FDDI by visiting the 3Com adapter page [3Com].
[Abramson 1970] N. Abramson, “The Aloha system,” AFIPS Conf. Proc., Vol. 37, 1970 Fall Joint Computer Confernce, AFIPS Press, Montvale, N.J., 1970, pp. 281-285.
[Abramson 1985] N. Abramson, “Development of the Alohanet,” IEEE Transactions on Information Theory,” Vol. IT-31, No. 3 (March 1985), pp. 119-123.
[Bertsekas 1992] D. Bertsekas and R. Gallager, Data Networks, Second Edition, Prentice Hall, Englewood Cliffs, New Jersey, 1992.
[3Com 1999] http://www.3com.com/products/nics.html
[Boggs 1988] D. Boggs, J. Mogul, and C. Kent, “Measured capacity of an Ethernet: myths and reality;” Proc ACM Sigcomm 1988, pp. 222 – 234
[IEEE 1998] IEEE, Token Ring Access Method (ISO/IEC 8802-5: 1998 and 8802-5 : 1998/Amd 1), 1998. See the 802.5 standards page at http://www.8025.org/802.5/documents/
[IEEE 1998b] IEEE, “Carrier sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications.” See the IEEE 802.3 publication catalog at http://standards.ieee.org/catalog/IEEE802.3.html
[Jain 1994] R. Jain, “FDDI Handbook : High-Speed Networking Using Fiber and Other Media,” Addison-Wesley (Reading MA, 1994).
[Kleinrock 1975] L. Kleinrock and F. A. Tobagi, “Packet Switching in Radio Channels: Part I — Carrier Sense Multiple-Access Modes and Their Throughput-Delay Characteristics,” IEEE Transactions on Communications, Vol. COM-23, No. 12, pp. 1400-1416, Dec. 1975.
[Lam 1980] S. Lam, A Carrier Sense Multiple Access Protocol for Local Networks,” Computer Networks, Volume 4, pp. 21-32, 1980.
[Metcalfe 1976] R. Metcalfe, D. Boggs, “Ethernet: Distributed packet switching for local computer networks,” Communications of the ACM, 19(7) (1976), pp. 395-404.
[Molle 87] M. Molle, “Space Time Analysis of CSMA Protocol,” IEEE Journal on Selected Areas in Communications, 1987.
[Pickholtz 1982] R. Pickholtz, D. Schilling, L. Milstein, “Theory of Spread Spectrum Communication – a Tutorial,” IEEE Transactions on Communications, Col. COM-30, No. 5 (May 1982), pp. 855-884.
[Rom 1990] R. Rom and M. Sidi, “Multiple Access Protocols: Performance and Analysis,” Springer-Verlag, New York, 1990.
[Spurgeon 1999] C. Spurgeon, “Charles Spurgeon’s Ethernet Web Site,” http://wwwhost.ots.utexas.edu/ethernet/ethernet-home.html
[Viterbi 1995] A. Viterbi, CDMA: Principles of Spread Spectrum Communication, Addison-Wesley, (Reading MA 1995).