WLAN Topologies

802.11 networks are flexible by design. You have the option of deploying three types of WLAN topologies:

• Independent basic service sets (IBSSs)
• Basic service sets (BSSs)
• Extended service sets (ESSs)

A service set is a logical grouping of devices. WLANs provide network access by broadcasting a signal across a wireless radio frequency (RF) carrier. A receiving station can be within range of a number of transmitters. The transmitter prefaces its transmissions with a service set identifier (SSID). The receiver uses the SSID to filter through the received signals and locate the one it wants to listen to.

IBSS
An IBSS consists of a group of 802.11 stations communicating directly with one another. An IBSS is also referred to as an ad-hoc network because it is essentially a simple peer-to-peer WLAN. Figure 2-1 illustrates how two stations equipped with 802.11 network interface cards (NICs) can form an IBSS and communicate directly with one another.

BSS
A BSS is a group of 802.11 stations communicating with one another. A BSS requires a specialized station known as an access point (AP). The AP is the central point of communications for all stations in a BSS. The client stations do not communicate directly other client stations. Rather, they communicate with the AP, and the AP forwards the frames to the destination stations. The AP might be equipped with an uplink port that connects the BSS to a wired network (for example, an Ethernet uplink). Because of this requirement, a BSS is also referred to as an infrastructure BSS. Figure 2-2 illustrates a typical infrastructure BSS.

ESS
Multiple infrastructure BSSs can be connected via their uplink interfaces. In the world of 802.11, the uplink interface connects the BSS to the distribution system (DS). The collection of BSSs interconnected via the DS is known as the ESS. Figure 2-3 shows a practical implementation of an ESS. The uplink to the DS does not have to be via a wired connection. The 802.11 specification leaves the potential for this link to be wireless. For the most part, DS uplinks are wired Ethernet.

Ethernet Technologies

802.3 Ethernet and the OSI Model

Diving deep in the OSI model is not the goal of this chapter, but you do need to focus on
Layer 2, the data link layer, to put Ethernet technologies into perspective. The data link layer has two sublayers, as illustrated in Figure 1-1:

• Data link sublayer— Also known as the MAC layer, this sublayer focuses on topology specific implementations. For example, 802.5 Token Ring networks have a different MAC than 802.3 Ethernet networks.
  

• Logical link (LLC) sublayer— Standard across all 802-based networks, this sublayer provides a simple frame protocol that provides connectionless frame delivery. There is no mechanism to notify the sender that the frame was or was not delivered.

The focus of the subsequent sections surrounds the MAC layer. This layer is unique to 802.3 networks and as such provides a reference point as you progress through the chapters on the wireless MAC.


The 802.3 Frame Format

Figure 1-2 depicts an Ethernet frame.

As Figure 1-2 illustrates, the Ethernet frame consists of the following fields:

• Preamble— The preamble is a set of 7 octets (an octet is a set of 8 bits) totaling 56 bits of alternating 1s and 0s. Each octet has the following bit pattern: 10101010. The preamble indicates to the receiving station that a frame is being transmitted on the medium. It is important to note that Ethernet topologies subsequent to 10 Mbps Ethernet still include the preamble but do not require one.
  

• Start of frame delimiter (SFD)— The SFD is an 8-bit field that has a bit pattern similar to the preamble, but the last 2 bits are both 1s (10101011). This pattern indicates to the receiving station that the frame's contents follow this field.
  

• Destination MAC address— The destination address field is a 48-bit value that indicates the destination station address of the frame.
  

• Source address— The source address field is a 48-bit value that indicates the station address of the sending station.

Type/length value (TLV)— The TLV field uses 16 bits to indicate what type of higher layer protocol is encapsulated in the data or payload field. The value contained in this field is also referred to as the Ethertype value. Table 1-1 lists some common Ethertype values.

Table 1-1. Some Common Ethernet Ethertypes

• Payload or data— The data or payload field carries upper-layer packets and must be a minimum of 46 bytes and a maximum of 1500 bytes in length. The minimum data or payload size is required to allow all stations a chance to receive the frame. This topic is discussed further in the section, "Ethernet Network Diameter and Ethernet Slot Time." If the data or payload is less than 46 bytes, the sending station pads the payload so it meets the minimum 46 bytes.
  

• Frame check sequence (FCS)— The FCS field contains a cyclic redundancy check (CRC) value calculated against the bit pattern of the frame. When the receiving station receives the frame, it calculates a CRC and compares it to what is in the FCS field. If the values match, the frame is considered error free (see Figure 1-3).

Ethernet Addressing

Ethernet addresses are 48-bit values that uniquely identify Ethernet stations on a LAN. Ethernet addresses are in part issued by a global authority, the IEEE, and in part by device vendors. The IEEE assigns unique 24-bit organizational unique identifiers (OUIs) to vendors. The OUI is the first 24 bits of the Ethernet address. The vendors themselves assign the remaining 24 bits. This process ensures that every Ethernet address is unique, and any station can connect to any network in world and be uniquely identified. Because this addressing describes a physical interface, it is also referred to as MAC addressing. For the most part, MAC addresses are expressed in hexadecimal form, with each byte separated by a dash or colon, or with every 2 bytes delimited with a period. For example, the following is an Ethernet address from a Cisco router:

00-03-6b-48-e9-20

You can also represent this value as 00:03:6b:48:e9:20 or 0003.6b48.e920

The IEEE has assigned the first 24 bits, 00-03-6b, to Cisco. The remaining 24 bits, 48-e9-20, have been assigned by Cisco to the device. The OUI of 00-03-6b allows the vendor to assign a range of addresses starting from 00-03-6b-00-00-00 to 00-03-6b-ff-ff-ff. This provides the vendor a total of 224 or 16,777,216 possible addresses.


CSMA/CD Architecture

The Ethernet networking standard is based on the CSMA/CD architecture. CSMA/CD is a halfduplex architecture, meaning only one station can transmit at a time. You can compare the CSMA/CD architecture to people communicating in a conference-call meeting:

• Each participant doesn't know when the other person is going to speak.
• A participant wanting to say something has to wait for the phone line to become quiet before she can start speaking.
• When the phone line becomes quiet, it is possible for two or more participants to start speaking at the same time.
• If two people speak at the same time, it is difficult for listeners to understand, so the speakers must stop talking and again wait for the line to become quiet before trying to speak again.

Consider Figure 1-4 where two stations are at extreme ends of the broadcast domain:

• Station transmits a frame and that is smaller than 512 bits.
• At the same moment, Station B begins transmitting a frame.
• Station A transmits the last bit of its frame.
• Station A does not detect a collision during transmission and discards the frame from its transmit buffer.
• Station A assumes that the destination station of its frame received the frame.
• Station A's frame collides with Station B's frame.
• Station A has already discarded the frame from its transmit buffer, so Station A has no frame to retransmit.

Unicast, Multicast, and Broadcast Frames

A station can address its frames for transmission using one of three methods:

• Broadcast addressing— The station sends the frame to all stations in the broadcast domain.
• Group or multicast addressing— The station addresses its frames to a subset of all stations in the broadcast domain that belong a predefined group.
• Unicast addressing— The station addresses its frames to a specific station.

Figure 1-5 depicts these addressing types. Ethernet networks use all three methods. No one method is a panacea. Each method has pros and cons for its use.

802.3u Fast Ethernet

As Ethernet became more accepted as a standard for data networking, users began demanding more bandwidth. To calm the screaming masses, the IEEE announced 802.3u, the standard for 100 Mbps Ethernet in 1995. Although there were a number of 100 Mbps solutions for Ethernet, two have become the most common options: 100BASE-TX and 100BASE-FX (both are collectively referred to as 100BASE-X). 100BASE-X technology is based on the non-IEEE standard FDDI (ANSI X3T9.5). FDDI was the de facto 100 Mbps standard before Fast Ethernet and had a number of advantages to shared Ethernet.

100BASE-TX applies the 100BASE-X specification to Category 5 twisted-pair cabling. 100BASE-TX is similar to 10BASE-T in many ways, but unlike 10BASE-T, 100BASE-TX requires Category 5 cabling. 100BASE-TX performs a great deal of high-frequency signaling that requires a higher grade of cable than the Category 3 required for 10BASE-T. 100BASETX also has the same distance limitation of roughly 100 m that 10BASE-T has, meaning the same cabling infrastructure can be leveraged (assuming it is Category 5 or better).

The network diameter and Ethernet slot time for Fast Ethernet networks change from Ethernet to 100BASE-X networks. The Ethernet slot time defines the maximum network diameter by stipulating that the diameter should not exceed the distance a 512-bit frame can travel before the transmitting station is done sending that frame. Fast Ethernet systems maintain the use of the 512-bit frame size to maintain backward compatibility with legacy Ethernet systems.

For Ethernet networks, the maximum diameter is 2800 m. With 100BASE-TX, the transmit operations occur 10 times faster than the transmit operations of Ethernet stations. Accordingly, for a sending station to detect a collision after sending the 512-bit frame, the frame can only travel one-tenth the distance. This limit reduces the maximum network diameter from 2800 m to roughly 200 m. The loss of distance does not pose a real issue because most Fast Ethernet deployments use 100BASE-TX, which has a maximum distance of 100 m anyway.

100BASE-FX is a variant of 100BASE-X that uses multimode fiber as the medium to transmit data. The network interface card (NIC) converts electric signals into pulses of light that are sent over the fiber medium to the receiving NIC. The receiving NIC then translates the light pulses back into electrical signals that the receiving station can process.

100BASE-FX uses the same encoding mechanism as 100BASE-TX, but that is where the similarities end between 100BASE-TX and 100BASE-FX. Because 100BASE-FX uses light to carry data through the medium, there is no electromagnetic interference to be concerned with. This setup allows for a more efficient signaling scheme. The maximum network diameter for 100BASE-FX is roughly 400 m in half-duplex mode. 100BASE-FX can also operate in full-duplex mode. (Duplex modes are discussed next.) Full-duplex operation essentially eliminates the issues surrounding collisions, so 100BASE-FX can safely extend to distances beyond 400 m. In fact, using standard 62.5/125 micron multimode fiber, 100BASEFX can extend to 2 km while in full-duplex mode. If connectivity requirements dictate distances beyond 2 km, single-mode transceivers are available that allow 100BASE-FX to operate over single-mode fiber to distances up to 40 km. The cost of single-mode transceivers and single-mode fiber is an order of magnitude more expensive than its multimode brethren, but the solution exists if needed.


Full-Duplex Operation

CSMA/CD is the methodology that half-duplex Ethernet and Fast Ethernet is based on. As described earlier, CSMA/CD is like a telephone conference call. Each participant must wait until the medium is available before he can speak. In 1995, the IEEE ratified 802.3x, which specifies a new methodology for transmission in Ethernet networks known as full-duplex operation. Full-duplex operation allows a station to send and receive frames simultaneously, allowing greater use of the medium and higher overall throughput (see Figure 1-8). Fullduplex operation significantly changes the requirements placed on end stations, however.


Full-duplex operation works only in a point-to-point environment. There can be only one other device in the collision domain. Stations connected to hubs, repeaters, and the like are unable to operate in full-duplex mode. Stations connected back-to-back or connected to Layer 2 switches (that support full-duplex mode) are able to use full-duplex mode.

The capability to transmit and receive at the same rate allows stations to better utilize the network medium. The bandwidth available to the station is theoretically doubled because the station has full access to the medium in the send direction and the receive direction. In the case of 100BASE-X, this access gives each station up to 200 Mbps of maximum bandwidth. For end stations, such as PCs, the truth is that few stations transmit and receive at the same time. Stations such as servers and networking infrastructure such as routers and switches can take advantage of full-duplex mode in a manner that end stations cannot. The devices aggregate sessions and connections from the edge of the network to the core and back. They send and receive traffic distributed in both the send and receive directions, so these links are able to really take advantage of the extra bandwidth that full-duplex operation provides.

Full-duplex operation allows Ethernet topologies to break free from the distance limitations that half-duplex operations impose on them. Ironically, only fiber-based interfaces can take advantage of additional distances (as 100BASE-FX does) because twisted-pair deployments are distance-limited by the physical medium itself and not the network diameter imposed by Ethernet or Fast Ethernet time slots.

Gigabit Ethernet

The jump from Ethernet to Fast Ethernet gave users 10 times more available bandwidth. Gigabit Ethernet, with a data rate of 1000 Mbps, offers the same proportioned jump for Fast Ethernet users, but the difference is 900 Mbps more available bandwidth as opposed to 90 Mbps. This substantial increase in bandwidth places a strain on developers who must solve network diameter issues and cabling issues. Gigabit Ethernet has two main areas:

• 1000BASE-T— Like its 10BASE-T and 100BASE-TX brethren, 1000BASE-T supports UTP cabling at a distance of up to 100 m.
  

• 1000BASE-X— 1000BASE-X has three subcategories:
  

1000BASE-SX— A fiber-optic–based medium designed for use over standard multimode fiber for short-haul runs up to 200 m.
1000BASE-LX— A fiber-optic–based medium designed for use over singlemode fiber for long runs of up to 10 km, although it is possible to use modeconditioned multimode fiber in some cases.
1000BASE-CX— A shielded copper medium designed for short patches between devices. 1000BASE-CX is limited to distances of 25 m.


802.3ab 1000BASE-T

The development of the 1000BASE-T standard stemmed from the efforts of Fast Ethernet development. The search for the ideal Fast Ethernet copper solution drove the adoption of 100BASE-TX. Although not well known, there were two other standards: 100BASE-T4 and
100BASE-T2. 100BASE-T4 was not a popular solution because it required the use of all four pairs of Category 3 or 5 cabling. Some installations wired only two-pair Category 3 or 5 cabling in accordance with the requirements of 10BASE-T. 100BASE-T4 also missed the mark by not supporting full-duplex operation.

100BASE-T2 was a more far-reaching specification, enabling 100 Mbps operation over Category 3 cabling using only two pairs. The problem is that no vendor ever implemented the standard. When the time came to develop the gigabit solution for the Ethernet standard, developers took the best of all the 100 Mbps standards and incorporated them into the 1000BASE-T specification.


802.3z 1000BASE-X

802.3z was ratified in 1999 and included in the 802.3 standard. 1000BASE-X is the specification for Gigabit Ethernet over a fiber-optic medium. The underlying technology itself is not new because it is based on the ANSI Fibre Channel standard (ANSI X3T11). 1000BASEX comes in three media types: 1000BASE-SX, 1000BASE-LX, and 1000BASE-CX. 1000BASESX is the most common and least expensive media, using standard multimode fiber. The low cost is not without shortcomings; 1000BASE-SX has a maximum distance of 220 m (compared with full-duplex 100BASE-FX at 2 km). 1000BASE-LX generally utilizes singlemode fiber and can span distances up to 5 km.

1000BASE-CX is the oddball of the three media types. It is a copper-based solution that requires precrimped shielded twisted-pair cabling. The connector is not the familiar RJ-45 of 10/100/1000BASE-T. Instead, you use either a DB-9 or HSSDC connector to terminate the two pairs of wire. 1000BASE-CX can span lengths of up to 25 m, relegating it to wiring closet patches. 1000BASE-CX is not all that common because 1000BASE-T provides the same function for a fraction of the price, and four times the cable length, using standard four-pair, Category 5 cabling.

Ethernet has evolved to support new requirements that users and network administrators demand. It continues to evolve beyond Gigabit Ethernet with its next iteration, 10 Gigabit Ethernet, on the horizon. Table 1-3 gives a summary of the Ethernet family of topologies and their media types. Each topology has a place in networking today, determined by requirements such as cost, required data rate, distance, and existing cable plant. Wired Ethernet shows that backward compatibility is what allows new topologies to prosper, develop, and become accepted standards.