Section 2.1: Network Technologies
Various network technologies can be used to establish switched connections within the campus network. These are: Ethernet, Fiber Distribution Data Interface (FDDI), Copper Distribution Data Interface (CDDI), Token Ring, and Asynchronous Transfer Mode (ATM) that can be used in a campus network. Ethernet is emerging as the most popular choice in installed networks because of its low cost, availability, and scalability to higher bandwidths. Ethernet scales to support increasing bandwidths, and should be chosen to match the need at each point in the campus network. As network bandwidth requirements grow, the links between access, distribution, and core layers can be scaled to match the load.
2.1.1: Ethernet
Ethernet is a LAN technology that provides shared media access to many connected stations. It is based on the Institute of Electrical and Electronics Engineers (IEEE) 802.3 standard and offers a bandwidth of 10 Mbps between end users. In its most basic form, Ethernet is a shared media that becomes both a collision and a broadcast domain. As the number of users on the shared media increases, so does the probability that a user is trying to transmit data at any given time. Ethernet is based on the carrier sense multiple access collision detect (CSMA/CD) technology, which requires that transmitting stations back off for a random period of time when a collision occurs.
In a campus network environment, Ethernet is usually used in the access layer, between end user devices and the access layer switch. Ethernet is not typically used at either the distribution or core layer.
2.1.1.1: Ethernet Switches
As the number of users on an Ethernet segment increases, the segment becomes less efficient. Ethernet switching addresses this problem by dynamically allocating a dedicated 10 Mbps bandwidth to each of its ports. The resulting increased network performance occurs by reducing the number of users connected to an Ethernet segment. To improve performance even further, an Ethernet switch can be implemented. An Ethernet switch provides all users with dedicated 10 Mbps connections. However, if an enterprise server is located elsewhere in the network, then all of the switched users must still share available bandwidth across the campus to reach it. A network design based on careful observation of traffic patterns and flows would thus need to be implemented.
Switched Ethernet removes the possibility of collisions, thus stations do not have to listen to each other in order to take a turn transmitting on the wire. Instead, stations can operate in full-duplex mode, transmitting and receiving simultaneously. This further increases network performance, with a net throughput of 10 Mbps in each direction, or 20 Mbps total on each port.
2.1.1.2: Ethernet Media
Coaxial cable was the first media system specified in the Ethernet standard. Coaxial Ethernet cable comes in two major categories: Thicknet (10Base5) and Thinnet (10Base2). These cables differed in their size and their length limitation. Although Ethernet coaxial cable lengths can be quite long, they susceptible to electromagnetic interference (EMI) and eavesdropping.
Table 2.1: Coaxial Cable for Ethernet
|
Cable |
Diameter |
Resistance |
Bandwidth |
Length |
|
Thinnet (10Base2) |
10 mm |
50 ohms |
10 Mbps |
185 m |
|
Thicknet (10Base5) |
5 mm |
50 ohms |
10 Mbps |
500 m |
Today most networks use twisted-pair media for connections to the desktop. Twisted-pair also comes in two major categories: Unshielded twisted-pair (UTP) and Shielded twisted-pair (STP). One pair of insulated copper wires twisted about each other forms a twisted-pair. The pairs are twisted top reduce interference and crosstalk. Both STP and UTP suffer from high attenuation, therefore these lines are usually restricted to an end-to-end distance of 100 meters between active devices. Furthermore, these cables are sensitive to EMI and eaves dropping. Most networks use 10BaseT UPT cable.
An alternative to twisted-pair is fiber optic cable (10BaseFL). Instead of transmitting electrical signals, as coaxial and twisted-pair cables do, fiber optic cable transmits light signals which are generated either by light emitting diodes (LEDs) or laser diodes (LDs). There are two major categories of fiber optic cables: multimode cables and single-made cables. Multimode cables transmit many wavelengths of the same light source (LDs) along multiple light paths. As a result the light pulse at the end of the cable is more blurred. Single-mode cables transmit a single wavelength light that is generated by LEDs along the same path. These cables support higher transmission speeds and longer distances but are more expensive. Because they do not carry electrical signals, fiber optic cables are immune to EMI and eavesdropping. They also have low attenuation which means they can be used to connect active devices that are up to 2 km apart. However, fiber optic devices are not cost effective while cable installation is complex.
Table 2.2: Twisted-Pair and Fiber Optic Cable for Ethernet
|
Cable |
Technology |
Bandwidth |
Cable Length |
|
Twisted-Pair |
(1000BaseT) |
1000 Mbps |
100 m |
|
Fiber Optic |
(1000BaseFL) |
1000 Mbps |
2,000 m |
2.1.2: Cisco Long Reach Ethernet (LRE)
Cisco LRE can be transported over lengthy distances over Category 1, 2, or 3 wiring.
A LRE connection requires the following tools:
• Cisco Catalyst 2900 LRE XL Switch: Combines 12 or 24 LRE connections at the building head-end
• Cisco 575 or 585 LRE CPE: Ends the LRE connection in the tenant room
• Cisco LRE 48 POTS Splitter: Divides POTS and LRE on 48 ports when a building is using current telephone wiring.
2.1.3: Fast Ethernet
To address the demand of modern networks for greater bandwidth, the networking industry developed a higher-speed Ethernet based on the existing Ethernet standards. Fast Ethernet operates at 100 Mbps and is based on the IEEE 802.3u standard. The Ethernet cabling schemes, CSMA/CD operation, and all upper-layer protocol operations have been maintained with Fast Ethernet. The net result is the same data link Media Access Control (MAC) layer merged with a new physical layer.
Furthermore, the Fast Ethernet specification is backward compatible with 10 Mbps Ethernet. Compatibility is possible because the two devices at each end of a network connection can automatically negotiate link capabilities so that they both can operate at a common level. This negotiation involves the detection and selection of the highest available bandwidth and half-duplex or full-duplex operation. For this reason, Fast Ethernet is also referred to as 10/100 Mbps Ethernet.
The larger bandwidth available with Fast Ethernet can support the aggregate traffic from multiple Ethernet segments in the access layer. Fast Ethernet can also be used to connect distribution layer switches to the core, with either single or multiple redundant links. It can also be used to connect faster end user workstations to the access layer switch, and to provide improved connectivity to enterprise servers. Therefore, Fast Ethernet can be successfully deployed at all layers within a campus network.
In addition, Cisco provides Fast EtherChannel (FEC), which allows several Fast Ethernet links to be bundled together for increased throughput. Fast EtherChannel (FEC) allows two to eight full-duplex Fast Ethernet links to act as a single physical link, for 400- to 1600-Mbps bandwidth. EtherChannel is discussed in more detail in Section 4.1.
Cabling for Fast Ethernet can be either UTP or fiber optic. Specifications for Fast Ethernet cables are shown in Table 2.3.
Table 2.3: Fast Ethernet Cabling and Distance Limitations
|
Technology |
Wiring Type |
Pairs |
Cable Length |
|
100BaseTX |
EIA/TIA Category 5 UTP |
2 |
100 m |
|
100BaseT2 |
EIA/TIA Category 3,4,5 UTP |
2 |
100 m |
|
100BaseT4 |
EIA/TIA Category 3,4,5 UTP |
4 |
100 m |
|
100BaseFX |
Multimode fiber (MMF) with 62.5 micron core; 1300 nm laser |
1 |
400 m (half-duplex) 2,000 m (full-duplex) |
|
Single-mode fiber (SMF) with 62.5 micron core; 1300 nm laser |
1 |
10,000 m |
2.1.4: Gigabit Ethernet
Gigabit Ethernetis an escalation of the Fast Ethernet standard using the same IEEE 802.3 Ethernet frame format. Gigabit Ethernet offers a throughput of 1,000 Mbps (1 Gbps). Like Fast Ethernet, Gigabit Ethernet is compatible with earlier Ethernet types. However, the physical layer has been modified to increase data transmission speeds: The IEEE 802.3 Ethernet standard and the American National Standards Institute (ANSI) X3T11 FibreChannel. IEEE 802.3 provided the foundation of frame format, CSMA/CD, full duplex, and other characteristics of Ethernet. FibreChannel provided a base of high-speed ASICs, optical components, and encoding/decoding and serialization mechanisms. The resulting protocol is termed IEEE 802.3z Gigabit Ethernet.
In a campus network, Gigabit Ethernet can be used in the switch block, the core block, and in the server block. In the switch block, it is used to connect access layer switches to distribution layer switches. In the core, it connects the distribution layer to the core switches, and also interconnects the core devices. For a server block, a Gigabit Ethernet switch in the server block can provide high-speed connections to individual servers.
Cisco has extended FEC to allow you to bundle several Gigabit Ethernet links. Gigabit EtherChannel (GEC) allows two to eight full-duplex Gigabit Ethernet connections to be aggregated, for up to 16 Gbps throughput.
Gigabit Ethernet supports several cabling types, referred to as 1000BaseX. Table 2.4 lists the cabling specifications for each type.
Table 2.4: Gigabit Ethernet Cabling and Distance Limitations
|
Technology |
Wiring Type |
Pairs |
Cable Length |
|
1000BaseCX |
Shielded Twisted Pair (STP) |
1 |
25 m |
|
1000BaseT |
EIA/TIA Category 5 UTP |
4 |
100 m |
|
1000BaseSX |
Multimode fiber (MMF) with 62.5 micron core; 850 nm laser |
1 |
275 m |
|
Multimode fiber (MMF) with 50 micron core; 1300 nm laser |
1 |
550 m | |
|
1000BaseLX/LH |
Multimode fiber (MMF) with 62.5 micron core; 1300 nm laser |
1 |
550 m |
|
Single-mode fiber (SMF) with 50 micron core; 1300 nm laser |
1 |
550 m | |
|
Single-mode fiber (SMF) with 9 micron core; 1300 nm laser |
1 |
10 km | |
|
1000BaseZX |
Single-mode fiber (SMF) with 9 micron core; 1550 nm laser |
1 |
70 km |
|
Single-mode fiber (SMF) with 8 micron core; 1550 nm laser |
1 |
100 km |
2.1.5: 10Gigabit Ethernet
Gigabit Ethernet has been further extended to 10Gigabit Ethernet, using the same IEEE 802.3 Ethernet frame format. 10Gigabit Ethernet offers a throughput of 10 Gbps and is compatible with earlier Ethernet types. However, it only functions over optical fiber, and only operates in full-duplex mode, thus making collision detection protocols (CSMA/CD) unnecessary.
In a campus network, 10Gigabit Ethernet can be used in the switch block, the core block, and in the server block. It can be used to connect access layer switches to distribution layer switches and distribution layer switches to the core switches; however, it's most practical application is to interconnect the core devices. For a server block, a 10Gigabit Ethernet switch in the server block can provide high-speed connections to individual servers.