Black Box Explains...Layer 2, 3, and 4 switches.
... more/see it now
E-Mail, Diagnostics, Word Processing, Database
Shells and Gateway Workstation Software
TR=Token Ring; A=ARCNET®; P=PhoneNET®
With the rapid development of computer networks over the last decade, high-end switching has become one of the most important functions on a network for moving data efficiently and quickly from one place to another.
Here’s how a switch works: As data passes through the switch, it examines addressing information attached to each data packet. From this information, the switch determines the packet’s destination on the network. It then creates a virtual link to the destination and sends the packet there.
The efficiency and speed of a switch depends on its algorithms, its switching fabric, and its processor. Its complexity is determined by the layer at which the switch operates in the OSI (Open Systems Interconnection) Reference Model (see above).
OSI is a layered network design framework that establishes a standard so that devices from different vendors work together. Network addresses are based on this OSI Model and are hierarchical. The more details that are included, the more specific the address becomes and the easier it is to find.
The Layer at which the switch operates is determined by how much addressing detail the switch reads as data passes through.
Switches can also be considered low end or high end. A low-end switch operates in Layer 2 of the OSI Model and can also operate in a combination of Layers 2 and 3. High-end switches operate in Layer 3, Layer 4, or a combination of the two.
Layer 2 Switches (The Data-Link Layer)
Layer 2 switches operate using physical network addresses. Physical addresses, also known as link-layer, hardware, or MAC-layer addresses, identify individual devices. Most hardware devices are permanently assigned this number during the manufacturing process.
Switches operating at Layer 2 are very fast because they’re just sorting physical addresses, but they usually aren’t very smart—that is, they don’t look at the data packet very closely to learn anything more about where it’s headed.
Layer 3 Switches (The Network Layer)
Layer 3 switches use network or IP addresses that identify locations on the network. They read network addresses more closely than Layer 2 switches—they identify network locations as well as the physical device. A location can be a LAN workstation, a location in a computer’s memory, or even a different packet of data traveling through a network.
Switches operating at Layer 3 are smarter than Layer 2 devices and incorporate routing functions to actively calculate the best way to send a packet to its destination. But although they’re smarter, they may not be as fast if their algorithms, fabric, and processor don’t support high speeds.
Layer 4 Switches (The Transport Layer)
Layer 4 of the OSI Model coordinates communications between systems. Layer 4 switches are capable of identifying which application protocols (HTTP, SNTP, FTP, and so forth) are included with each packet, and they use this information to hand off the packet to the appropriate higher-layer software. Layer 4 switches make packet-forwarding decisions based not only on the MAC address and IP address, but also on the application to which a packet belongs.
Because Layer 4 devices enable you to establish priorities for network traffic based on application, you can assign a high priority to packets belonging to vital in-house applications such as Peoplesoft, with different forwarding rules for low-priority packets such as generic HTTP-based Internet traffic.
Layer 4 switches also provide an effective wire-speed security shield for your network because any company- or industry-specific protocols can be confined to only authorized switched ports or users. This security feature is often reinforced with traffic filtering and forwarding features. collapse
Black Box Explains...Ethernet.
If you have an existing network, there’s a 90% chance it’s Ethernet. If you’re installing a new network, there’s a 98% chance it’s Ethernet—the Ethernet standard is... more/see it nowthe overwhelming favorite network standard today.
Ethernet was developed by Xerox®, DEC®, and Intel® in the mid-1970s as a 10-Mbps (Megabits per second) networking protocol—very fast for its day—operating over a heavy coax cable (Standard Ethernet).
Today, although many networks have migrated to Fast Ethernet (100 Mbps) or even Gigabit Ethernet (1000 Mbps), 10-Mbps Ethernet is still in widespread use and forms the basis of most networks.
Ethernet is defined by international standards, specifically IEEE 802.3. It enables the connection of up to 1024 nodes over coax, twisted-pair, or fiber optic cable. Most new installations today use economical, lightweight cables such as Category 5 unshielded twisted-pair cable and fiber optic cable.
How Ethernet Works
Ethernet signals are transmitted from a station serially, one bit at a time, to every other station on the network.
Ethernet uses a broadcast access method called Carrier Sense Multiple Access/Collision Detection (CSMA/CD) in which every computer on the network hears every transmission, but each computer listens only to transmissions intended for it.
Each computer can send a message anytime it likes without having to wait for network permission. The signal it sends travels to every computer on the network. Every computer hears the message, but only the computer for which the message is intended recognizes it. This computer recognizes the message because the message contains its address. The message also contains the address of the sending computer so the message can be acknowledged.
If two computers send messages at the same moment, a collision occurs, interfering with the signals. A computer can tell if a collision has occurred when it doesn’t hear its own message within a given amount of time. When a collision occurs, each of the colliding computers waits a random amount of time before resending the message.
The process of collision detection and retransmission is handled by the Ethernet adapter itself and doesn’t involve the computer. The process of collision resolution takes only a fraction of a second under most circumstances. Collisions are normal and expected events on an Ethernet network. As more computers are added to the network and the traffic level increases, more collisions occur as part of normal operation. However, if the network gets too crowded, collisions increase to the point where they slow down the network considerably.
Standard (Thick) Ethernet (10BASE5)
Thin Ethernet (ThinNet) (10BASE2)
- Uses thick coax cable with N-type connectors for a backbone and a transceiver cable with 9-pin connectors from the transceiver to the NIC.
- Both ends of each segment should be terminated with a 50-ohm resistor.
- Maximum segment length is 500 meters.
- Maximum total length is 2500 meters.
- Maximum length of transceiver cable is 50 meters.
- Minimum distance between transceivers is 2.5 meters.
- No more than 100 transceiver connections per segment are allowed.
Twisted-Pair Ethernet (10BASE-T)
- Uses "Thin" coax cable.
- The maximum length of one segment is 185 meters.
- The maximum number of segments is five.
- The maximum total length of all segments is 925 meters.
- The minimum distance between T-connectors is 0.5 meters.
- No more than 30 connections per segment are allowed.
- T-connectors must be plugged directly into each device.
Fiber Optic Ethernet (10BASE-FL, FOIRL)
- Uses 22 to 26 AWG unshielded twisted-pair cable (for best results, use Category 4 or 5 unshielded twisted pair).
- The maximum length of one segment is 100 meters.
- Devices are connected to a 10BASE-T hub in a star configuration.
- Devices with standard AUI connectors may be attached via a 10BASE-T transceiver.
- Uses 50-, 62.5-, or 100-micron duplex multimode fiber optic cable (62.5 micron is recommended).
- The maximum length of one 10BASE-FL (the new standard for fiber optic connections) segment is 2 kilometers.
- The maximum length of one FOIRL (the standard that preceded the new 10BASE-FL) segment is 1 kilometer.
Black Box Explains...Single-strand fiber WDM.
Traditional fiber optic media converters perform a useful function but don’t really reduce the amount of cable needed to send data on a fiber segment. They still require two strands... more/see it nowof glass to send transmit and receive signals for fiber media communications. Wouldn’t it be better to combine these two logical communication paths within one strand?
That’s exactly what single-strand fiber conversion does. It compresses the transmit and receive wavelengths into one single-mode fiber strand.
The conversion is done with Wave-Division Multiplexing (WDM) technology. WDM technology increases the information-carrying capacity of optical fiber by transmitting two signals simultaneously at different wavelengths on the same fiber. The way it usually works is that one unit transmits at 1310 nm and receives at 1550 nm. The other unit transmits at 1550 nm and receives at 1310 nm. The two wavelengths operate independently and don’t interfere with each other. This bidirectional traffic flow effectively converts a single fiber into a pair of “virtual fibers,” each driven independently at different wavelengths.
Although most implementations of WDM on single-strand fiber offer two channels, four-channel versions are just being introduced, and versions offering as many as 10 channels with Gigabit capacity are on the horizon.
WDM on single-strand fiber is most often used for point-to-point links on a long-distance network. It’s also used to increase network capacity or relieve network congestion. collapse
Black Box Explains…Media converters that also work as switches.
Media converters transparently convert the incoming electrical signal from one cable type and then transmit it over another type—thick coax to Thin, UTP to fiber, and so on. Traditionally, media... more/see it nowconverters were purely Layer 1 devices that only converted electrical signals and physical media and didn’t do anything to the data coming through the link.
Today’s media converters, however, are often more advanced Layer 2 Ethernet devices that, like traditional media converters, provide Layer 1 electrical and physical conversion. But, unlike traditional media converters, they also provide Layer 2 services and route Ethernet packets based on MAC address. These media converters are often called media converter switches, switching media converters, or Layer 2 media converters. They enable you to have multiple connections rather than just one simple in-and-out connection. And because they’re switches, they increase network efficiency.
Media converters are often used to connect newer 100-Mbps, Gigabit Ethernet, or ATM equipment to existing networks, which are generally 10BASE-T, 100BASE-T, or a mixture of both. They can also be used in pairs to insert a fiber segment into copper networks to increase cabling distances and enhance immunity to electromagnetic interference.
Rent an apartment…
Media converters are available in standalone models that convert between two different media types and in chassis-based models that house many media converters in a a single chassis.
Standalone models convert between two media. But, like a small apartment, they can be outgrown.
Consider your current and future applications before selecting a media converter. A good way to anticipate future network requirements is to choose media converters that work as standalone devices but can be rackmounted if needed later.
…or buy a house.
Chassis-based or modular media converter systems are normally rackmountable and have slots to house media converter modules. Like a well-planned house, the chassis gives you room to grow. These are used when many Ethernet segments of different media types need to be connected in a central location. Modules are available for the same conversions performed by the standalone converters, and they enable you to mix different media types such as 10BASE-T, 100BASE-TX, 100BASE-FX, ATM, and Gigabit modules. Although enterprise-level chassis-based systems generally have modules that can only be used in a chassis, many midrange systems feature modules that can be used individually or in a chassis. collapse
Black Box Explains...LAN switches.
Rush hour-all day, every day.
Applications such as document imaging, video/multimedia production, and intranetworking are very demanding. They generate huge data files that often must be transferred... more/see it nowbetween stations based on strict timing requirements. If such traffic is not transmitted efficiently, you end up with jerky video, on-screen graphics that take forever to load, or other irritating, debilitating problems.
These problems arise because in traditional LANs, only one network node transmits data at a time while all other stations listen. This works in conventional, server-based LANs where multiple workstations share files or applications housed on a central server. But if a network has several servers, or if it supports high-bandwidth, peer-to-peer applications such as videoconferencing, the one-station-at-a-time model just doesn’t work.
Ideally, each LAN workstation should be configured with its own dedicated LAN cable segment. But that’s neither practical nor affordable. A far more reasonable solution is a network designed to provide clear paths from each workstation to its destination on demand, whether that destination is another workstation or server.
These vehicles clear the lanes.
Unlike bridges and routers, which process data packets on an individual, first-come, first-served basis, switches maintain multiple, simultaneous data conversions among attached LAN segments.
From the perspective of an end-user workstation, a switched circuit appears to be a dedicated connection-a direct, full-speed LAN link to an attached server or other remote LAN node. Although this technique is somewhat different from what a LAN bridge or router does, switching hubs are based on similar technologies.
Which route will you choose?
Switching hubs that use bridging technologies are called Layer 2 switches-a reference to Layer 2 or the Data-Link Layer of the OSI Model. These switches operate using the MAC addresses in Layer 2 and are transparent to network protocols. Switches that use routing technologies are known as Layer 3 switches, referring to Layer 3—the Network Layer—of the OSI Model. These switches, like routers, represent the next higher level of intelligence in the hardware hierarchy. Rather than passing packets based on MAC addresses, these switches look into the data structure and route it based on the network addresses found in Layer 3. They are also dependent on the network protocol.
Layer 2 switches connect different parts of the same network as determined by the network number contained with the data packet. Layer 3 switches connect LANs or LAN segments with different network numbers.
If you’re subdividing an existing LAN, obviously you’re dealing with only one network and one network number, so you can install a Layer 2 switch wherever it will segment network traffic the best, and you don’t have to reconfigure the LAN. However, if you use a Layer 3 switch, you’ll have to reconfigure the segments to ensure that each has a different network number.
Similarly, if you’re connecting existing networks, you have to examine the currently configured network numbers before adding a switch. If the network numbers are the same, you need to use a Layer 2 switch. If they’re different, you must use a Layer 3 switch.
When dealing with multiple existing networks, you’ll find they usually use different network numbers. In this case, it’s preferable to use a Layer 3 switch (or possibly even a full-featured router) to avoid reconfiguring the network.
But what if you’re designing a network from scratch and can choose either type of switch? Your decision should be based on the expected complexity of your LAN. Layer 3 routing technology is well suited for complex networks. Layer 2 switches are recommended for smaller, less complex networks. collapse
Black Box Explains...Layer 2, 3, and 4 switching.
The Open Systems Interconnection (OSI) Reference Model provides a layered network design framework that establishes a standard so that devices from different vendors work together.
Layer 2 (The Data-Link Layer)
Layer 2... more/see it nowswitches operate using physical network addresses. Physical addresses, also known as link-layer, hardware, or MAC-layer addresses, identify individual devices. Most hardware devices are permanently assigned this number during the manufacturing process.
Switches operating at Layer 2 are very fast because they’re just sorting physical addresses, but they usually arent very smart.
Layer 3 (The Network Layer)
Layer 3 switches use network or IP addresses that identify locations on the network. Physical addresses identify devices; network addresses identify locations. A location can be a LAN workstation, a location in a computer’s memory, or even a packet of data traveling through a network.
Network addresses are hierarchical. The more details included, the more specific the address becomes and the easier it is to find.
Switches operating at Layer 3 are smarter than Layer 2 devices and incorporate routing functions to actively calculate the best way to send a packet to its destination. However, because Layer 3 Switches take the extra time to read more details of a network address, they are sometimes much slower than Layer 2 Switches.
Layer 4 (The Transport Layer)
Layer 4 of the OSI Model coordinates communications between systems. Layer 4 identifies which application protocols (HTTP, SNTP, FTP, etc.) are included with each packet and uses this information to hand off the packet to the appropriate higher-layer software. Layer 4 switches make packet forwarding decisions based not only on the MAC address and IP address, but also on the application a packet belongs to.
Because Layer 4 devices enable you to establish priorities for network traffic based on application, you can assign a high priority to packets belonging to vital in-house applications, such as Peoplesoft®, with different forwarding rules for low-priority packets, such as generic HTTP-based Internet traffic.
Layer 4 switches also provide an effective wire-speed security shield for your network. collapse
Black Box Explains...DIN rails.
A DIN rail is an industry-standard metal rail, usually installed inside an electrical enclosure, which serves as a mount for small electrical devices specially designed for use with DIN rails.... more/see it nowThese devices snap right onto the rails, sometimes requiring a set screw, and are then wired together.
Many different devices are available for mounting on DIN rails: terminal blocks, interface converters, media converter switches, repeaters, surge protectors, PLCs, fuses, or power supplies, just to name a few.
DIN rails are a space-saving way to accommodate components. And because DIN rail devices are so easy to install, replace, maintain, and inspect, this is an exceptionally convenient system that has become very popular in recent years.
A standard DIN rail is 35-mm wide with raised-lip edges, its dimensions outlined by the Deutsche Institut für Normung, a German standardization body. Rails are generally available in aluminum or steel and may be cut for installation. Depending on the requirements of the mounted components, the rail may need to be grounded. collapse
Black Box Explains…Energy-Efficient Ethernet.
The IEEE 802.3az Ethernet standard, ratified in 2010, provides a standardized way for some Ethernet devices to reduce power consumption. Energy-Efficient Ethernet devices have a low-power idle (LPI) mode that... more/see it nowcan cut power use by 50% or more during periods of low data activity. Because energy-efficient Ethernet devices scale down power consumption when the load is lower, they save both the energy used to power processors and the energy used to cool them.
These energy savings are currently available for 100BASE-TX, 1000BASE-T, and 10GBASE-T Ethernet as well as some backplane Ethernet. 802.3az can be found on most types of network equipment, including NICs, switches, routers, and media converters. Because these devices are totally backwards compatible with other Ethernet devices, all you need to do to reap energy savings is to swap out devices.
Black Box Explains...High-speed networking.
What are my choices for high-speed
... more/see it nownetworking?
| 100BASE-T | ATM
| Gibabit Ethernet
Switched Ethernet relies on centralized multiport switches
to provide a physical link between multiple LAN segments.
Inside each intelligent switch, high-speed circuitry supports
wire-speed virtual connections between all the segments
for maximum bandwidth allocation on demand. Adding new segments
to a switch increases the aggregate network speed while
reducing overall congestion, so Switched Ethernet provides
superior configuration flexibility. It also gives you an
excellent migration path from 10- to 100-Mbps Ethernet,
since both segments can often operate via the same switch.
Benefits of Switched Ethernet—It’s
a cost-effective technique for increasing the overall network
throughput and reducing congestion on a 10-Mbps network.
Other than the addition of the switching hub, the Ethernet
network remains the same—the same network interface
cards, the same client software, the same LAN cabling.
100BASE-T retains the familiar CSMA/CD media access technique
used in 10-Mbps Ethernet networks. It also supports a broad
range of cabling options: two standards for twisted pair
and one for fiber. 100BASE-TX supports 2-pair Category 5
UTP or Type 1 STP cable. 100BASE-FX enables fiber optic
links via duplex multimode fiber cable.
Benefits of 100BASE-T—It
retains CSMA/CD, so existing network management systems
don’t need to be rewritten. It can easily be integrated
into existing 10-Mbps Ethernet LANs, so your previous investment
is saved (see Figures 1 and 2).
It’s also backed by hundreds of manufacturers in the
high-speed networking industry, including Black Box!
||100BASE-T (IEEE 802.3u)
of this Standard
||Category 5 (2-Pair)
||Duplex Multimode or Single-Mode
|| Category 5—100
||Backbone using Ethernet
switches to provide increased throughput. Small to medium
workgroups using applications (i.e. CAD, CAM) that output
huge data files.
Asynchronous Transfer Mode (ATM) is a cell-based fast-packet
communication technique that supports data-transfer rates
ranging from sub-T1 speeds (less than 1.544 Mbps) up to
Like other packet-switching services (Frame Relay, SMDS),
ATM achieves its high speeds in part by transmitting data
in fixed-size cells and dispensing with error-correction
protocols. Instead, it relies on the inherent integrity
of digital lines to ensure data integrity.
Benefits of ATM—Networks
are extremely versatile. An ATM network can be treated as
a single network, whether it connects points in a building
or across the country. Its fixed-length cell-relay operation,
the signaling technology of the future, offers more predictable
performance than variable-length frames. And it can be integrated
into an existing network as needed without having to upgrade
the entire LAN.
Like Ethernet and Fast Ethernet before it, Gigabit Ethernet
works with earlier versions of the IEEE 802.3 standard—both
10 and 100 Mbps— although some equipment will need
to be upgraded. The Gigabit Ethernet standard (IEEE 802.3z)
was approved in June 1998, and its speed of 1 Gbps is a
tenfold increase over Fast Ethernet.
There are two basic types of Gigabit Ethernet: shared
and switched. Shared Gigabit Ethernet
is a higher-speed version of 10/100BASE-T using CSMA/CD
Medium Access Control. Switched Gigabit Ethernet uses Logical
Link Control (LLC). Gigabit Ethernet increases frame sizes
from 64 bytes to 512 bytes minimum, and from 1514 bytes
to 9000 bytes maximum.
Benefits of Gigabit Ethernet—It
solves bandwidth problems. Its primary use is for backbones.
The medium is fiber or Category 5e 100-ohm cable. collapse
Black Box Explains…SFP compatibility.
Standards for SFP fiber optic media are published in the SFP Multi-Source Agreement, which specifies size, connectors, and signaling for SFPs, with the idea that all SFPs are compatible with... more/see it nowdevices that have appropriate SFP slots. These standards, which also extend to SFP+ and XFP transceivers, enable users to mix and match components from different vendors to meet their own particular requirements.
However, some major manufacturers, notably Cisco®, HP®, and 3Com®, sell network devices with SFP slots that lock out transceivers from other vendors. Because the price of SFPs—especially Gigabit SFPs and 10GBASE SFP+ and XFP transceivers—can add significantly to the price of a switch, this lock-out scheme raises hardware costs and limits transceiver choices.
Many vendors don’t advertise that SFP slots on their devices don’t accept standard SFPs from other vendors. This can lead to unpleasant surprises when a device simply refuses to communicate with an SFP.
Another game that some vendors play is to build devices that accept open-standard SFPs, but refuse to support those devices when SFPs from another vendor are used with them.
The only way around this “lock-in” practice is to only buy network devices that accept standard SFPs from all vendors and to buy from vendors that support their devices no matter whose SFPs are used with them. Questions? Call our FREE Tech Support at 724-746-5500.