2.3.7 DSL

2.3.7 DSL
DSL, Digital Subscriber Line, uses existing copper phone lines. DSL is available only in certain areas, and you must be within a short distance of a switching station. Speeds can vary based upon type of DSL but are typically around 9Mbps (the theoretical maximum is 52Mbps). DSL is typically less expensive than even ISDN in terms of hardware, setup, and service costs, yet the need to be within a few miles of a switching station is a big deterrent.
There are several types of DSL to choose from, and not all types are available in all markets. The types available include:
• Asymmetric DSL (ADSL)—uses existing copper phone lines
• High-bit-rate DSL (HDSL)—requires two wire pairs
• ISDN DSL (IDSL)—uses existing ISDN facilities
• Rate Adaptive DSL (RADSL)—adjusts the speed based on signal quality
• Symmetric DSL (SDSL)—a version of HDSL using a single pair of wires (and providing slower rates)
• Very-high-bit-rate DSL (VDSL)—transmits over short distances; the connection rate increases as the distance decreases
To illustrate the performance possible with the different types and the way it varies, Table 2.2 shows the transmission rates and distances for various DSL implementations.

2.3.6 ATM

2.3.6 ATM
ATM, Asynchronous Transfer Mode, is a high-bandwidth switching technology developed by the ITU Telecommunications Standards Sector (ITU-TSS). ATM uses 53-byte cells for all transmissions. Because ATM cells are uniform in length, switching mechanisms can operate with a high level of efficiency. This high efficiency results in high data-transfer rates. Some ATM systems can operate at 622Mbps; a typical working speed for an ATM, however, is around 155Mbps.
The unit of transmission for ATM is called a cell. All cells are 53 bytes long and consist of a 5-byte header and 48 bytes of data. The "Asynchronous" aspect refers to the fact that transmission time slots don't occur periodically but are granted at irregular intervals. Traffic that is time-critical, such as voice or video, can be given priority over data traffic that can be delayed slightly with no ill effect. Devices communicate on ATM networks by establishing a virtual path, within which virtual circuits can be established.

2.3.5 Frame Relay

2.3.5 Frame Relay
Frame Relay, a packet-switching protocol supporting T1 and T3, was designed to support the Broadband Integrated Services Digital Network (B-ISDN). The specifications for Frame Relay address some of the limitations of X.25. As with X.25, Frame Relay is a packet-switching network service, but Frame Relay was designed around newer, faster fiber-optic networks. Unlike X.25, Frame Relay assumes a more reliable network. This enables Frame Relay to eliminate much of the X.25 overhead required to provide reliable service on less reliable networks. Frame Relay relies on higher-level network protocol layers to provide flow and error control. To use Frame Relay, you must have special Frame-Relay-compatible connectivity devices (such as Frame-Relay-compatible routers and bridges).
Frame Relay typically is implemented as a public data network and therefore is regarded as a WAN protocol. Frame Relay provides permanent virtual circuits, which supply permanent virtual pathways for WAN connections. Frame Relay services typically are implemented at line speeds from 56Kbps up to 1.544Mbps (T1). Customers typically purchase access to a specific amount of bandwidth on a Frame Relay service, for which the customer is guaranteed access.

2.3.4 X.25

2.3.4 X.25
X.25 is a packet-switching standard widely used in WANs. The X.25 standard was developed by the International Telegraph and Telephone Consultative Committee (CCITT), which has been renamed the International Telecommunications Union (ITU). The standard, referred to as Recommendation X.25, was first introduced in 1974, and it provides to networks the options of permanent or switched virtual circuits. X.25 is required to provide reliable service and end-to-end flow control. Because each device on a network can operate more than one virtual circuit, X.25 must provide error and flow control for each virtual circuit.
A big advantage of X.25 is that it is used internationally, while the major drawback is that error checking and flow control slow down X.25. Traditionally, networks utilizing it are implemented with line speeds of up to 64Kbps. These speeds are inadequate to provide most LAN services, which typically require speeds of 10Mbps or better. X.25 networks, therefore, are poor choices for providing LAN application services in a WAN environment.

2.3.3 Tx Connections

2.3.3 Tx Connections
A T1 line is a dedicated line that operates across 24 channels at 1.544Mbps. The European counterpart to this is E1, which uses 32 channels and can run at 2.048Mbps. A T2 connection (rarely used) adds more channels (96) and can operate at 6.312Mbps. A T3 line (E3 being the European equivalent) is a dedicated line of 672 channels able to run at speeds of 43Mbps. A T4 line jumps to 4,032 channels and speeds of over 274Mbps. Of the dedicated-phone-line options, T1 and T3 are the most commonly implemented.

Very few private networks require the capacity of a T3 line, and many do not even need the full capacity of a T1. The channels of a T1 or T3 line thus can be subdivided or combined for fractional or multiple levels of service. For instance, one channel of a T1 line's 24-channel bandwidth can transmit at 64Kbps. This single-channel service is called DS-0. DS-1 service is a full T1 line. DS-1C is two T1 lines, DS-2 is four T1 lines, and DS-3 is a full T3 line (equivalent to 28 T1 lines).

Bridges

2.2.2 Bridges
While hubs are used to build a network at a single site, bridges are used to build a network at two sites—or to join two networks . A bridge operates by looking at the header of the data that comes to it. If the data is for the network on which the bridge resides, the bridge leaves the data alone. If the data is for another network, the bridge gets rid of the data by sending it to a predefined location. An example would be two networks, one in New York and the other in Chicago, that have a bridge at each location (on each network). If a user sends a message in New York and it is not for another user in New York, it must be for a user in Chicago, so it is sent there. Likewise, if a user in Chicago sends a message and it is not for another user in Chicago, it must be for a user in New York.

A bridge can never be used with more than two sites. If San Francisco were added into the mix, the bridge at Chicago could not determine whether to send the message to San Francisco or to Chicago and could send it to only one location.
The biggest advantages to bridges are that they are reasonably cheap, and they work with all protocols by dropping down to and concentrating on the physical addresses of devices. Physical addresses give bridges the ability to work with NetBEUI (NetBIOS Extended User Interface) and non-routable protocols as easily as they work with TCP/IP. Remote bridges are nothing more than bridges that connect two LANs into a WAN and filter signals.
It is important to understand that a bridge—like a hub—receives every data packet sent on the network. The bridge then looks at the header and at an internal table (known as the forwarding database, or routing table) and determines if it should leave the packet alone or send the packet out to the address it has. In this capacity, a bridge is used to expand the geographic scope of the network to another location. The opposite could also be true in that a bridge could be used to divide one network into two segments to reduce traffic throughout the whole network.
To visualize the latter situation, suppose that a company has two large departments: Manufacturing and Sales. Every piece of data generated by Manufacturing is sent throughout the network, as is every piece of data generated by Sales. If the network could be divided into two segments with a bridge between Sales and Manufacturing, the network traffic could arguably be cut in half. All the Manufacturing traffic would stay on the Manufacturing segment, and all the Sales traffic would stay on the Sales segment. Data would cross over through the bridge only when Sales requested data from or sent data to Manufacturing, or vice versa.

hubs

2.2.1 Hubs
Hubs are devices used to build networks utilizing a star topology, as shown in Figure 2.1. Hubs make it easy to add workstations to the network and to reconfigure the network at any time by simply unplugging and plugging in patch cables.



Figure 2.1 With a star topology, all devices run to a central device—a hub.
The three hub types are passive, active, and intelligent. Passive hubs allow for connections and central wiring only. Active hubs amplify the signals coming in and filter out noise. Intelligent hubs provide either switching capabilities or management features.
Switching hubs provide quick routing of signals between hub ports in order to direct data where it needs to go and reduce the bandwidth of sending the data to all locations. Switching hubs are always intelligent hubs, but intelligent hubs are not always switching hubs.

In the absence of switching, a hub sends all traffic it receives to all ports.
Hubs are occasionally known as concentrators and range in size from 4 ports to 16 ports or more. Cascading allows numerous hubs to be connected to form larger networks. Where switching is employed, it is possible for a hub to perform some of the functions of a bridge—but this is typical only if multiple networks are within a limited geographic scope.

networking devices

2.2 Networking Devices
A network can be as small as two computers talking together in a peer-to-peer relationship, or as large as the Internet—with unlimited possibilities between the two ends of the spectrum. All networks, regardless of size, have the following items in common:
• An operating system on the client or host that allows for the use of networking redirectors
• A networking protocol—a common language—through which communication can take place. Every workstation must run its own stack or use the stack of a server (as in a proxy server) to be able to communicate.
• Applications that utilize the network—email, FTP, etc.
• Network interface cards (NICs) installed in each machine
• Cabling
The cabling can be of various types, or even wireless. Cable types are tested heavily in the Network+ exam. The i-Net+ exam picks up in content where Network+ leaves off and looks at the connectivity devices used between the hosts to build the network. In particular, you must know four connectivity devices—hubs, bridges, routers, and gateways—each of which is examined in the following sections.