Bandwidth is a measure of number of bits transmitted per second. How much Bandwidth can we really expect? The amount of bandwidth achi...

Bandwidth is a measure of number of bits transmitted per second.

How much Bandwidth can we really expect?

The amount of bandwidth achieved generally depends on which side of the firewall is that. Many corporate users are already in bandwidth heaven, at least on their LANs. However outside the firewall, home users and mobile users are probably in bandwidth hell. The bandwidth you get at work is primarily a function of the capacity of LANs, WANs and the links that connect them and bandwidth at home is primarily dependent on link that connects to an access point or POP (point of presence).

So to understand bandwidth, the performance at five potential chokepoints are considered.

The State of LAN:

LANs are not the bottleneck in the quest for bandwidth heaven. However except for ATM and switched Ethernet, all the LAN technologies use some form of contention to access the broadcast medium. Also ATM technology can be used in both LAN and WANs.

The State of WAN:

WAN performance is primarily affected by two factors: 1) the switches that route data across networks and 2) the type of wide area cabling infrastructure. For ex, the faster switches require a fiber optic cabling infrastructure to achieve maximum speeds. Thus the physical world still matters when it comes to networks.

As a result, Internet backbone providers are now stepping up to ANSI’s Synchronous Optical Network (Sonet) transmission standards for high-speed fiber optic links. The Sonet standard is specified in multiples of 51.84 Mbits/s. All the major carriers are deploying Sonet worldwide.

Modern WANs make use of packet switching technology to provide the link layer on top of Sonet. Packet switches break data streams into packets that they can launch into the network. The address headers are used to direct the packets to its destination.

Three leading packet switching technologies are –
  1.  Frame Relay
  2.  Switched Multi Megabit Data Services (SMDS)
  3.  Asynchronous Transfer Mode
Frame Relay:

Frame relay is currently the most popular packet switching technology for WANs.

Frame relay can route variable length packets over existing routers, switches and other HDLC based equipment, it only requires a software upgrade. Frame relay achieves its magic by bypassing error checks at each network segment (the midpoints). It relies on endpoints to provide end-to-end error checking. The demand for frame relay is also very strong.


SMDS (Switched Multimegabit Data Services) was designed to fill the gap for high-speed WAN services until ATM become widely available. It allows variable length packets that can be broken into ATM size fixed cells to facilitate transmission. It obtains its throughput by not providing support for virtual circuits in contrast to ATM and SMDS, which supports virtual circuits.


It is a packet switching protocol that achieves very high speed by using fixed length data cells or packets on top of virtual circuits. Permanent virtual circuits are statically assigned and Switched Virtual Circuits are dynamic. In either case, a virtual circuit can guarantee quality of service including bandwidth and priority.

ATM was designed to mix different types of traffic – including data, voice and video. The small fixed sized cell makes it possible to implement very high-speed switches in hardware. ATM’s bandwidth is demand based and scalable, meaning that each node can access the network at the speeds required by the application. ATM promises to provide seamless networking and remove the distinctions between LANs and WANs.


Broadband Integrated Services Digital Network is a cell switched WAN that supports speeds of over 1.54 Mbits/s. The B-ISDN standard defines an ATM adaptation layer (AAL) that is responsible for mapping data, voice and video information to and from ATM defined cell formats.

The State of LAN-to-WAN Interconnect:

LANs connect to WANs via routers. The routers connect to either public or private WANs via leased lines. The Switching technology is either Frame Relay or Point-to-Point. Some of the larger corporations are using ATM over Sonet for their LAN-to-WAN connections.

The state of Home-to-WAN Connection:

To extend the WAN into home requires a solution to the impedance mismatch created by the ‘last mile’ of telephone cables. But over the past decade the long distance carriers replaced their copper wire backbones with a network of superfast, high-bandwidth, low tariff fiber-optic cables.

The two new technologies from the local telephone companies that promise to speed up the local loops without replacing the existing wiring – Integrated Services Digital Network (ISDN) and High Bit Rate Digital Subscriber Line (HDSL).


ISDN is the digital telephony that supports the high-speed transfer of voice and data over telephone lines. The most prevalent of the ISDN service is the Basic Rate Interface (BRI). ISDN BRI is the digitization of telephone line. It is five times faster than anything that can be done with a modem today. In addition it provides a low-cost form of communication because it charges by connect time rather than by packet. It higher speeds can reduce the connect time and thus save money. Though ISDN provides more number of benefits the drawback is that it has poor serviceability.


It was developed as a way to use existing telephone copper twisted pair to deliver low-cost video on demand in competition with cable modems. The beauty of HDSL is that it offers repeaterless spans of 12000 feets over existing copper loops. A variation of HDSL called ADSL (Asymmetric Digital Subscriber Line) has more bandwidth going in to the home that out.

Cable Modems:

The telephone companies face stiff competition with the cable TV companies in solving the last mile problem. The cable companies introduce a cheap cable modem that can provide more than 10 Mbits/s of bi-directional bandwidth by emulating Ethernet over a cable. Computer can be connected to the cable mode via an Ethernet adapter.

Even though the modems are bi-directional, most home cable systems are unidirectional. Generally the cable modems are used to download information from WAN at Ethernet speeds and a plain old telephone modem to upload information. A new start up called @Home is partnering with the major cable companies in providing a high speed internet point of presence.
The State of Wireless - to – WAN Connection:

Wireless networks promise to provide the ultimate connection to the digital highway by letting us communicate anywhere anytime. The state of today’s wireless to WAN connection can be seen as follows:

Circuit Switched Cellular:

It simply provides a modem that can be used to connect a laptop with a cellular phone. So connection can be made with any phone on the nationwide cellular network. And the cellular phone can be used for voice. Of course, the cellular rates must be paid by the minutes, which is where the fun stops.

Cellular Digital Packet Data:

This transmits data packets over unused portions of the existing cellular network. CPDD can dynamically pick up the open voice channels and use them for data traffic. CPDD modems do not require a cellular phone since it has got inbuilt radio transceivers and it requires pay-by-packet.

Private Packet Radio Providers:

They offer a nationwide wireless alternative to the cellular network. The two leading providers in US are Ram Mobile Data and Ardis.

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