Cable Modem

2.2.8 Cable Modem
Coaxial cable is widely used in urban areas to distribute television signals. Network access is available from some cable television networks. This allows for greater bandwidth than the conventional telephone local loop.
Enhanced cable modems enable two-way, high-speed data transmissions using the same coaxial lines that transmit cable television. Some cable service providers are promising data speeds up to 6.5 times that of T1 leased lines. This speed makes cable an attractive medium for transferring large amounts of digital information quickly, including video clips, audio files, and large amounts of data. Information that would take two minutes to download using ISDN BRI can be downloaded in two seconds through a cable modem connection.
Cable modems provide an always-on connection and a simple installation. An always-on cable connection means that connected computers are vulnerable to a security breach at all times and need to be suitably secured with firewalls. To address security concerns, cable modem services provide capabilities for using Virtual Private Network (VPN) connections to a VPN server, which is typically located at the corporate site.
A cable modem is capable of delivering up to 30 to 40 Mbps of data on one 6 MHz cable channel. This is almost 500 times faster than a 56 Kbps modem.
With a cable modem, a subscriber can continue to receive cable television service while simultaneously receiving data to a personal computer. This is accomplished with the help of a simple one-to-two splitter.
Cable modem subscribers must use the ISP associated with the service provider. All the local subscribers share the same cable bandwidth. As more users join the service, available bandwidth may be below the expected rate.

DSL

2.2.7 DSL
Digital Subscriber Line (DSL) technology is a broadband technology that uses existing twisted-pair telephone lines to transport high-bandwidth data to service subscribers. DSL service is considered broadband, as opposed to the baseband service for typical LANs. Broadband refers to a technique which uses multiple frequencies within the same physical medium to transmit data. The term xDSL covers a number of similar yet competing forms of DSL technologies:

• Asymmetric DSL (ADSL)
• Symmetric DSL (SDSL)
• High Bit Rate DSL (HDSL)
• ISDN (like) DSL (IDSL)
• Consumer DSL (CDSL), also called DSL-lite or G.lite

DSL technology allows the service provider to offer high-speed network services to customers, utilizing installed local loop copper lines. DSL technology allows the local loop line to be used for normal telephone voice connection and an always-on connection for instant network connectivity. Multiple DSL subscriber lines are multiplexed into a single, high capacity link by the use of a DSL Access Multiplexer (DSLAM) at the provider location. DSLAMs incorporate TDM technology to aggregate many subscriber lines into a less cumbersome single medium, generally a T3/DS3 connection. Current DSL technologies are using sophisticated coding and modulation techniques to achieve data rates up to 8.192 Mbps.
The voice channel of a standard consumer telephone covers the frequency range of 330 Hz to 3.3 KHz. A frequency range, or window, of 4 KHz is regarded as the requirements for any voice transmission on the local loop. DSL technologies place upload (upstream) and download (downstream) data transmissions at frequencies above this 4 KHz window. This technique is what allows both voice and data transmissions to occur simultaneously on a DSL service.
The two basic types of DSL technologies are asymmetric (ADSL) and symmetric (SDSL). All forms of DSL service are categorized as ADSL or SDSL and there are several varieties of each type. Asymmetric service provides higher download or downstream bandwidth to the user than upload bandwidth. Symmetric service provides the same capacity in both directions.
Not all DSL technologies allow the use of a telephone. SDSL is called dry copper because it does not have a ring tone and does not offer telephone service on the same line. Therefore a separate line is required for the SDSL service.
The different varieties of DSL provide different bandwidths, with capabilities exceeding those of a T1 or E1 leased line. The transfer rates are dependent on the actual length of the local loop and the type and condition of its cabling. For satisfactory service, the loop must be less than 5.5 kilometers (3.5 miles). DSL availability is far from universal, and there are a wide variety of types, standards, and emerging standards. It is not a popular choice for enterprise computer departments to support home workers. Generally, a subscriber cannot choose to connect to the enterprise network directly, but must first connect to an Internet service provider (ISP). From here, an IP connection is made through the Internet to the enterprise. Thus, security risks are incurred. To address security concerns, DSL services provide capabilities for using Virtual Private Network (VPN) connections to a VPN server, which is typically located at the corporate site. 

Frame Relay / ATM

2.2.5 Frame Relay
With increasing demand for higher bandwidth and lower latency packet switching, communications providers introduced Frame Relay. Although the network layout appears similar to that for X.25, available data rates are commonly up to 4 Mbps, with some providers offering even higher rates.
Frame Relay differs from X.25 in several aspects. Most importantly, it is a much simpler protocol that works at the data link layer rather than the network layer.
Frame Relay implements no error or flow control. The simplified handling of frames leads to reduced latency, and measures taken to avoid frame build-up at intermediate switches help reduce jitter.
Most Frame Relay connections are PVCs rather than SVCs. The connection to the network edge is often a leased line but dialup connections are available from some providers using ISDN lines. The ISDN D channel is used to set up an SVC on one or more B channels. Frame Relay tariffs are based on the capacity of the connecting port at the network edge. Additional factors are the agreed capacity and committed information rate (CIR) of the various PVCs through the port.
Frame Relay provides permanent shared medium bandwidth connectivity that carries both voice and data traffic. Frame Relay is ideal for connecting enterprise LANs. The router on the LAN needs only a single interface, even when multiple VCs are used. The short-leased line to the Frame Relay network edge allows cost-effective connections between widely scattered LANs.
2.2.6 ATM
Communications providers saw a need for a permanent shared network technology that offered very low latency and jitter at much higher bandwidths. Their solution was Asynchronous Transfer Mode (ATM). ATM has data rates beyond 155 Mbps. As with the other shared technologies, such as X.25 and Frame Relay, diagrams for ATM WANs look the same.
ATM is a technology that is capable of transferring voice, video, and data through private and public networks. It is built on a cell-based architecture rather than on a frame-based architecture. ATM cells are always a fixed length of 53 bytes. The 53 byte ATM cell contains a 5 byte ATM header followed by 48 bytes of ATM payload. Small, fixed-length cells are well suited for carrying voice and video traffic because this traffic is intolerant of delay. Video and voice traffic do not have to wait for a larger data packet to be transmitted.
The 53 byte ATM cell is less efficient than the bigger frames and packets of Frame Relay and X.25. Furthermore, the ATM cell has at least 5 bytes of overhead for each 48-byte payload. When the cell is carrying segmented network layer packets, the overhead will be higher because the ATM switch must be able to reassemble the packets at the destination. A typical ATM line needs almost 20% greater bandwidth than Frame Relay to carry the same volume of network layer data.
ATM offers both PVCs and SVCs, although PVCs are more common with WANs.
As with other shared technologies, ATM allows multiple virtual circuits on a single leased line connection to the network edge.

Leased Line / X.25 /

2.2.3 Leased Line
When permanent dedicated connections are required, leased lines are used with capacities ranging up to 2.5 Gbps.
A point-to-point link provides a pre-established WAN communications path from the customer premises through the provider network to a remote destination. Point-to-point lines are usually leased from a carrier and are called leased lines. Leased lines are available in different capacities. These dedicated circuits are generally priced based on bandwidth required and distance between the two connected points. Point-to-point links are generally more expensive than shared services such as Frame Relay. The cost of leased-line solutions can become significant when they are used to connect many sites. There are times when cost of the leased line is outweighed by the benefits. The dedicated capacity gives no latency or jitter between the endpoints. Constant availability is essential for some applications such as electronic commerce.
A router serial port is required for each leased-line connection. A CSU/DSU and the actual circuit from the service provider are also required.
Leased lines are used extensively for building WANs and give permanent dedicated capacity. They have been the traditional connection of choice but have a number of disadvantages. WAN traffic is often variable and leased lines have a fixed capacity. This results in the bandwidth of the line seldom being exactly what is needed. In addition, each end point would need an interface on the router which would increase equipment costs. Any changes to the leased line generally require a site visit by the carrier to change capacity.
Leased lines provide direct point-to-point connections between enterprise LANs and connect individual branches to a packet-switched network. Several connections can be multiplexed over a leased line, resulting in shorter links and fewer required interfaces.

2.2.4   X.25
In response to the expense of leased lines, telecommunications providers introduced packet-switched networks using shared lines to reduce costs. The first of these packet-switched networks was standardized as the X.25 group of protocols. X.25 provides a low bit rate shared variable capacity that may be either switched or permanent.
X.25 is a network-layer protocol and subscribers are provided with a network address. Virtual circuits can be established through the network with call request packets to the target address. The resulting SVC is identified by a channel number. Data packets labeled with the channel number are delivered to the corresponding address. Multiple channels can be active on a single connection.
Subscribers connect to the X.25 network with either leased lines or dialup connections. X.25 networks can also have pre-established channels between subscribers that provide a PVC.
X.25 can be very cost effective because tariffs are based on the amount of data delivered rather than connection time or distance. Data can be delivered at any rate up to the connection capacity. This provides some flexibility. X.25 networks are usually low capacity, with a maximum of 48 kbps. In addition, the data packets are subject to the delays typical of shared networks.
X.25 technology is no longer widely available as a WAN technology in the US. Frame Relay has replaced X.25 at many service provider locations.
Typical X.25 applications are point-of-sale card readers. These readers use X.25 in dialup mode to validate transactions on a central computer. Some enterprises also use X.25 based value-added networks (VAN) to transfer Electronic Data Interchange (EDI) invoices, bills of lading, and other commercial documents. For these applications, the low bandwidth and high latency are not a concern, because the low cost makes the use of X.25 affordable.

WAN Technologies / Analog Dialup / ISDN

2.2 WAN Technologies
2.2.1 Analog Dialup
When intermittent, low-volume data transfers are needed, modems and analog dialed telephone lines provide low capacity and dedicated switched connections.
Traditional telephony uses a copper cable, called the local loop, to connect the telephone handset in the subscriber premises to the public switched telephone network (PSTN). The signal on the local loop during a call is a continuously varying electronic signal that is a translation of the subscriber voice.
The local loop is not suitable for direct transport of binary computer data, but a modem can send computer data through the voice telephone network. The modem modulates the binary data into an analog signal at the source and demodulates the analog signal at the destination to binary data.
The physical characteristics of the local loop and its connection to the PSTN limit the rate of the signal. The upper limit is around 33 kbps. The rate can be increased to around 56 kbps if the signal is coming directly through a digital connection.
For small businesses, this can be adequate for the exchange of sales figures, prices, routine reports, and email. Using automatic dialup at night or on weekends for large file transfers and data backup can take advantage of lower off-peak tariffs (line charges). Tariffs are based on the distance between the endpoints, time of day, and the duration of the call.
The advantages of modem and analog lines are simplicity, availability, and low implementation cost. The disadvantages are the low data rates and a relatively long connection time. The dedicated circuit provided by dialup will have little delay or jitter for point-to-point traffic, but voice or video traffic will not operate adequately at relatively low bit rates.
2.2.2 ISDN
The internal connections, or trunks, of the PSTN have changed from carrying analog frequency-division multiplexed signals, to time-division multiplexed (TDM) digital signals. An obvious next step is to enable the local loop to carry digital signals that result in higher capacity switched connections.
Integrated Services Digital Network (ISDN) turns the local loop into a TDM digital connection. The connection uses 64 kbps bearer channels (B) for carrying voice or data and a signaling, delta channel (D) for call set-up and other purposes.
Basic Rate Interface (BRI) ISDN is intended for the home and small enterprise and provides two 64 kbps B channels and a 16 kbps D channel. For larger installations, Primary Rate Interface (PRI) ISDN is available. PRI delivers twenty-three 64 kbps B channels and one 64 kbps D channel in North America, for a total bit rate of up to 1.544 Mbps. This includes some additional overhead for synchronization. In Europe, Australia, and other parts of the world, ISDN PRI provides thirty B channels and one D channel for a total bit rate of up to 2.048 Mbps, including synchronization overhead. In North America PRI corresponds to a T1 connection. The rate of international PRI corresponds to an E1 connection.
The BRI D channel is underutilized, as it has only two B channels to control. Some providers allow the D channel to carry data at low bit rates such as X.25 connections at 9.6 kbps.
For small WANs, the BRI ISDN can provide an ideal connection mechanism. BRI has a call setup time that is less than a second, and its 64 kbps B channel provide greater capacity than an analog modem link. If greater capacity is required, a second B channel can be activated to provide a total of 128 kbps. Although inadequate for video, this would permit several simultaneous voice conversations in addition to data traffic.
Another common application of ISDN is to provide additional capacity as needed on a leased line connection. The leased line is sized to carry average traffic loads while ISDN is added during peak demand periods. ISDN is also used as a backup in the case of a failure of the leased line. ISDN tariffs are based on a per-B channel basis and are similar to those of analog voice connections.
With PRI ISDN, multiple B channels can be connected between two end points. This allows for video conferencing and high bandwidth data connections with no latency or jitter. Multiple connections can become very expensive over long distances.

WAN link options

2.1.6 WAN link options
Circuit switching establishes a dedicated physical connection for voice or data between a sender and receiver. Before communication can start, it is necessary to establish the connection by setting the switches. This is done by the telephone system, using the dialed number. ISDN is used on digital lines as well as on voice-grade lines.
To avoid the delays associated with setting up a connection, telephone service providers also offer permanent circuits. These dedicated or leased lines offer higher bandwidth than is available with a switched circuit. Examples of circuit-switched connections include:

• Plain Old Telephone System (POTS)
• ISDN Basic Rate Interface (BRI)
• ISDN Primary Rate Interface (PRI)

Many WAN users do not make efficient use of the fixed bandwidth that is available with dedicated, switched, or permanent circuits, because the data flow fluctuates. Communications providers have data networks available to more appropriately service these users. In these networks, the data is transmitted in labeled cells, frames, or packets through a packet-switched network. Because the internal links between the switches are shared between many users, the costs of packet switching are lower than those of circuit switching. Delays (latency) and variability of delay (jitter) are greater in packet-switched than in circuit-switched networks. This is because the links are shared and packets must be entirely received at one switch before moving to the next. Despite the latency and jitter inherent in shared networks, modern technology allows satisfactory transport of voice and even video communications on these networks.
Packet-switched networks may establish routes through the switches for particular end-to-end connections. Routes established when the switches are started are PVCs. Routes established on demand are SVCs. If the routing is not pre-established and is worked out by each switch for each packet, the network is called connectionless.
To connect to a packet-switched network, a subscriber needs a local loop to the nearest location where the provider makes the service available. This is called the point-of-presence (POP) of the service. Normally this will be a dedicated leased line. This line will be much shorter than a leased line directly connected to the subscriber locations, and often carries several VCs. Since it is likely that not all the VCs will require maximum demand simultaneously, the capacity of the leased line can be smaller than the sum of the individual VCs. Examples of packet or cell switched connections include:

• Frame Relay
• X.25
• ATM 

Packet and circuit switching

2.1.5 Packet and circuit switching 

Packet-switched networks were developed to overcome the expense of public circuit-switched networks and to provide a more cost-effective WAN technology.
When a subscriber makes a telephone call, the dialed number is used to set switches in the exchanges along the route of the call so that there is a continuous circuit from the originating caller to that of the called party. Because of the switching operation used to establish the circuit, the telephone system is called a circuit-switched network. If the telephones are replaced with modems, then the switched circuit is able to carry computer data.
The internal path taken by the circuit between exchanges is shared by a number of conversations. Time division multiplexing (TDM) is used to give each conversation a share of the connection in turn. TDM assures that a fixed capacity connection is made available to the subscriber.
If the circuit carries computer data, the usage of this fixed capacity may not be efficient. For example, if the circuit is used to access the Internet, there will be a burst of activity on the circuit while a web page is transferred. This could be followed by no activity while the user reads the page and then another burst of activity while the next page is transferred. This variation in usage between none and maximum is typical of computer network traffic. Because the subscriber has sole use of the fixed capacity allocation, switched circuits are generally an expensive way of moving data.
An alternative is to allocate the capacity to the traffic only when it is needed, and share the available capacity between many users. With a circuit-switched connection, the data bits put on the circuit are automatically delivered to the far end because the circuit is already established. If the circuit is to be shared, there must be some mechanism to label the bits so that the system knows where to deliver them. It is difficult to label individual bits, therefore they are gathered into groups called cells, frames, or packets. The packet passes from exchange to exchange for delivery through the provider network. Networks that implement this system are called packet-switched networks.
The links that connect the switches in the provider network belong to an individual subscriber during data transfer, therefore many subscribers can share the link. Costs can be significantly lower than a dedicated circuit-switched connection. Data on packet-switched networks are subject to unpredictable delays when individual packets wait for other subscriber packets to be transmitted by a switch.
The switches in a packet-switched network determine, from addressing information in each packet, which link the packet must be sent on next. There are two approaches to this link determination, connectionless or connection-oriented. Connectionless systems, such as the Internet, carry full addressing information in each packet. Each switch must evaluate the address to determine where to send the packet. Connection-oriented systems predetermine the route for a packet, and each packet need only carry an identifier. In the case of Frame Relay, these are called Data Link Control Identifiers (DLCI). The switch determines the onward route by looking up the identifier in tables held in memory. The set of entries in the tables identifies a particular route or circuit through the system. If this circuit is only physically in existence while a packet is traveling through it, it is called a Virtual Circuit (VC).
The table entries that constitute a VC can be established by sending a connection request through the network. In this case the resulting circuit is called a Switched Virtual Circuit (SVC). Data that is to travel on SVCs must wait until the table entries have been set up. Once established, the SVC may be in operation for hours, days or weeks. Where a circuit is required to be always available, a Permanent Virtual Circuit (PVC) will be established. Table entries are loaded by the switches at boot time so the PVC is always available.