Private WAN Infrastructures (2.2.2)
In this topic, private WAN infrastructures are discussed including leased lines, dialup access, ISDN, Frame Relay, ATM, MPLS, and Ethernet WANs, and VSAT.
Leased Lines (18.104.22.168)
When permanent dedicated connections are required, a point-to-point link is used to provide a pre-established WAN communications path from the customer premises to the provider network. Point-to-point lines are usually leased from a service provider and are called leased lines.
Leased lines have existed since the early 1950s, and for this reason are referred to by different names, such as leased circuits, serial link, serial line, point-to-point link, and T1/E1 or T3/E3 lines. The term leased line refers to the fact that the organization pays a monthly lease fee to a service provider to use the line. Leased lines are available in different capacities and are generally priced based on the bandwidth required and the distance between the two connected points.
In North America, service providers use the T-carrier system to define the digital transmission capability of a serial copper media link, while Europe uses the E-carrier system, as shown in Figure 2-13.
Figure 2-13 Sample Leased Line Topology
For instance, a T1 link supports 1.544 Mbps, an E1 supports 2.048 Mbps, a T3 supports 43.7 Mbps, and an E3 connection supports 34.368 Mbps. Optical Carrier (OC) transmission rates are used to define the digital transmitting capacity of a fiber-optic network.
The advantages of leased lines include
- Simplicity: Point-to-point communication links require minimal expertise to install and maintain.
- Quality: Point-to-point communication links usually offer high service quality, if they have adequate bandwidth. The dedicated capacity removes latency or jitter between the endpoints.
- Availability: Constant availability is essential for some applications, such as e-commerce. Point-to-point communication links provide permanent, dedicated capacity, which is required for VoIP or Video over IP.
The disadvantages of leased lines include
- Cost: Point-to-point links are generally the most expensive type of WAN access. The cost of leased line solutions can become significant when they are used to connect many sites over increasing distances. In addition, each endpoint requires an interface on the router, which increases equipment costs.
- Limited flexibility: WAN traffic is often variable, and leased lines have a fixed capacity, so that the bandwidth of the line seldom matches the need exactly. Any change to the leased line generally requires a site visit by ISP personnel to adjust capacity.
The Layer 2 protocol is usually HDLC or PPP.
Dialup WAN access may be required when no other WAN technology is available. For example, a remote location could use a modem and analog dialed telephone lines to provide low-capacity and dedicated switched connections. Dialup access is suitable when intermittent, low-volume data transfers are needed.
Traditional telephony uses a copper cable for the local loop to connect the telephone handset in the subscriber premises to the CO. The signal on the local loop during a call is a continuously varying electronic signal that is a translation of the subscriber voice into an analog signal.
Traditional local loops can transport binary computer data through the voice telephone network using a modem. The modem modulates the binary data into an analog signal at the source and demodulates the analog signal to binary data at the destination. The physical characteristics of the local loop and its connection to the PSTN limit the rate of the signal to less than 56 Kbps.
For small businesses, these relatively low-speed dialup connections are 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 (toll 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 has little delay or jitter for point-to-point traffic, but voice or video traffic does not operate adequately at these low bit rates.
Figure 2-14 displays a sample topology of two remote sites interconnecting with dialup modems.
Figure 2-14 Sample Dialup Topology
Integrated Services Digital Network (ISDN) is a circuit-switching technology that enables the local loop of a PSTN to carry digital signals, resulting in higher-capacity switched connections.
ISDN changes the internal connections of the PSTN from carrying analog signals to time-division multiplexed (TDM) digital signals. TDM allows two or more signals, or bit streams, to be transferred as subchannels in one communication channel. The signals appear to transfer simultaneously; but physically, the signals are taking turns on the channel.
Figure 2-15 displays a sample ISDN topology. The ISDN connection may require a terminal adapter (TA), which is a device used to connect ISDN Basic Rate Interface (BRI) connections to a router.
Figure 2-15 Sample ISDN Topology
ISDN turns the local loop into a TDM digital connection. This change enables the local loop to carry digital signals that result in higher-capacity switched connections. The connection uses 64-Kbps bearer channels (B) for carrying voice or data and a signaling delta channel (D) for call setup and other purposes.
There are two types of ISDN interfaces:
Basic Rate Interface (BRI): ISDN BRI is intended for the home and small enterprise and provides two 64-Kbps B channels and one 16-Kbps D channel. The BRI D channel is designed for control and often underused, because it has only two B channels to control (Figure 2-16).
Figure 2-16 ISDN BRI
Primary Rate Interface (PRI): ISDN is also available for larger installations. In North America, PRI delivers 23 B channels with 64 Kbps and 1 D channel with 64 Kbps 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 30 B channels and 1 D channel, for a total bit rate of up to 2.048 Mbps, including synchronization overhead (see Figure 2-17).
Figure 2-17 ISDN PRI
BRI has a call setup time that is less than a second, and the 64-Kbps B channel provides 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 permits 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 if the leased line fails. 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 endpoints. This allows for videoconferencing and high-bandwidth data connections with no latency or jitter. However, multiple connections can be very expensive over long distances.
Frame Relay (22.214.171.124)
Frame Relay is a simple Layer 2 nonbroadcast multiaccess (NBMA) WAN technology used to interconnect enterprise LANs. A single router interface can be used to connect to multiple sites using PVCs. PVCs are used to carry both voice and data traffic between a source and destination, and support data rates up to 4 Mbps, with some providers offering even higher rates.
An edge router only requires a single interface, even when multiple virtual circuits (VCs) are used. The short-leased line to the Frame Relay network edge allows cost-effective connections between widely scattered LANs.
Frame Relay creates PVCs, which are uniquely identified by a data-link connection identifier (DLCI). The PVCs and DLCIs ensure bidirectional communication from one DTE device to another.
For instance, in the example in Figure 2-18 R1 will use DLCI 102 to reach R2, while R2 will use DLCI 201 to reach R1.
Figure 2-18 Sample Frame Relay Topology
Asynchronous Transfer Mode (ATM) technology 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 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 larger data packets to be transmitted.
The 53-byte ATM cell is less efficient than the bigger frames and packets of Frame Relay. 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 is higher because the ATM switch must be able to reassemble the packets at the destination. A typical ATM line needs almost 20 percent greater bandwidth than Frame Relay to carry the same volume of network layer data.
ATM was designed to be extremely scalable and to support link speeds of T1/E1 to OC-12 (622 Mbps) and faster.
ATM offers both PVCs and SVCs, although PVCs are more common with WANs. As with other shared technologies, ATM allows multiple VCs on a single leased-line connection to the network edge.
In the example in Figure 2-19, the ATM switch transmits four different traffic flows consisting of video, VoIP, web, and email.
Figure 2-19 Sample ATM Topology
Ethernet WAN (126.96.36.199)
Ethernet was originally developed to be a LAN access technology. At that time however, it really was not suitable as a WAN access technology because the maximum cable length supported was only up to a kilometer. However, newer Ethernet standards using fiber-optic cables have made Ethernet a reasonable WAN access option. For instance, the IEEE 1000BASE-LX standard supports fiber-optic cable lengths of 5 km, while the IEEE 1000BASE-ZX standard supports up to 70 km cable lengths.
Service providers now offer Ethernet WAN service using fiber-optic cabling. The Ethernet WAN service can go by many names, including Metropolitan Ethernet (MetroE), Ethernet over MPLS (EoMPLS), and Virtual Private LAN Service (VPLS).
Figure 2-20 displays a sample Ethernet WAN topology.
Figure 2-20 Sample Ethernet WAN Topology
Benefits of Ethernet WAN include
- Reduced expenses and administration: Ethernet WAN provides a switched, high-bandwidth Layer 2 network capable of managing data, voice, and video all on the same infrastructure. This characteristic increases bandwidth and eliminates expensive conversions to other WAN technologies. The technology enables businesses to inexpensively connect numerous sites, in a metropolitan area, to each other and to the Internet.
- Easy integration with existing networks: Ethernet WAN connects easily to existing Ethernet LANs, reducing installation costs and time.
- Enhanced business productivity: Ethernet WAN enables businesses to take advantage of productivity-enhancing IP applications that are difficult to implement on TDM or Frame Relay networks, such as hosted IP communications, VoIP, and streaming and broadcast video.
Multiprotocol Label Switching (MPLS) is a multiprotocol high-performance WAN technology that directs data from one router to the next based on short path labels rather than IP network addresses.
MPLS has several defining characteristics. It is multiprotocol, meaning it has the ability to carry any payload including IPv4, IPv6, Ethernet, ATM, DSL, and Frame Relay traffic. It uses labels that tell a router what to do with a packet. The labels identify paths between distant routers rather than endpoints, and while MPLS actually routes IPv4 and IPv6 packets, everything else is switched.
MPLS is a service provider technology. Leased lines deliver bits between sites, and Frame Relay and Ethernet WAN deliver frames between sites. However, MPLS can deliver any type of packet between sites. MPLS can encapsulate packets of various network protocols. It supports a wide range of WAN technologies, including T-carrier / E-carrier links, Carrier Ethernet, ATM, Frame Relay, and DSL.
The sample topology in Figure 2-21 illustrates how MPLS is used.
Figure 2-21 Sample MPLS Topology
Notice that the different sites can connect to the MPLS cloud using different access technologies. In the figure, CE refers to the customer edge, PE is the provider edge router, which adds and removes labels, while P is an internal provider router, which switches MPLS labeled packets.
All private WAN technologies discussed so far used either copper or fiber-optic media. What if an organization needs connectivity in a remote location where there are no service providers that offer WAN service?
Very small aperture terminal (VSAT) is a solution that creates a private WAN using satellite communications. A VSAT is a small satellite dish similar to those used for home Internet and TV. VSATs create a private WAN while providing connectivity to remote locations.
Specifically, a router connects to a satellite dish that is pointed to a service provider’s satellite in a geosynchronous orbit in space. The signals must travel approximately 35,786 km (22,236 miles) to the satellite and back.
The example in Figure 2-22 displays a VSAT dish on the roofs of the buildings communicating with a satellite dish thousands of kilometers away in space.
Figure 2-22 Sample VSAT Topology