Before you start
Objectives: Learn about different technologies and services which can be utilized to connect to the WAN.
Prerequisites: You have to know what a WAN is, and why it is used.
Key terms: WAN, PSTN, ISDN, ATM, Frame Relay, SONET, MPLS
Public Switched Telephone Network (PSTN)
To connect one site (location) to another, over WAN, we could use Public Switched Telephone Network (PSTN). PSTN network is a circuit switched network and which is used for placing telephone calls. When the call is placed a dedicated circuit is established which remains in place throughout the call. To use PSTN to connect to WAN, on our data networks we would install a server with a modem, at each location. Our network would generate digital data that is sent to the Remote Access Server (RAS). The RAS uses modem to translate that data into analog data for transmission on the PSTN. On the other end, analog data received from the PSTN is converted by the modem back to digital data for connection to our network. We often think of the PSTN as a big analog network. In reality the Central Office (CO) converts analog signals received from our network to digital data for transmission trough the WAN. Typically they will use fiber optics within the WAN to send data from one location to another. So, when we use dial-up network do dial-in to a remote location, we are taking a digital signal, converting it to analog, which gets converted to digital signal on the WAN, which is converted back to analog, in order to get to the modem on the other side, where it is converted back to the digital signal to access our remote network.
Using PSTN for data access has a limitation of 56 Kbps. This is very slow when compared to the speed typically used on LANs (100 Mbps or even 1 Gbps). Because of this, it is only implemented when we have relatively small amount of data to send. The 56 Kbps limitation comes from the local loop that connect our modem to the telephone network. A single dial-up telephone line has 64 Kbps worth of data that can be sent, and this is analog data. When we send digital data, we need some additional overhead taking our effective data transfer rate down to the 56 Kbps.
Integrated Services Digital Network (ISDN)
Another method we can use for WAN is ISDN. It was more common in Europe when compared to USA. ISDN is a local loop technology, meaning that ISDN operates on our connection to a WAN service. ISDN is also a circuit switching technology. ISDN is not used within WAN, but is rather the technology that connects our location to the WAN service.
ISDN has two main implementations. The first is called Basic Rate Interface (BRI). BRI uses the Plain Old Telephone Service Line (POTS). These are the regular telephone lines that are already present on our locations. BRI might be used for small busines or a home network. Because BRI uses the same type of wiring, we can simply switch our telephone service to the ISDN service provider. We can then send digital signals over the existing wiring. BRI uses four wires which typically already exist in traditional telephone installations. The only difference is that an RJ 45 connector is typically used so that we don’t plug the regular telephone into a ISDN line. ISDN works by taking the regular copper cable that’s used for telephone line and dividing it into channels. This allows us to send multiple streams of data along the same physical wire. With ISDN BRI we have two data channels which are referred to as B-channels. These are 64 Kbps each. We also have a third channel which is referred to as a D-channel, which is used for control information, such as setting up a call and taking down a call. D-channel uses 16 Kbps. With ISDN BRI we can use each channel separately, for example, one channel for a telephone call, and the second for data. We can also bind two channels together getting the 128 Kbps for data. The two B-channels are the two channels which we can actually use to send data. So, the ISDN BRI has a maximum of 128 Kbps for data in total. Although, we can sometimes hear the value of 144 Kbps which is the total number of all three channels.
The second implementation of ISDN is Primary Rate Interface (PRI). In America, PRI uses a T1 line which provides 23 B-channels and one D-channel, with all channels being 64 Kbps. With ISDN PRI in America we have a total of 1.544 Mbps. In Europe, PRI uses E1 line with 30 B-channels, and one D-channel, all being 64 Kbps, maximum speed being 2.048 Mbps. When we use ISDN PRI we will probably need to get a new line installed into our location. We can’t use the traditional telephone wiring.
Frame Relay is actually a protocol used to connect to a WAN using leased lines. Frame Relay uses T1, T3 lines, or other, to connect our locations to the WAN. Frame Relay is a packet switching technology. It supports variable-sized data units called frames. With Frame Relay a permanent virtual circuit is established trough the WAN to that destination. The circuit is not a physical path trough the network, it is only virtual. Because the circuit is permanent, there is no call setup required.
Virtual circuits can be configured in two different ways. One way is a Point-to-Point circuit, in which we connect two locations. In that case all data sent will arrive at that single destination. If we need to connect a third location this way, we would need a second virtual connection to arrive to a second destination. As we add more locations, the number of virtual connections increases between locations, and this is only if we want one way of communication between locations. If we want to have a two way communication between locations, we would need additional circuits at each location.
The other way of defining virtual circuits is to use a multipoint connection. In that case a single virtual circuit can connect to multiple locations within a WAN. When implementing a Frame Relay network we need a router and a CSU / DSU. When we sign up for a Frame Relay service, we get a Committed Information Rate (CIR) which is a level of service that defines how much data we can send trough the WAN. When congestion within a WAN is low, we can probably go higher than our CIR and we get more bandwidth between our locations. As congestion occurs, packets will be dropped within the WAN, but we are guarantied to have a bandwidth at or above the CIR, but not below that. However, the Frame Relay WAN itself will drop packets as congestion occurs. Frame Relay networks provide error detection, but not error recovery.
Asynchronous Transfer Mode (ATM)
ATM is a packet switching technology that takes data and divides it into packets called cells. Each cell has a fixed length of 53 bytes. Because of the fixed length, ATM can simply send data at a constant rate. ATM also establishes a virtual circuit between two locations. ATM is connection-oriented (compared to Frame Relay which is connectionless).
If we have an IP network that is connected to the ATM WAN, we would have IP packets of various lengths. Each packet would have to be divided into a cell for transmission on ATM WAN. On the other side, the data would have to be reassembled into a packet.Information within a cell identifies a path to follow trough the WAN. Switches within a WAN read the information within a cell header to identify where to send a particular cell. Unlike Frame Relay which drops packets when it gets congested, ATM includes mechanisms to help ensure the delivery of cells trough the network.
Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH)
SONET is actually a subset of SDH specification. SONET is a standard for transmitting data over optical medium. SONET is a transport protocol which defines the structure of WAN and how information pass in WAN. SONET is a packet switching technology. It uses different frame sizes based on the bandwidth.
SONET is often combined with other types of WAN access. The reason for that is that SONET is a transport protocol which can carry other types of traffic such as ATM, Ethernet and IP. SONET typically refers to the underline technology that transports information trough the WAN. SONET uses fiber optic in a series of rings that are inter connected throughout the SONET network. There are typically two rings for every connection. Data flows in one direction on one ring, and another direction on another ring. Devices connect both rings together as well as separate rings. If there is a break in one ring, data can be routed trough the other ring. This way we have a redundancy of service throughout WAN. The data rates in SONET networks can reach from 51 Mbps to several hundred Gbps.
Multiprotocol Label Switching (MPLS)
MPLS is actually a data classification and data carrying mechanism used in WAN. MLPS is a packet switching technology. It is a connection-oriented protocol. It can be utilized in order to prioritize traffic in the WAN. A normal IP packet consist of the packet payload (data), together with the header information (IP info and MAC info), meaning between Network and Data Link layers. With MPLS, labels are inserted between the IP header and a MAC address. Labels identify characteristics about the data. Special routers on the edge of MPLS network insert labels within an IP packet. Switches within a WAN cloud then use label information to route data to the destination. At the end device, the labels are removed and the packet is sent on the normal network. Labels in MPLS networks can be used to prioritize data as it moves trough the cloud. It can identify data that is more sensitive and has a higher priority.
The type of WAN service we sign up depends on the speed of the connection we want, as well as of the type of services that are available for us. Of the services that we have mentioned, using the PSTN or ISDN BRI offers low data transfer rates and are typically not used for connecting two remote sites together. Frame Relay and ATM are more common services that we can use to provide WAN connectivity for our network. SONET is typically used within WAN for moving data over fiber optic links. MPLS is a service that let us prioritize traffic and is used within WAN for moving packets based on labels.