WAN Operations [2.1.2]
This topic introduces common WAN terminology and devices and differentiates between circuit-switch and packet-switch networks.
WANs in the OSI Model [2.1.2.1]
As shown in Figure 2-6, WAN operations focus primarily on the physical layer [OSI Layer 1] and the data link layer [OSI Layer 2].
WAN access standards typically describe both physical layer delivery methods and data link layer requirements, including physical addressing, flow control, and encapsulation.
WAN access standards are defined and managed by a number of recognized authorities, including the
- Telecommunication Industry Association and the Electronic Industries Alliance [TIA/EIA]
- International Organization for Standardization [ISO]
- Institute of Electrical and Electronics Engineers [IEEE]
Layer 1 protocols describe how to provide electrical, mechanical, operational, and functional connections to the services of a communications service provider.
Layer 2 protocols define how data is encapsulated for transmission toward a remote location, and the mechanisms for transferring the resulting frames. A variety of different technologies are used, such as the Point-to-Point Protocol [PPP], Frame Relay, and Asynchronous Transfer Mode [ATM]. Some of these protocols use the same basic framing or a subset of the High-Level Data Link Control [HDLC] mechanism.
Most WAN links are point to point. For this reason, the address field in the Layer 2 frame is usually not used.
Common WAN Terminology [2.1.2.2]
One primary difference between a WAN and a LAN is that an organization must subscribe to an outside WAN service provider and use the WAN carrier network services to interconnect its sites and users. A WAN uses data links provided by carrier services to access the Internet and connect different locations of an organization to each other, to locations of other organizations, to external services, and to remote users.
The physical layer of a WAN describes the physical connections between the company network and the service provider network. As illustrated in Figure 2-7, common terminology is used to describe WAN components and reference points.
Specifically, these terms include
- Customer premises equipment [CPE]: The devices and inside wiring located on the enterprise edge connecting to a carrier link. The subscriber either owns the CPE or leases the CPE from the service provider. A subscriber, in this context, is a company that arranges for WAN services from a service provider.
- Data communications equipment [DCE]: Also called data circuit-terminating equipment, the DCE consists of devices that put data on the local loop. The DCE primarily provides an interface to connect subscribers to a communication link on the WAN cloud.
- Data terminal equipment [DTE]: The customer devices that pass the data from a customer network or host computer for transmission over the WAN. The DTE connects to the local loop through the DCE.
- Demarcation point: A point established in a building or complex to separate customer equipment from service provider equipment. Physically, the demarcation point is the cabling junction box, located on the customer premises, that connects the CPE wiring to the local loop. It is usually placed for easy access by a technician. The demarcation point is the place where the responsibility for the connection changes from the user to the service provider. When problems arise, it is necessary to determine whether the user or the service provider is responsible for troubleshooting or repair.
- Local loop: The actual copper or fiber cable that connects the CPE to the CO of the service provider. The local loop is also sometimes called the “last mile.”
- Central office [CO]: The CO is the local service provider facility or building that connects the CPE to the provider network.
- Toll network: This consists of the long-haul, all-digital, fiber-optic communications lines, switches, routers, and other equipment inside the WAN provider network.
WAN Devices [2.1.2.3]
As illustrated in Figure 2-8, there are various methods, and therefore devices, that are used to access the WAN connection. Service providers also have specific WAN devices within their network and devices that are required to interconnect to other WAN providers.
The example in the figure identifies the following WAN devices:
- Dialup modem: Considered to be a legacy WAN technology, a voiceband modem converts [i.e., modulates] the digital signals produced by a computer into voice frequencies that can be transmitted over the analog lines of the public telephone network. On the other side of the connection, another modem converts the sounds back into a digital signal [i.e., demodulates] for input to a computer or network connection.
- Access server: Devices used to concentrate the dial-in and dial-out user communications of dialup modems. Considered to be a legacy technology, an access server may have a mixture of analog and digital interfaces and support hundreds of simultaneous users.
- Broadband modem: A type of digital modem used with high-speed DSL or cable Internet service. Both operate in a similar manner to the voiceband modem, but use higher broadband frequencies and transmission speeds.
- Channel service unit / data service unit [CSU/DSU]: Digital leased lines require a CSU and a DSU. A CSU/DSU can be a separate device like a modem or it can be an interface on a router. The CSU provides termination for the digital signal and ensures connection integrity through error correction and line monitoring. The DSU converts the line frames into frames that the LAN can interpret and vice versa.
- WAN switch: A multiport internetworking device used in service provider networks. These devices typically switch traffic, such as Frame Relay or ATM and operate at Layer 2.
- Router: This is a CPE device that provides internetworking and WAN access interface ports used to connect to the service provider network. These interfaces may be serial connections, Ethernet, or other WAN interfaces. With some types of WAN interfaces, an external device, such as a DSU/CSU or modem [analog, cable, or DSL], is required to connect the router to the local service provider.
- Core router/multilayer switch: These are the routers and multilayer switches that reside within the service provider WAN backbone. To fulfill this role, the devices must be able to support routing protocols being used in the core and multiple high speed interfaces used in the WAN core backbone. They must also be able to forward IP packets at full speed on all of those interfaces. Key core routers interconnect to other provider core routers.
The type of devices used depends on the WAN technology implemented. These WAN technologies are implemented over either circuit-switched or packet-switched networks.
Circuit-Switched Networks [2.1.2.4]
A circuit-switched network is one that establishes a dedicated circuit [or channel] between nodes and terminals before the users may communicate. As illustrated in Figure 2-9, circuit switching dynamically establishes a dedicated virtual connection for voice or data between a sender and a receiver. Before communication can start, it is necessary to establish the connection through the network of the service provider. It is important to note that the circuit must remain established and never change or communication will be terminated.
As an example, 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 caller to 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.
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 is 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.
The two most common types of circuit-switched WAN technologies are the public switched telephone network [PSTN] and the Integrated Services Digital Network [ISDN].
Packet-Switched Networks [2.1.2.5]
In contrast to circuit switching, packet switching splits traffic data into packets that are routed over a shared network. Packet-switching networks do not require a circuit to be established, and they allow many pairs of nodes to communicate over the same channel.
The routers in a packet-switched network [PSN] determine the links that packets must be sent over based on the addressing information in each packet. The following are two approaches to this link determination:
- Connectionless systems: Full addressing information must be carried in each packet. Each router must evaluate the address to determine where to send the packet. An example of a connectionless system is the Internet.
- Connection-oriented systems: The network predetermines the route for a packet, and each packet only has to carry an identifier. The router 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. When the circuit is established temporarily while a packet is traveling through it, and then breaks down again, it is called a virtual circuit [VC]. An example of a connection-oriented system is Frame Relay. In the case of Frame Relay, the identifiers used are called data-link connection identifiers [DLCIs].
Because the internal links between the switches are shared between many users, the cost of packet switching is lower than that of circuit-switching. However, delays [latency] and variability of delay [jitter] are greater in packet-switched networks than in circuit-switched networks. This is because the links are shared and because 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 video communications on these networks.