WANs operate at OSI Layer 1 and Layer 2, specifically the Physical Layer and the Data Link Layer. To review, OSI Layer 1 describes how to provide electrical, mechanical and operational connections, while Layer 2 defines how data is encapsulated for transmission to a remote location and the mechanisms for transferring the resulting frames. Different technologies are used, such as Frame Relay and ATM. WAN access standards are defined and managed by a number of recognized authorities like, International Organization of Standardization (ISO), Telecommunication Industry Association (TIA) and Electronic Industries Alliance (EIA).
When you prepare for the CCNA exam, you must be able to describe some specific WAN terms. Some terms are related to the Physical Layer, some to the Data Link Layer and some about the WAN Switching methods. In this lesson we will discuss the terms related to the Physical Layer and the ones related to WAN Switching. The Data Link layer, specifically the encapsulation types and frame formats will be left for another lesson.
At the Physical Layer of a WAN you must be able to recognize and describe some key concepts:
- Customer Premises Equipment (CPE) – is the device located in the premises of the subscriber (company) and connected to a service provider or carrier. The subscriber can either own or lease the equipment from the carrier. Example of CPE equipments are telephones, DSL and Cable modems, set-top boxes.
- Data Communications Equipment (DCE) – sometimes called data circuit-terminating equipment, the DCE provides an interface to connect subscribers to the WAN cloud and consists of devices that put data on the local loop. Usually, the DCE equipment is a modem.
- Data Terminal Equipment (DTE) – are the customer devices that pass the data from its network for transmission over the WAN. The DTE connects to the local loop through the DCE. The DTE device is usually a router.
- Demarcation Point – is the border which separates the customer equipment from the service provider equipment. Physically, the Demarcation Point is usually the cabling junction box from the customer premises.
- Local Loop – is the copper or fiber cable that connects the CPE at the subscriber to the Central Office (CO) of the service provider. Sometimes is called “last-mile”.
- Central Office (CO) – is the service provider’s facility where local telephone cables link to long-haul, all-digital, fiber-optic communications lines through a system of switches and other equipments.
Several types of devices are used in WANs:
- Modem – used to modulate an analog carrier signal to encode digital information and also demodulate the carrier signal to decode the transmitted information.
- CSU/DSU – used by digital lines such as T1 or T3. The CSU provides termination for the digital signal and the DSU converts the line frames so the LAN can interpret them and vice versa.
- WAN Switch – is a multiport internetworking device used by carriers able to switch WAN traffic types such as Frame Relay, ATM or X.25. They operate at the Data Link Layer.
- Router – provides internetworking and WAN access interface ports used to connect to the service provider network, such as serial interfaces. Some types of interfaces require an external device such as a DSU/CSU or modem (analog, cable, DSL).
- Core Router – is the router located in the middle or backbone of the WAN. The router must be able to support multiple types of interfaces and must be able to forward IP packets at full speed on those interfaces. The router must also support the routing protocols used in the core.
As we already told you, several authorities are handling the WAN standards. These protocols establish the codes and electrical parameters used by devices to communicate with each other. We will cover the most important ones, as requested by the Cisco CCNA exam.
- EIA/TIA-232 – this protocol is able to signal speeds of up to 64kb/s using a 25-pin D-connector over short distances. It is also known as RS-232. The ITU-T V.24 specification is effectively the same.
- EIA/TIA-449/530 – this is the faster version of EIA/TIA-232, being able to carry frames at speeds up to 2Mb/s. It uses a 36-pin D-connector and is also able to reach longer destinations. Also know as RS422 and RS-423.
- EIA/TIA-612/613 – describes the High-Speed Serial Interface (HSSI) protocol, which provides speeds up to 52Mb/s using a 60-pin D-connector.
- V.35 – ITU-T standard for synchronous connections between a network access device and a packet network using a 34-pin rectangular connector. Originally designed to support speeds up to 48kb/s, now supports speeds of up to 2.048Mb/s.
- X.21 – ITU-T standard for synchronous digital communications. Uses a 15-pin D-connector.
WAN protocols are not able to run over LAN technologies, such as Ethernet and vice versa because the frame formats, encapsulation methods and the signaling at the physical layer differ from WAN to LAN.
WAN connections are generally grouped in three connection types: Point-to-Point, Circuit-switched and Packet-switches. In your preparation for the CCNA exam you must study Circuit-switched and Packet-switched technologies.
Circuit-switched networks are networks that are establishing a dedicated circuit between nodes and terminals before users may communicate. Let’s take for example a connection between two modems. The first one tries to make a connection with the other one by dialing its number. The dialed number is used to set the switches in the exchanges along the route of the call so that there is a continuous circuit between the two modems. However, the internal path between the exchanges is shared with a number of other connections. Time-division multiplexing (TDM) assures that a fixed capacity is allocated to each connection. Examples of Circuit-switched networks are the Public Switched Telephone Network (PSTN) and Integrated Services Digital Network (ISDN).
Packet-switched networks, unlike circuit-switched, route packets over a shared network. They do not require a circuit to be established and they allow multiple devices to communicate over the same channel. The switches are able to determine the link required to be used for forwarding the packet to the next switch or device using the addressing information found in each packet. The link determination can be made in two ways: connectionless or connection-oriented. Connectionless systems carry full addressing information in each packet and each switch must evaluate the address to be able to figure out where to send it, while connection-oriented systems are able to predetermine the route for a packet and each packet carries only an identifier. In packet switched networks, because the internal links are shared by many users, you may encounter delays and jitter (variability of delay).
The predetermined routes in a packet-switched network are called virtual circuits (VCs). A VC is a logical circuit between two network devices through the shared internal network. There are two types of VCs:
- Permanent Virtual Circuit (PVC) – PVCs are used when data transfer between devices is constant.
- Switched Virtual Circuit (SVC) – is an on-demand VC. The virtual circuit comes up when a data transfer begins and terminates when the transfer is complete. The first phase in a SVC is to establish the VC itself and then the data transfer begins. When the transfer completes, the VC disconnects from the remote device. SVCs are cheaper than PVCs.
Examples of packet-switched connections include: X.25, Frame Relay and ATM.
We will go deeper in this CCNA topic by studying different circuit-switched and packet-switched connections.
Analog dialup and ISDN are examples of circuit switched connections.
Analog dialup is using the traditional telephony copper cables. The data is modulated and demodulated by a modem and sent through the local loop to the CO. Advantages of using analog dialup are simplicity, available and low implement costs, but the main disadvantage is the low data rate, 56kb/s due to the PSTN network limitations. If your company needs a data connection only occasionally and is not sending or receiving large amounts of data, this connection may be adequate. However, voice and video traffic is not supposed to work at these lot bit rates.
Integrated Services Digital Network, or ISDN for short, are also circuit-switched networks. They allow a local loop to carry digital signals, resulting in higher data rates. ISDN turns the analog signals to time-division multiplexed (TDM) digital signals. TDM allows two or more signals to be transferred as subchannels in one communication channel. ISDN connections are using 64kb/s bearer channels (B) for 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) – provides two 64 kb/s B channels and a 16kb/s D channel.
- Primary Rate Interface (PRI) – allows for higher speeds. In North America, an ISDN PRI deliver 23 B channels at 64kb/s and one D channel, also at 64kb/s, totaling to 1.544MB/s, corresponding to a T1 connection. In the other parts of the world, PRI provides 30 B channels and one D channel, totaling to 2.048MB/s, corresponding to an E1 or a J1 connection.
Common packet-switched technologies used today include X.25, Frame Relay and ATM.
X.25 is a packet-switched technology mainly used these days for point-of-sale (POS) card readers. X.25 works at the Network Layer. Subscribers are provided with a network address. SVCs are established using call request packets and are identified using a channel number. Available speeds vary from 2400b/s to 2Mb/s, but they rarely exceed above 64kb/s. Due to the speed limitations, this type of networks are now being replaced by other technologies such as Frame Relay, ATM and ADSL.
Frame Relay is a protocol that works at the Data Link Layer, implementing flow control. Frame relay offers speeds up to 4Mb/s, and is able to carry both voice and data traffic. The VCs in Frame Relay are identified using an unique DLCI. You will find more details about Frame Relay in its dedicated lesson.
Asynchronous Transfer Mode (ATM) has a cell-based architecture. You may find someone referring to ATM as a cell-switched network. ATM cell are always 53 bytes, containing a 5 byte ATM header followed by 48 bytes of ATM payload. ATM networks are suitable for voice, video and data traffic.
However, due to its small cells, it’s is less efficient than X.25 and Frame Relay and you need at least 20% more bandwidth than Frame Relay to carry the same amount of data. The biggest advantage of ATM is that it is able to operate at very high speeds – from T1/E1 to OC-12 (622Mb/s) and higher. In most cases, ATM uses PVCs, but can be configured as a SVCs too and allows multiple VCs on a single leased-line connection.
This concludes our lesson today. We hope you find it useful in your preparation for the CCNA exam. Understanding these key WAN concepts is very important, as well as for taking the CCNA certification but for real-life scenarios too.