Interfaces and Modules

Interfaces and Modules


  • Select the components required to meet a network specification
  • Describe the purpose and functions of various network devices

Networks can be connected to Cisco hardware in a variety of ways. Communication lines can be terminated to a Cisco device via hardware interfaces and modules. Interfaces provide a physical point of interaction between two networks. That hardware interface includes the cable, plug, socket, and signal that sync up together to communicate among devices.

Certain Cisco routers are built as fixed-port routers or fixed configuration routers and do not allow for additional network module installations. Cisco also offers modular-port routers. Modular-port routers allow for future system upgrades by mounting network modules in available spaces to accommodate changing network environments. A module is a self-contained component. A Cisco network module has built-in hardware interfaces to add alternate connection options on a network. Because they are modular components, Cisco routers can be upgraded easily and with minimal expense to the company. This chapter reviews the variety of physical connection types on LAN and WAN devices.

LAN Interfaces

Local area network (LAN) interfaces are used to provide a point of interconnection between Cisco switches and other network devices. Cisco provides a wide selection of switches that can be implemented on a LAN and offer end-user connectivity. In Chapter 3, “Data Link Networking Concepts,” Layer 2 switches were introduced. The Cisco 2950 series switch family includes various models with different interface options, such as the Cisco 2950-12 and the Cisco 2950-24. The 2950-12 has 12 built-in ethernet ports, whereas the 2950-24 has 24 builtin ethernet ports. Certain models also have Gigabit Ethernet slots.

If Gigabit Ethernet ports are included, the front panel of a 2950 model has 10/100 ethernet ports on the left side of the switch and 2 Gigabit Ethernet slots that accommodate LAN interface modules on the right. The 10/100 ports allow for either a 10Mbps or 100Mbps connection speed. Media connects to an ethernet switch port via an RJ-45 connector. As far as the switch is concerned, each ethernet port is designated as a numbered interface for identification. The top left port is labeled 1 by the switch. Each interface begins with a 0/#, where the # sign equals the port number on the switch. The top left port is then named 0/1 on that switch. Given this formula, the bottom left port is labeled 0/2.

For the CCNA exam, know how the interfaces are labeled for an ethernet port. This information is necessary for any switch configuration exercises.

The Gigabit Ethernet slots are available for Gigabit Interface Converters (GBICs). A GBIC interface module can be inserted into the Gigabit Ethernet slot to allow for different media connections to that port. The physical media can range from copper to single-mode fiber. A GBIC is also hot swappable, so you can remove and replace it without shutting off power to the switch. This helps to avoid interruption of service to that switch. The back panel of the 2950 includes power input and the switch’s console port. The console port has an RJ-45 connector and is connected to a terminal with a rollover cable for initial switch configuration.

WAN Interfaces

Wide area network (WAN) interfaces are also used to provide a point of interconnection between Cisco routers and other network devices. Types of WAN interfaces include

  • Basic Rate Interface (BRI)
  • Synchronous Serial
  • Asynchronous Serial
  • High-Speed Serial Interface (HSSI)
  • T1 Controller Card

BRI is an Integrated Services Digital Network (ISDN) service that consists of two 64Kbps bearer (B) channels and one 16Kbps data channel. Voice, video, and data traffic can be carried

over the B-channels. Signals between telephone company switches use the D-channel. Cisco offers an 8-port ISDN-BRI with a built-in Network Termination Type 1 (NT-1) Network Module for router installation

For the exam you should know that BRI is an ISDN line that consists of two 64Kbps bearer (B) channels and one 16Kbps data (D) channel.

The NT-1 is a telephone company requirement for an ISDN line connection. This network module has a BRI U interface, which means that the NT-1 is built in on the network module and does not require a separate NT-1 device.

Synchronous Serial
A synchronous serial interface synchronizes clocks for the bit stream of both the sending and receiving end of a serial link. This allows for the data rate to be adjusted if necessary to ensure that both ends of a serial link are functioning at the same speed.

Asynchronous Serial
An asynchronous serial interface does the opposite of a synchronous serial interface. It does not synchronize the clocks for the bit stream of the sending and receiving end of a serial link. Cisco offers a 4-port asynchronous/synchronous serial network module. With the asynchronous/synchronous serial network module, each port can be configured individually as either asynchronous or synchronous, depending on your network setup.

High-speed serial interfaces offer up to 52Mbps transmission rates to the WAN from a Cisco router. The higher speed capacity is relevant if the corporate backbone requires high-speed Internet access and VPN connectivity. Cisco offers a 2-port HSSI port adapter.

T1 Controller Card
Also referred to as a digital signal level 1 (DS1) service, a T1 is a connecting line that offers a 1.544Mbps data transmission speed. A single T1 line consists of 24 digital signal level 0 (DS0) channels that are 64Kbps each and an additional 8Kbps that is reserved for management overhead. A T1 controller card can be installed in a router’s T1 slot to communicate with and control the 24 DS0 channels.

Data Communications Equipment (DCE)
Data Communications Equipment (DCE) or Data Circuit-Terminating Equipment (DCE) is the term used to identify a device that connects the Data Terminal Equipment (DTE) to a service provider’s communications line. The DCE side of a connection sets the clock speed for a serial connection.
DCE equipment may consist of a

  • Modem
  • Channel Service Unit/Data Service Unit (CSU/DSU)
  • Basic Rate Interface Network Termination Type 1 (BRI NT-1)

Modems convert a digital signal into an analog signal for transmission over a telephone line. The signal is converted back into a digital format when it reaches the device on the other end of that telephone line. A Channel Service Unit/Data Service Unit (CSU/DSU) serves as the intermediary between the service provider and the WAN router. In most cases, the CSU/DSU provides the clock speed for the router. A CSU/DSU may be a separate unit or it could be incorporated into a WAN interface card (WIC). If it is not built in on a Cisco router via a BRI-U (Basic Rate Interface-User) interface, the service provider requires separate BRI NT-1 hardware as a termination point for the communications line. The BRI NT-1 then connects to the Cisco router.

A modem, CSU/DSU, or BRI NT-1 can be used to connect a WAN router at the customer premise to a
service provider.

Data Terminal Equipment (DTE)
Data Terminal Equipment is the term used to identify a device at the user end of a network and is connected to the service provider via the DCE device. DTE equipment may consist of a

  • Router
  • PC
  • Server

In Figure 6.1, the service provider, whom you will most likely hear called a telco, brings a communication line from its central office (CO) to the customer and terminates its line to the CSU/DSU. The CSU/DSU is then connected to the customer router. The point at which the telco terminates its line to the customer is called a demarcation point or demarc. Customerowned equipment, such as the router and typically the CSU/DSU, is referred to as customer premise equipment (CPE).


Cabling between the CSU/DSU and router is decided by the type of CSU/DSU that is deployed on that network. If a WIC functions as a CSU/DSU, then the CPE is a telco jack and a Category 5 or Category 6 cable is used with an RJ-45 connector. If a WIC does not function as the CSU/DSU, there are several types of connections possible between a CSU/DSU and the DTE device. With routers, typically a DB-60 connector is used to connect to the router while one of the following connectors is used to connect to the CSU/DSU:

  • EIA/TIA-232
  • EIA/TIA-449
  • EIA/TIA-530
  • V.35
  • X.21

The Electronic Industries Association/Telecommunications Industry Association (EIA/TIA) formed a standards body, which developed the 232, 449, and 530 cables. The V.35 and X.21 cables were developed by the International Telecommunication Union (ITU). Your best bet is to position the CSU/DSU as close to the router as possible. This requires the shortest amount of cable and therefore ensures maximum speeds.

For the exam you should know the difference between DCE and DTE equipment. Also, know that the DCE side of a connection must provide the clocking for the serial connection.

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