DDS Equipment Overview
Before discussing DDS line operation, several devices should be examined:
• Channel service unit (CSU)
• Office channel unit-data port (OCU-DP)
• Multijunction unit (MJU)
You can use a CSU in DDS and numerous other technologies. Typically referred to in tandem with DSUs (CSU/DSU), you can use these devices in DDS, FT1/T1, FE1/E1, and several other technologies. The CSU is the interface that communicates directly with the service provider network (see Figure
4-2). The CSU terminates the DDS circuit, allows for maintenance through loopback and test patterns, and formats the information transferred to it from the DSU for transmission over the installed network.
Figure 4-2 Identifying the CSU Portion of DDS Equipment
To Customer DTE Equipment
To Customer DTE Equipment
To Service
Provider Network -►
To Service
Provider Network -►
In the United States, the CSU is also responsible for power surge, power cross, and lightning protection. A power cross is something like a power line detaching and landing across pole telephone drops. The protection is two fold because the CSU protects your equipment from a power spike coming from the service provider, and it also protects sensitive service provider equipment from any type of power spikes that may originate from your side of the network. This type of protection was made necessary by the divestiture of the AT&T in 1984.
DSUs are devices that connect to the customer's DTE. For example, it connects to a router that does not have DDS support or to a PC at your desk. The purpose of the DSU is to convert the unipolar signal from something like a computer into a bipolar balanced signal so that it can be transmitted through the service provider's network (see Figure 4-3). The difference between unipolar and bipolar signals is the fact the unipolar signals (uni) only have a single polarity, and bipolar signals have two different polarity states.
Figure 4-3 Identifying the DSU Portion of DDS Equipment
To Customer DTE Equipment
To Customer DTE Equipment
To Service
Provider Network ->
To Service
Provider Network ->
DSUs can connect to your equipment in several different ways, including V.35 and X.21. A V.35 or X.21 connection identifies that the CSU/DSU is a separate piece of equipment that terminates the DDS circuit. A CSU/DSU can also be an expansion card directly configured in the Cisco router in use.
Prior to the divestiture, the CSU and DSU were separate boxes, both owned by the RBOC. Since that time, they have become the responsibility of the customer. You typically see the CSU and DSU within the same unit, such as a router with DDS support. Don't let the terms DSU/CSU and CSU/DSU fool you, they are the same thing. It just depends on who you are talking to. If you are speaking with a service provider representative, chances are they say CSU/DSU because the CSU is from their vantage point. Most customers, on the other hand, refer to DSU/CSU for the exact same reason. Either way is correct.
The OCU-DP provides the four-wire DDS interface to the subscriber and is usually a card located in a T1 channel bank at the CO. If there are multiple DDS circuits with loop rates lower than 56 kbps or 64 kbps, a Subrate Data Multiplexer (SRDM) combines them into a single digital service 0 (DS0) for multiplexing into a digital signal level 1 (DS-1) signal. The channel bank acts as a T1 multiplexer that can provide multiple DDS, ISDN-BRI, and subrate FT1s (fractional). These services are combined, or multiplexed, into a single DS-1 signal. In turn, these DS-1 links are normally multiplexed into a higher bandwidth link such as a DS-3 through a M13 MUX or a Digital Access and Crossconnect System (DACS), as shown in Figure 4-4.
Figure 4-4 Multiplexing at the Central Office for DDS
T1 Channel Bank With OCU-DP
Figure 4-4 Multiplexing at the Central Office for DDS
T1 Channel Bank With OCU-DP
- DDS Circuit-----------
The definition of multiplexing is the action of taking multiple circuits and combining them into a larger circuit with more bandwidth. This is where the term time-division multiplexing (TDM) comes from. TDM and 24 DS0s with 8 kbps of overhead are combined to form a DS-1 signal. Each DS0 is allotted a moment in time to transmit information through the circuit. Refer to Chapter 5, "T1 Technology," for more information on T1 circuits and TDM.
OCU-DP cards can provide variable rates of service up to 64 kbps, depending on the subscriber's needs. Most of the time, DDS is thought of as a point-to-point, nailed architecture. The term nailed refers to a circuit that is always connected and always being paid for. However, some hardware can also provide a point-to-multipoint service for DDS by using a polling mechanism. Figure 4-5 shows a DDS deployment with an MJU.
When a point-to-multipoint configuration is required, the service provider will use an MJU. In communication downstream from the primary CSU/DSU, data can be sent to one or more of the connected CSU/DSUs at the remote ends of the circuit. DDS does not employ any type of mechanism to avoid data corruption from multiple devices transmitting simultaneously or to ensure that one device does not monopolize the circuit bandwidth. These functions are left up to the customer's CPE.
Referencing Figure 4-5, CSU/DSU A acts as the hub for the connection. It can communicate individually with each remote CSU/DSU (B-E) or broadcast to them all at once. Furthermore, communication is between one of the remote devices and the hub. Communication between remote devices such as B and E is not necessarily specified. This topology is common for local stores that are uploading their receipts for the day to a regional sales center.
Figure 4-5 Point-to-Multipoint DDS Connection
Figure 4-5 Point-to-Multipoint DDS Connection
Continue reading here: Line Coding
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