E1 Physical Characteristics
Similar to T1 technology, an E1 refers to the carrier facilities associated with the digital signal. E1 circuits are also dedicated point-to-point synchronous circuits. Similar to T1, E1 circuits are equally susceptible to timing issues associated with multiple clock sources. Don't confuse the synchronous circuit with the plesiochronous network structure. Remember that plesiochronous networks contain multiple synchronous circuits with different clock sources.
An E1 circuit differs in several ways from a T1 circuit. The first, and most noticeable, is the allocation of DS0s. An E1 circuit has 32 DS0s, instead of 24 DS0s, which gives it a total bandwidth of 2.048 Mbps. Out of these 32 DS0s, 30 or 31 of them might be for bearer traffic (depending whether or not you use timeslot #16 [TS16] for signaling or bearer traffic).
An argument can be made as to whether E1 circuits are common channel signaling (CCS) or channel associated signaling (CAS). By definition, CCS states that the signaling is not intrusive to the data, but CAS states that signaling is intrusive, or non-separable from the data. The fact is, you can use E1 circuits in several different ways. Basic E1 circuits are typically found in CAS configurations. When the circuit is set up as CAS, TS16 splits between the circuit timeslots at timed intervals and the bits for one timeslot cannot be shared with another.
However, when you use an E1 circuit in a CCS configuration, all the timeslots use or share TS16 as a common resource. For example, if you use the E1 circuit as a Signaling System 7 (SS7) signaling link or as an Integrated Services Digital Network Primary Rate Interface (ISDN PRI) the circuit can be considered CCS. The reason for the CCS classification is that in both instances, TS16 can provide signaling for any of the other timeslots on the circuit. Although the signaling is physically in-band, the signaling is logically out-of-band because the signaling is carried over a separate timeslot.
You do not have to use TS16 for signaling, particularly in the cases of ISDN PRI and SS7. It is common in both of these instances to have E1 circuits with 31 channels available for bearer traffic. ISDN PRI circuits can be deployed in groups with something called non-facility associated signaling (NFAS). NFAS allows a single ISDN D-channel to provide call control for up to 479 B-channels. So, in a NFAS group with 5 PRIs, the first PRI uses TS16 as the NFAS D-channel. You can use TS16 in PRIs 2-5 for bearer traffic.
SS7 is a similar case to ISDN. A single signaling link can provide call control for thousands of timeslots (trunks). Within the first E1 span, circuits can use TS16 as the SS7 signaling link, and subsequent E1 circuits can use TS16 for bearer traffic.
There might be other instances that allow you to reclaim the 16th timeslot for bearer traffic, particularly if you are using the same vendor's equipment on both ends of the circuit and a proprietary signaling format has been put in place. However, these proprietary signaling formats are not nearly as common as the usage described with NFAS or SS7. Figure 6-1 shows the allocation of the channels for an E1 circuit.
Figure 6-1 DS0 Allocation on an E1 Circuit
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Figure 6-1 DS0 Allocation on an E1 Circuit
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Timing and Synchronization------
Signaling -
Timing and Synchronization------
Signaling -
E1 circuits, similar to T1 circuits, are referred to as co-directional interfaces. A co-directional circuit is defined when timing information and the bearer traffic take the same physical path between nodes and in the same direction. Figure 6-2 shows a co-directional circuit deployed as two copper pairs of wiring. Remember that because of time-division multiplexing (TDM), it is possible to carry 32 timeslots over just four wires.
Figure 6-2 Co-Directional Circuit Between Endpoints
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E1 circuits are deployed throughout the international community on one of three types of medium:
• 120 ohms unbalanced twisted copper cable
• 75 ohms balanced coax cable with Bayonet Neil Concelman (BNC) connectors
Continue reading here: DB15 connector
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