High-speed SS7 - Signaling System 7 - Technology Information

Communications News, August, 1999 by Bruce Ramsay, Emma Masters

Calculate the transformation of communications.

Our increasing reliance on telecommunications is straining telephone networks that were not designed to carry the huge amounts of data that traverse them today. Consequently, the amount of SS7 traffic being generated by telecommunications networks is climbing fast. Why is this?

For one, the Telecommunications Act of 1996 was aimed at deregulating the market and increasing competition between service providers. There are two significant rulings that resulted from the Act. First, one carrier should not have an appreciable cost advantage over any other when competing for the same customer. Second, cost recovery should not have a negative effect on the ability of a carrier to earn a normal return on investment.

With incumbent and competitive local exchange carriers (CLECs) now unable to compete effectively on price, they must find other ways to distinguish themselves. Thus, they're aiming to introduce unique services and deploy them faster than their competitors. To do this, they are using advanced intelligent network (AIN) functionality, which increases the amount of SS7 traffic on the network.

The FCC ordered that local number portability (LNP) be implemented within the top 100 major metropolitan areas nationwide between October 1, 1997, and December 31, 1998. This order mandated that wireless carriers must route ported number calls by the December 31 deadline and provide full number portability among their own networks within six months of this deadline. With approximately two billion calls per day being made in the U.S. and the prediction that in several years almost every call will have a database dip, some RBOCs (regional Bell operating companies) are anticipating 20% growth in SS7 links solely to deal with LNP.

Operators will therefore have to plan their SS7 signaling capacity to cope with additional demands imposed by the new services. In some cases, the additional capacity could be met with extra low-speed links. However, where the limits of STP port and link-set capacity have already been reached, a better alternative would be to replace low-speed links with new high-speed SS7 links.

LOW-SPEED VS. HIGH-SPEED 557

There are a number of differences between the low-speed SST and high-speed SS7 technologies. The differences are in the lower two layers of the protocol stack (i.e., the MTP-1 and MTP-2 layers). A comparison of the two technologies is made here.

The low-speed signaling data link is a full-duplex, digital transmission channel operating at 64 or 56 kbps. The packets carried using this protocol are variable length and carried down a single clear channel link. This means that the link must be dedicated to the SS7 traffic and cannot be used to carry any other data. MTP-1 is the lowest level of this protocol and is responsible for the physical connection to the transport links. MTP-2, the next layer up, is responsible for transferring traffic between two SS7 components.

The initial deployment of high-speed SS7 will operate at a speed of 1.544 Mbps. The U.S. market has adopted the asynchronous transfer mode (ATM) over T1 standard for high-speed SS7, the only exception being AT&T which adopted the clear channel method developed by Lucent. As such, only the ATM version of high-speed SS7 is covered here.

With the high-speed SS7 system, the MTP-1 and the MTP-2 layers are replaced. The MTP-2 layer is replaced with the signaling ATM adaption layer (SAAL), and the MTP-1 layer is replaced with ATM.

The data is packaged into a CPCS-PDU (common part convergence sublayer-protocol data unit) by adding the service-specific connection-oriented protocol-protocol data unit (SSCOP-PDU) trailer and the ATM adaption layer (AAL) CPCS PDU trailer. (Both are shown in the figure.) This adds eight octets of header detail onto the size of the message. The protocol data unit is then split into 48 octet sections so that it will fit into the payload of the ATM cell.

PROBLEMS INVOLVED

While one high-speed link can theoretically replace up to 24 low-speed links, in practice the replacement ratio is variable. Where low-speed SS7 is a variable-length packet communications protocol, high-speed SS7 is fixed length. This implies that, depending on the traffic profile, the efficiency of the new high-speed links can vary.

Each ATM cell in the high-speed link is fixed at 48 octets, so very short message signaling units (MSUs) are less efficient because they do not utilize the entire available payload. As the length of the MSU increases, the efficiency increases until the point that the MSU becomes too large for one packet and the efficiency drops again.

Deciding which links should be replaced can be difficult. The replacement ratio can vary depending on the type of traffic that must be transported. If an incorrect decision is made, the new high-speed link (a) may not be sufficient to cope with the data that it is expected to transport or (b) may be vastly underused. This is why proper profiling of traffic is essential when deciding which links should be replaced.