Siemens TIM 4R-IE GPRS Telecontrol: Node Limits and MSC Setup

Overview

This technical reference defines the connection-capacity rules, protocol requirements, and hardware selection logic for scaling a Siemens Industrial Remote Communication solution built on GPRS telecontrol. The architecture serves a 145-node deployment combining 5 remote substations, 80 power meter stations, and 60 flow meter stations distributed up to 14 km from a central control room. The configuration uses the Telecontrol Professional software package, TIM 3V-IE (or TIM 3V-IE Advance) station modules, TIM 4R-IE central modules, and either MD720-3 serial GPRS modems or MD741-1 EGPRS Ethernet modems. The reference quantifies three independent ceilings — TIM 4R-IE active connections, TIM MSC partner database size, and SCALANCE S6xx IPsec tunnel count — then derives the number of central TIM 4R-IE modules and SCALANCE S6xx VPN appliances required to absorb the 145 stations. It also documents the three valid network patterns, the modem selection logic, the IP / NAT / DNS planning constraints, and a step-by-step commissioning sequence with a verification matrix.

Operators planning a multi-cluster GPRS telecontrol deployment on Siemens hardware routinely underestimate the binding constraint: a single TIM 4R-IE accepts only 64 simultaneously active MSC connections, the partner database is capped at 128, and a single SCALANCE S6xx terminates only 128 IPsec tunnels. For 145 stations, those numbers force a minimum of three central TIM 4R-IE modules and, if a VPN-fronted architecture is chosen, two SCALANCE S6xx appliances on two separate DSL lines.

System Requirements and Topology Constraints

The reference deployment is split into three physical clusters of remote stations, all reporting back to a single central control room (CCR) over a public GPRS bearer that is terminated at the CCR on a DSL router:

At the CCR, the DSL router performs the address translation and the GPRS-to-Ethernet bridging. Behind the DSL router sit the central TIM 4R-IE modules and the Telecontrol Professional host PC. Telecontrol Professional polls each connected TIM 4R-IE, buffers the data, and exposes it to the SCADA layer above. The GPRS network itself uses the standard GSM packet-switched domain defined by 3GPP — GPRS is often called 2.5G and historically delivered theoretical peak rates up to 171.2 kbps across all eight timeslots, with typical real-world throughput of 30-80 kbps depending on coding scheme, signal conditions, and base-station loading (GPRS overview).

MSC Protocol and WAN Requirements

MSC is the Siemens telecontrol protocol that runs between TIM modules. It is an encrypted, connection-oriented protocol that carries S7-compatible payloads, time-stamped events, and buffered frames. Within the Telecontrol Professional stack, MSC is the only protocol family that supports secure wide-area operation between a station TIM and a central TIM. If MSC is not enabled on the WAN-facing interface, the TIM will not accept a connection request from a peer across a routed or VPN network.

Key facts on MSC support across TIM variants:

• TIM 3V-IE (standard): MSC is not supported on the Ethernet port. MSC is only available over the serial port to an MD720-3 GPRS modem, where MSC rides the GSM/GPRS bearer as a serial-tunneled protocol.
• TIM 3V-IE Advance: MSC is supported on the Ethernet port, so the station can present MSC directly to a router or SCALANCE M over Ethernet and forward it to the central TIM 4R-IE.
• TIM 4R-IE (central): MSC supported on both Ethernet interfaces, with redundant LAN ports for the dual-network architectures commonly used at the CCR.

Each open MSC connection consumes one connection slot in the central TIM 4R-IE. The slot is reserved from the moment the remote station authenticates until the connection is explicitly closed, or until the connection times out under the configured watchdog. For sizing purposes, an "active connection" means any partner whose state is established or maintained, including those that are temporarily idle but not yet torn down.

Connection and Tunnel Capacity: The Hard Numbers

Three independent ceilings apply to a Telecontrol Professional deployment, and they are easy to confuse. The binding limit for any given architecture is the smallest of the three that is actually in the path of traffic.

Reference: Siemens Industry Online Support entry 54066139 covers the GPRS/Internet communication overview and the configuration rules referenced throughout this article.

Sizing Formulas and the 145-Node Calculation

For the reference 145-node deployment, the constraint chain is:

1. Total remote stations: N = 145
2. Active-connection ceiling per central TIM 4R-IE: C_T = 64
3. Minimum number of central TIM 4R-IE modules: TIM_min = ceil(N / C_T) = ceil(145 / 64) = 3
4. Active-connection headroom with 3 modules: H_active = 3 × 64 − 145 = 47 spare slots
5. Partner-database ceiling per TIM: P_T = 128
6. Partner count per TIM in a 3-way split: P_per_TIM = ceil(145 / 3) = 49 partners (well within 128)
7. SCALANCE S6xx tunnel ceiling: T_S6xx = 128
8. Minimum SCALANCE S6xx appliances if VPN-fronted: S6xx_min = ceil(N / T_S6xx) = ceil(145 / 128) = 2

Two TIM 4R-IE modules can carry at most 2 × 64 = 128 active MSC connections, leaving 17 stations unreachable. Three TIM 4R-IE modules provide 192 connection slots — 47 slots of headroom over the 145-station requirement. A 70-70 split across two TIM 4R-IE behind one DSL line works only when N ≤ 128; for N = 145, three central modules are mandatory.

The 128-tunnel SCALANCE S6xx ceiling becomes binding if the remote stations are fronted by SCALANCE M routers rather than MD720-3 modems. With 145 remote sites, a single SCALANCE S6xx cannot terminate all 145 IPsec tunnels. The two practical options are:

• Two SCALANCE S6xx appliances, each terminating up to 128 of the 145 remote VPN endpoints (with overlap designed in), behind two DSL lines.
• A SCALANCE S627 (the higher-end S6xx variant) used for the bulk of the VPN mesh, with the long tail of stations fronted by a separate S612 cluster.

Note that two SCALANCE S6xx cannot be placed behind a single DSL connection as a true redundant pair — the IPsec IKE negotiation cannot be split across two endpoints terminating the same set of remote peers on a single public IP. Plan one SCALANCE S per public IP, i.e., one SCALANCE S per DSL line.

Hardware Selection Matrix

Order numbers above are typical MLFB references for the variants commonly deployed in Telecontrol Professional projects; verify the exact part number against the Siemens Industry Mall at the time of procurement, since Siemens frequently refreshes the order numbers when firmware revisions change. The TIM 4R-IE has two independent Ethernet interfaces that are designed to be used in a redundant LAN topology (e.g., one to the SCADA LAN, one to the DSL router LAN) so that a single cable or switch failure does not isolate the central module from either side.

Modem Selection: MD720-3 vs MD741-1

The MD720-3 is a GPRS-only modem that connects to the TIM 3V-IE over RS-232. It supports GPRS class 8/10 and is the workhorse of the legacy Telecontrol Professional deployments — the variant is mature, well-supported, and the GPRS radio characteristics are well known in the field. The MD741-1 is the EGPRS-capable successor, supporting GPRS class 12 and EDGE modulation, and connects over Ethernet rather than serial. Choose the modem based on the following decision matrix:

When MD741-1 is in front of a SCALANCE S6xx, the IPsec tunnel terminates on the SCALANCE S, not on the TIM. The TIM only sees a default gateway. The MD741-1 then simply acts as the IP bearer for the SCALANCE S IPsec endpoint. This is the cleanest way to scale past the TIM 4R-IE's 64-connection ceiling, because the IPsec tunnel is not an MSC connection — it is a layer-3 transport that can carry many TCP sessions inside.

Network Architecture Patterns

Three patterns are valid for a GPRS telecontrol deployment. The choice determines which capacity ceiling binds.

Pattern A — TIM + MD720-3, MSC over GPRS

Each remote station has a TIM 3V-IE (or Advance) with an MD720-3 on its serial port. The central TIM 4R-IE accepts MSC connections from the MD720-3 modems over the public GPRS network. This pattern works for plain TIM 3V-IE stations because MSC travels over the GSM/GPRS bearer through the MD720-3's serial protocol — MSC on Ethernet is not required. The pattern is simple, proven, and avoids the need for any IPsec / VPN infrastructure. Its downsides are: (a) the 64-connection limit on the central TIM 4R-IE binds, (b) the GPRS link is not encrypted at the IP layer (only at the MSC layer above it), and (c) the throughput ceiling is the GPRS class 8/10 ceiling of the MD720-3.

Pattern B — TIM Advance + DSL, MSC over IP

Each remote station has a TIM 3V-IE Advance and a DSL router. MSC terminates on the TIM Advance's Ethernet port and travels over the Internet to the central TIM 4R-IE. This requires VPN or another secure transport, typically provided by SCALANCE S6xx appliances at the station and at the CCR. The pattern is appropriate when the stations are in fixed locations with DSL coverage. The 64-connection TIM 4R-IE limit still binds, so three central TIMs are still required for 145 stations.

Pattern C — SCALANCE S + MD741-1, IPsec mesh, TIM unaware

Each remote site has a SCALANCE S6xx behind an MD741-1 EGPRS modem. All remote SCALANCE S units establish IPsec tunnels to a central SCALANCE S6xx (or pair of S6xx) at the CCR. The SCALANCE S units act purely as the VPN layer; the TIM modules at the station and the central TIM 4R-IE see only a default gateway and an Ethernet frame. This is the only pattern that scales past the MSC partner-database limit of 128 per TIM, because the IPsec tunnel is not an MSC connection — it is a layer-3 transport that can carry an unlimited number of TCP sessions inside.

The three patterns can be mixed in a single project. Substations with high data volume and low node count often use Pattern A or B; the long tail of low-volume meter stations is the natural fit for Pattern C, where the SCALANCE S6xx absorbs the connection overhead and the central TIM 4R-IE only sees the polling sessions that pass through the VPN.

IP Planning, NAT, and DNS

When Pattern B or Pattern C is used, every IPsec tunnel must terminate on a remote network with a unique subnet. The internal IP subnets behind each tunnel must be different. The standard remedies for overlapping subnets are:

• 1:1 NAT (one-to-one network address translation) at the SCALANCE S to remap the remote subnet into a non-overlapping range visible to the central TIM.
• Re-IP the remote subnets during commissioning so the engineering ranges are unique across the fleet.
• Hub-and-spoke addressing plan with a private range (e.g., 10.0.0.0/8) chopped into /24 or /28 subnets, one per station, allocated from a central registry.

For DNS, SCALANCE S6xx firmware V3 and later supports DynDNS as a client. A SCALANCE S6xx V3 device can register its dynamic public IP with a DynDNS service, but it cannot resolve DNS names on its own. As a consequence, SCALANCE-S-to-SCALANCE-S VPN connections with both endpoints on dynamic IPs will not auto-negotiate — the central SCALANCE S cannot resolve the DynDNS name to find the peer's current public IP. Workarounds:

• Use static public IPs on the central SCALANCE S (and on the DSL line it sits behind).
• Use a fixed-IP SIM at the remote stations so the MD741-1 always presents the same public address.
• Terminate the IPsec on a third-party firewall / router that does support DNS-resolved peers, and place the SCALANCE S downstream of it.

Subnet sizing cheat sheet for 145 stations using a /28 per station and a /16 hub address space:

The /16 hub block plus the /20 remote block leaves ~50,000 addresses of headroom for future expansion without re-numbering. The 14-usable-hosts-per-/28 is enough for a TIM, an S7 CP, a SCALANCE S, an MD741-1, and a few spare addresses for engineering access.

Commissioning, Verification, and Troubleshooting

Commissioning Procedure

1. Verify the central TIM 4R-IE count satisfies ceil(N / 64) = 3 for N = 145 stations. Pre-install the Telecontrol Professional license for three TIM 4R-IE connections.
2. Provision the DSL line(s) and assign static public IPs to the central SCALANCE S6xx endpoints (or to the DSL router that fronts them).
3. Configure the SCALANCE S6xx IPsec tunnel table. Do not exceed 128 tunnels per S6xx. With 145 stations, plan a 73/72 split across two S6xx appliances on two DSL lines.
4. Install TIM 3V-IE Advance (or SCALANCE M + MD741-1) at each remote station. Pre-configure the SIM in the MD741-1 or MD720-3 and verify the bearer is up before mounting on site.
5. Program the MSC partner table in each central TIM 4R-IE. Verify the partner count does not exceed 128 per TIM. For 3 TIMs covering 145 stations, the per-TIM partner count is approximately 49.
6. On each central TIM 4R-IE, confirm the active-connection count stays ≤ 64. Use TIM diagnostics page Diagnostics > Connections or TIA Portal online diagnostics.
7. If stations use S7-300 / S7-400 PLCs behind a CP (e.g., CP 343-1 Lean on an S7-300 station), reload the PLC after changing the default gateway in HW Config so the CP picks up the new default route to the central TIM. Without this reload, the buffered events at the station will not flush even after the link is restored.
8. From the Telecontrol Professional PC, force a test frame on each partner. Verify it lands in the Telecontrol buffer and is then forwarded to the SCADA layer above. Confirm time-stamping is consistent across all three TIM 4R-IE modules.
9. Pull the DSL line on one S6xx and confirm the second S6xx continues serving its 72-73 stations with no loss of buffered events. The TIM at the station buffers during the link outage and flushes on link restoration in correct sequence; the SCADA layer should not see a gap.
10. Run a 24-hour soak test with all 145 stations online. Capture peak active-connection count on each TIM 4R-IE, peak IPsec tunnel count on each SCALANCE S6xx, and GPRS link-up / link-down counters on each MD720-3 or MD741-1.

Verification and Acceptance

Acceptance is signed off only when all checks pass over a continuous 24-hour window with all 145 stations simultaneously online and a synthetic event storm applied at hour 12 (e.g., 50 stations report a digital event within 60 seconds).

Troubleshooting Matrix

Capacity Planning: Throughput, Latency, SIM, and APN

For planning purposes, GPRS class 8 with a single timeslot upstream delivers a theoretical maximum of ~13.4 kbps per station, with typical application throughput in the 8-10 kbps range after RLC/MAC overhead. EGPRS (class 12, 4-up/4-down with EDGE modulation) raises the practical upstream ceiling to ~60-80 kbps. The round-trip poll cycle of 15 s quoted for typical 1 KB S7 polls fits comfortably in both bearers, but the moment a fleet exceeds ~80 stations, aggregate GPRS backhaul on the central TIM can saturate a single DSL uplink. Plan for:

• Throughput_station ≈ 1 KB / 15 s ≈ 0.53 kbps average
• Throughput_fleet ≈ N × Throughput_station
• For N = 145, fleet average ≈ 77 kbps, well within a single DSL line, but burst peaks during event storms can push to 4-6× the average.

Latency budget for a single poll (one station):

The PDP (Packet Data Protocol) context attach time is the long pole in the latency budget. PDP contexts on idle stations time out at the SGSN after a provider-configured idle period (commonly 30 minutes to 2 hours). When the station needs to send data after idle, it must re-attach, which can take 1.5-3 s. To avoid this, configure the MD720-3 / MD741-1 to send periodic keep-alive frames (e.g., one frame every 10 minutes) so the PDP context stays hot.

SIM and APN selection determines the addressing and routing available to each station:

For Pattern C with SCALANCE S6xx IPsec tunnels, the remote stations must accept inbound IKE packets. That requires either a public fixed IP on the SIM or a private APN with inbound allowed through the carrier's network. Confirm inbound IPsec is permitted by the carrier before commissioning — many M2M APN contracts are outbound-only and silently drop inbound IKE packets.

When to Escalate to Siemens Support

For a 145-node, multi-cluster GPRS telecontrol deployment, the sizing rules above are necessary but not sufficient. Specific firmware versions of TIM 4R-IE and SCALANCE S6xx can move the connection and tunnel ceilings. For example, SCALANCE S6xx V3 lifted the DynDNS client and tunnel-count limits relative to V2, but the 128-tunnel ceiling remained. Open a Siemens Technical Support request through the Siemens Industry Online Support portal and request:

• Confirmation of the exact connection / partner ceiling for the firmware version you plan to deploy on the TIM 4R-IE.
• Validation of the 73/72 split across two SCALANCE S6xx on two DSL lines for 145 stations.
• Confirmation of the Telecontrol Professional license model for three central TIM 4R-IE modules.
• Review of the project-specific IP plan to confirm no subnet collisions will block 1:1 NAT at the SCALANCE S6xx.

FAQ