Architecture · Topic 2 of 6
Transport Technologies, Mobility and Routed Optical Networks
Describe xDSL, DWDM, DOCSIS, TDM and xPON transport, the 5G packet core, RAN xhaul for vRAN and ORAN, and routed optical networking with coherent optics.
- 5 min
- xDSL uses copper phone lines and ends at a DSLAM.
- DOCSIS runs over coax, with a CMTS or CCAP at the provider and a cable modem at the customer.
- xPON uses passive splitters, with an OLT at the provider and an ONT at the customer.
What it is
Providers reach customers and carry traffic over several different physical technologies, and the core has to join them.
Some technologies serve homes and businesses at the edge. Others, such as DWDM, move huge volumes over long fibre routes. Mobile networks add their own radio and core elements on top.
You need to describe what each technology is and where it sits, not to configure it.
How it works
Each technology has a customer side device and a provider side device.
Access technologies
| Technology | Medium | Provider side | Customer side |
|---|---|---|---|
| xDSL | Copper telephone line | DSLAM | DSL modem |
| DOCSIS | Coaxial cable in a hybrid fibre coax network | CMTS or CCAP | Cable modem |
| xPON | Fibre with passive splitters | OLT | ONT or ONU |
| TDM | Dedicated time slots, such as E1, T1 or SONET and SDH | Multiplexer | Multiplexer or router |
xDSL is a family of standards. ADSL gives more downstream than upstream speed, and VDSL2 is faster but works over shorter loops.
DOCSIS is the standard for data over cable television plant. DOCSIS 3.0 introduced channel bonding, and DOCSIS 3.1 added OFDM. Remote PHY moves the physical layer closer to the customer, into a node in the field.
xPON shares one fibre among many customers through passive optical splitters. Downstream traffic is broadcast to all ONTs.
Upstream traffic uses time slots so that ONTs do not collide. GPON and XGS-PON are common variants, and EPON is the Ethernet based family.
TDM gives each circuit fixed time slots. An E1 has 32 time slots and a T1 has 24. Legacy circuits can cross a packet core with circuit emulation, which wraps the bits in a pseudowire.
DWDM
Dense wavelength division multiplexing (DWDM) puts many wavelengths of light on one fibre. Each wavelength carries its own signal, so a single pair of fibres can carry many channels.
Wavelengths sit on an ITU grid with fixed channel spacing, for example 50 GHz or 100 GHz. Optical amplifiers boost the signal along the route, and a ROADM adds or drops wavelengths at a site without converting the signal to electrical form.
Mobility: packet core and RAN
A mobile network has a radio access network (RAN) and a packet core. In 4G the core is the EPC. In 5G the core is the 5G core, which is cloud native and built from services such as AMF, SMF and UPF.
The RAN has three parts: the radio unit (RU), the distributed unit (DU) and the centralised unit (CU). The transport between them has names:
| Segment | Between | Name |
|---|---|---|
| Fronthaul | RU and DU | Very strict latency and timing |
| Midhaul | DU and CU | Less strict |
| Backhaul | CU and packet core | Typical IP and MPLS transport |
Xhaul is the term for one converged transport network that carries all three. A router based xhaul often uses MPLS or SR with Ethernet, and it must also deliver timing, usually through PTP and SyncE.
vRAN runs the DU and CU functions as software on standard servers. ORAN (Open RAN) defines open interfaces between RU, DU and CU, so the parts can come from different vendors.
Routed optical network
A routed optical network moves the optics into the router. Coherent pluggable optics, such as 400G ZR and ZR+, are fitted into routers, so no separate transponder shelf is needed.
The IP layer and the optical layer become one network. This reduces equipment, power and the number of devices to manage. Optical health can be read on the router.
RP/0/RP0/CPU0:R1# show controllers optics 0/0/0/10That is an IOS XR command, and the port address is an example. It shows power levels and alarms for the optic.
What to remember for the exam
Seven facts cover most questions on this topic.
- xDSL uses copper phone lines and ends at a DSLAM.
- DOCSIS runs over coax, with a CMTS or CCAP at the provider and a cable modem at the customer.
- xPON uses passive splitters, with an OLT at the provider and an ONT at the customer.
- DWDM carries many wavelengths on one fibre, and a ROADM adds or drops them.
- Fronthaul links RU and DU, midhaul links DU and CU, and backhaul links CU and the core.
- vRAN virtualises DU and CU, and ORAN adds open interfaces between the RAN parts.
- A routed optical network puts coherent pluggable optics into routers.
Match each technology to its provider side device: DSLAM, CMTS, OLT. This pairing is a common question.
Common mistakes
These three mistakes appear often.
Mixing up fronthaul and backhaul. Fronthaul is the radio side, between RU and DU. Backhaul is the link towards the packet core.
Assuming xPON is active. The splitters are passive and need no power. The OLT and the ONTs are the active devices.
Treating vRAN and ORAN as the same thing. vRAN is about running functions as software. ORAN is about open interfaces between vendors.
Timing matters as much as bandwidth in mobile transport, so expect PTP and SyncE to appear next to fronthaul.
Check yourself
Work through these three questions.
Which device sits at the provider end of an xPON network?
The OLT sits at the provider end, and the ONT or ONU is at the customer.
What does the term xhaul mean in 5G transport?
It is one converged transport network that carries fronthaul, midhaul and backhaul.
What changes in a routed optical network?
Coherent pluggable optics, such as 400G ZR and ZR+, are fitted into the routers, so a separate transponder is not needed.
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