TYPICAL BACKHAUL SCENARIOS
Scenario 1 Protocol Stack – End-End Carrier Ethernet
The protocol stack for this scenario assumes that the provider’s Carrier Ethernet service operates between the MASG, which is co-located with the operator’s core network, and the CSG, which is co-located with the cell site. This model does not illustrate scenarios in which an operator provides their own access backhaul links to an aggregation site from where it passes traffic to the provider via a CSG.
Provider S-tags are therefore pushed/popped at the CSG and MASG and the provider’s services operates end-end between those devices.
Connection resilience options on backhaul links in this scenario include G.8031/G.8032, LAG (Link Aggregation Groups) and STP. LAG provides redundancy by allowing multiple Ethernet physical links to be ‘bound’ into a Link Aggregation Group. If a link in a LAG fails then its functions can be taken over by another bound member of the group.
LTE Backhaul
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NGMN Scenario 2: Carrier Ethernet + L2/L3 VPN
In this scenario Carrier Ethernet is used across the access part of the link and MPLS is employed in the Metro/
Aggregation portion. An E-LAN or E-Line virtual connection is created and travels over Ethernet or WDM bearers between the CSG and the 2nd Aggregation point whilst an MPLS L2 or L3 VPN is created between there and the MASG.
S1 backhaul interfaces would be switched all the way through to the EPC using customer-specified VLANs. Inter-eNB X2 interfaces exist as separate EVCs that must be switched as far as the 2nd Aggregation point (the end of the Carrier Ethernet EVC) before being forwarded to the target eNB.
Typical Backhaul Scenarios
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Scenario 2a Protocol Stack – L2 VPN
The protocol stack for this scenario assumes that the provider’s Carrier Ethernet service operates between the CSG, which is co-located with the cell site, and the 2nd Aggregation point, which would be located at a Transmission High Site or other aggregation node. An MPLS service is employed to support the metro portion of the network and a VPN will be created (at either Layer 2) to carry a pseudowire to serve the connection.
Carrier Ethernet provider S-tags are therefore pushed/popped at the CSG and the 2nd Aggregation point. Ethernet frames are switched onto a pseudowire at the 2nd Aggregation point and the MASG.
Connection resilience options on access backhaul links in this scenario include G.8031/G.8032 and LAG and STP, on the second mile link to the 2nd Aggregation point these are joined by STP.
A VSI is employed to forward traffic across the appropriate MPLS LSP. A VSI is a virtualized instance of an Ethernet switch and operates by learning the MAC addresses available via each connected pseudowire; the VSI will therefore determine which MPLS PW to forward traffic through based on the destination MAC address carried in each frame.
Bridges are used to forward traffic at the Ethernet layer in any intermediate metro network nodes. The MPLS provider will create an LSP between the 2nd Aggregation point and the MASG to carry the end-end backhaul connection.
Control protocol options, which allow for the automated discovery and administration of MPLS nodes, for the MPLS metro portion of the network include BGP, IGP (an Interior Gateway Protocol of some sort, such as OSPF or RIP) and RSVP-TE (Resource Reservation Protocol - Traffic Engineering). Network service options, which provide the means to automatically create MPLS paths, include T-LDP (Targeted Label Distribution Protocol).
End-End connectivity is provided by a concatenation of a Carrier Ethernet EVC and an MPLS LSP.
LTE Backhaul
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Scenario 2b Protocol Stack – L3 VPN
The protocol stack for this scenario assumes that the provider’s Carrier Ethernet service operates between the CSG, which is co-located with the cell site, and the 2nd Aggregation point, which would be located at a Transmission High Site or other aggregation node. An MPLS service is employed to support the metro portion of the network and a VPN will be created (at Layer 3) that travels over an LSP.
Carrier Ethernet provider S-tags are therefore pushed/popped at the CSG and the 2nd Aggregation point. Ethernet frames are routed across an LSP between the 2nd Aggregation point and the MASG.
Connection resilience options on access backhaul links in this scenario include G.8031/G.8032 and LAG and STP, on the second mile link to the 2nd Aggregation point these are joined by STP.
A VRF is employed to route traffic across the appropriate MPLS LSP. A VRF is a virtualized instance of an IP Router, a router running VRFs can create multiple route tables, each specific to a particular VRF and each dealing with its own specific subset of router traffic. In this scenario, a separate VRF would be created by the MPLS provider to support each customer, allowing them each to receive an individually configured service over the common resources of the MPLS network. The MPLS provider will create an LSP between the 2nd Aggregation point and the MASG to carry the end-end backhaul connection.
Control protocol options, which allow for the automated discovery and administration of MPLS nodes, for the MPLS metro portion of the network include BGP, IGP, such as OSPF or RIP, and RSVP-TE. Network service options, which provide the means to automatically create MPLS paths, include MP-BGP (Multi Protocol BGP).
End-End connectivity is provided by a concatenation of a Carrier Ethernet EVC and an MPLS LSP.
Typical Backhaul Scenarios
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Backhaul Example – Virgin Media
Virgin Media are supplying cellular backhaul services via a Sync-E based aggregation network solution in which local aggregation sites are connected to the cellular core network via a Carrier Ethernet EVPL E-Line service.
Based on information published on the Virgin Media Business site that describes their Sync-E cellular backhaul service and on some assumptions, the likely configuration of the backhaul service is as shown in the diagram, although this will be subject to correction for individual operators’ solutions.
LTE Backhaul
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Protocol Stack – via operator-owned microwave
Many networks have traditionally used their own operator-owned microwave network for first stage access backhaul.
If this arrangement were to be retained for LTE backhaul via Virgin Media then the first mile access link can be expected to be Carrier Ethernet over dual TDM/GigE Ethernet microwave. The CSG could therefore be expected to be deployed at a THS (Transmission High Site) or other aggregation point and provide connectivity into the Virgin Media backhaul network.
The Virgin Media backhaul aggregation network is based on Carrier Ethernet.
End-End connectivity between eNBs and EPC elements could be based on creating dedicated customer VLANs to serve individual sites or clusters of sites. Ethernet frames travelling between eNBs and the EPC will therefore be Dot 1q tagged with the ID of the appropriate customer VLAN. Virgin Media will push provider S-Tags into each frame at the MASG and pop them at the CSG (and vice versa) to facilitate transport across the aggregation network.
Typical Backhaul Scenarios
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Protocol Stack – via VM-supplied GigE Tail Circuit
An alternative to the use of operator-owned microwave backhaul would be the provision of fibre-based GigE tail circuits to selected cell sites by an access provider.
An example of the possible configuration of protocol stacks for cell sites served by this backhaul model is shown in the diagram.
It has been assumed in this example that the fibre tail circuit links will be used in the same way as the operator-owned microwave links and Ethernet frames will only be tagged with C-Tags at as they travel over the tail circuit. Other configurations, in which the S-tag is retained over the tail circuit all the way to the cell site are also possible.
LTE Backhaul
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Sync Distribution
The distribution of synchronization in the Virgin Media backhaul model is based on Sync-E.
Operator’s core networks can be expected to continue to use their existing sync distribution methods and will supply sync feeds to the core network gateway.
Sync is distributed through the Virgin Media backhaul aggregation network by Sync-E on inter-switch links.
Sync-E can also be used to distribute sync directly to cell sites over VM-supplied fibre tail circuits.
Sync-E distribution via microwave was not initially supported by most microwave equipment vendors, so in early implementations operator-owned microwave links used the TDM portion of their signals to carry a standard E1 sync signal, which could then be supplied to cell site equipment in the usual manner.
As Sync-E does not provide Time of Day references, it is expected that individual base stations will continue to require connectivity to an NTP server for periodic ToD fixes. If phase synchronization ever becomes a requirement (as it is in LTE-TDD and SFN networks) it is expected that an IEEE 1588v2/PTP system may need to be deployed, if such a system isn’t already in use.