Link aggregation in microwave links

Link aggregation in microwave links

Link aggregation, load balancing, link bonding are computer networking umbrella terms to describe various methods of bundling (aggregating) parallel network connections into a single logical link. It allows the increase of throughput beyond what a single connection could sustain, as well as provides redundancy in case one of the links fails.

Microwave links are limited in their throughput capabilities by many factors, such as antenna sizes, highest achievable modulation, maximum radio bandwidth, etc. Therefore microwave point-to-point (PtP) links generally provide smaller bitrate compared to the regular guided 1Gbps link rate (1000BASE-T, 1000BASE-SX or 1000BASE-LX). In microwave point-to-point links, aggregation by means of two or more parallel links allows enhancing the overall microwave link throughput. Such an approach is made at the expense of additional bandwidth on the RF spectrum, as well extra equipment (e.g. additional pair of radios, couplers or OMTs, or antennas). Designations “2+0”, “3+0”, etc. are commonly used for aggregation/protection scenarios; where the first character (before “+”) stands for number of parallel links, and the second character stands for the number of redundant links (used in protection configurations). In purely aggregation scenarios second character is commonly “0”.

A majority of modern microwave radios imply built-in Layer 1 or Layer 2 aggregation capabilities mostly for 2+0 scenarios. If XPIC technology is supported, a single RF channel may be utilized for two parallel wireless links, by means of parallel transmission on vertical and horizontal polarizations independently. However, some microwave radios may not have built-in aggregation functionality or it may provide several limitations for the network designer. Thus aggregation implemented on external equipment is also widely used in the combination with PtP microwave links.

Types of aggregation

Aggregation can be implemented at one of the lowest three layers of the OSI model:

Link bonding – Layer 1 aggregation, which is based on frame or bit aggregation. From the network point of view, link bonding is a single physical channel. Link bonding is Layer 1 aggregation of frames, which is delivered as a single physical link to Layer 2. This aggregation type is most efficient in terms of load distribution between aggregated links. Unlike Layer 2 aggregation, Link Bonding does not require diversity in MAC-MAC (or IP-IP) connections.

Link aggregation – Layer 2 aggregation, which is based on the IEEE 802.3ad standard. Link aggregation control protocol (LACP) is the most popular Layer 2 aggregation protocol. Load distribution is achieved by distributing different connections between available aggregated links by means of MAC-MAC address hashing. Some implementations also allow Layer 3 and Layer 4 hashing (e.g. IP address. TCP/UDP ports). Layer 2 aggregation may bring along some challenges, which will be discussed in the second chapter of this whitepaper.

Load balancing - Traffic segmentation based on pre-defined traffic path configuration, usually implemented in routers or Layer 3 switches. Traffic segmentation can be defined by VLANs, IP addresses, ports, etc, depending on options in the device. Load balancing may also be performed by dynamic routing protocols (e.g. OSPF).

The implementation of built-in and external aggregation have their pros and cons:

Built-in aggregation for microwave radio links is the recommended approach. This scenario does not require complex external equipment and its advanced configuration; configuration is relatively easy and well documented in the manual; allows using XPIC technology in some radio models; finally, the built-in link aggregation engine gets direct feedback about the wireless interface status and is capable of adopting load balancing to the unstable wireless link condition (e.g. frequent modulation downshifting). However, built-in aggregation on microwave radios often has very limited number of parallel links (usually two parallel links), lacks configuration flexibility and may bring some challenges to integration into the existing network.

Figure 1 – Built-in aggregation with two parallel microwave links

In turn, aggregation running on external switches allows a higher number of parallel aggregated links (LACP allows up to 8 parallel links), allows more flexible configuration, and may ensure easier integration into the existing network, because all complex functionality is shifted to the CPE side. Moreover, it is a good solution for the radios that do not employ advanced Ethernet settings, including built-in aggregation possibility. On the other side, it lacks advantages of the built-in aggregation and requires out-of-band management implementation, hence the radio management traffic and the aggregated user traffic must run on two different cables. Within the radio unit out-of- band management may be implemented by means of VLANs.

Figure 2 – External aggregation with two parallel microwave links

SAF Tehnika offers different kinds of equipment with Layer 1 or Layer 2 built-in aggregation functionality, as well external aggregation capabilities. Please find the aggregation possibilities for different types of SAF Tehnika equipment in the table below:

* - In most cases, it is not used in Layer 2, 3 switches or routers, however, it is possible.
** - Only if out-of-band management firmware is used.

Link aggregation techniques are a good way to overcome capacity limitations in the microwave links brought by factors like antennas sizes, maximal modulation, channel bandwidth, etc. In the second chapter of this whitepaper, several link aggregation challenges in the microwave links will be discussed.