Understanding UNI (User Network Interface)

Dear Friends, 

The year 2018 is coming to a close and this is the right time to write something on request of somebody. I had provided an article explaining Provider Bridge and Provider Edge concept where I touch-based upon the aspects of bridging and tunneling. Well I believe the article needed a prequel and that was to understand the different interface types that we have in the data networking. 

The networking that we do for data services are very dynamic and there can be several combinations of port-types and service types with domains realizing one important aspect and that is to carry the traffic from one point to another with reliability, security and prioritization. 

There are two kinds of interfaces that we have

UNI ----- User Network Interface

NNI ------ Network Network Interface. 

Today my post will concentrate only on the UNI aspect as I do not want to make this a very lengthy post. People may get grossly bored to read a lengthy post as for one I have noticed, the new generation lacks some kind of patience. 

What is UNI? 

In a very short and crisp definition we can say that UNI is the interface that connects a customer to the network. UNI port is an entity of the provider, however it interacts with a customer port or an interface. Please note that I have mentioned the terms port and interface separately. The reason being that a particular physical port may carry several logical interfaces that may be of UNI nature. 

Figure-1 Representation of a UNI

The figure above visualizes the definition of UNI. 

However, the concept of UNI is deep and needs more elaboration. For this we need to understand the concept of VLANs. A VLAN is a concept where we can break the broadcast domains to different smaller domains without involving the routing. You must be knowing that a switch breaks collision domains and the router breaks broadcast domains. However, what can be done in order to break a broadcast domain within a switching network without involving routers is to use VLAN. 

The VLAN gives a kind of identification to the packet or frame that is coming to the switching device as to which service that is should follow. VLANs are of 4 bytes and that are appended to a standard 802.3 frame. 

We see the structure of VLAN below. Disected. 

Figure-2 Dissection of the structure of VLAN

The VLAN is appended in the frame and it is providing the identification of the frame to follow a service. Here the main important thing is to see how the VLAN is appended in the frame. The frame can be untagged (without a Vlan) or it can be tagged (with a VLAN ID). Basis on this we have the two different types of interfaces of UNI. 

Figure-3: Types of UNI port

The types of UNI are the the basis of how the VLAN is treated in the service and the traffic is mapped. This is the thing that we will see in detail below. 

1. Access UNI port:

Imagine a situation where there are three customers who are all sending untagged frames. However the WAN which is carrying the traffic is a common WAN. How are we going to segregate this traffic. There is one way to do it by leaving it on the Mac Learning and bridging concept. The concept over here is that every device will have a different MAC and based on the MAC filter there will be communication and we will have one common LAN. However, this aspect has some disadvantages. 

Figure-4: Situation where three customers carry different traffic

> What happens in the unlearned state? 

> How will the traffic be treated? 

> What happens if one customer is sending a broadcast? This will affect the performance of all other customers. 

However, we cannot impose the condition on the customer to tag these frames. If we do that we are putting conditions on the customer and this way we will have to put similar conditions on all other customers, which is not a good idea. 

Here the thing is to have an access UNI port. 

The access UNI port accepts an untagged port on the LAN and then appends the VLAN on the frame of customer and then forwards it to the WAN. 

The flow is explained in the figure below. 

Figure 5: How the access VLAN UNI works

Here you see that the customer 1 sends the untagged frame and then it reaches the access port. Here the access port adds a VLAN ID to the frame and this travels through the WAN. On the other side when the frame is coming out the VLAN is matched and stripped and the original frame is received by the end. 

Points to note: 

1. Access VLAN ports in the UNI append VLAN in the ingress and match and strip the VLAN in teh egress. 

2. This action is like attaching a unique envelope to the untagged frame and taking it out on the other end without distorting any information. 

In our case we can have the following scenario for our three customers. 

Figure-6: How the three customers will be segregated

Here we see that every customer is connected to a different access port and so we put the access VLAN accordingly to identify the customer. Also note that the customer 2 may have a different drop point than customer 1 so this has to be organized in that way. 

2. Trunk Port: 

Once we understand the Access VLAN port functioning understanding trunk port is very easy. The trunk port in this case is our WAN. The WAN port is not adding or stripping any VLAN however it does a validation of VLAN that it has to pass. In our case we will put a validation that the trunk link should only pass VLAN 100, 200 and 300. 

The algorithm is the trunk port accepts frames that are already tagged and then matches them with the validation. Based on that it forwards. 

So friends, this is about the UNI port. We shall see the functioning of the NNI port in a later post. Understanding the UNI interface is very important in order to do selection of interfaces in data traffic planning. Therefore, be careful in deciding the same. 

Till then have a fantastic New Years' Eve.... 

Cheers and a Happy New Year, 

Kalyan