As talked about in the previous post the difference between a wired LAN and a Wireless Local Area Networks (WLANs) is that the Layer-1 transmission medium of a traditional wired local area network (LAN) (CAT-5 cable) is replaced with Radio Frequency (RF) transmissions.
What follows is the pimping of Cisco Aironet products and where they fit into three main wireless categories:
Wireless in-building LANs for client Access: The Cisco Aironet products can plug into an existing wired infrastructure and function like an overlay to the existing LAN or even replace the wired LAN.
Wireless Building-to-Building bridges: The Cisco Aironet products can provide wireless bridging to connect two or more networks that are physically separated to be connected on one LAN without the time or expense required to get physical lines to be installed.
Wireless mesh networks: Mesh networking is a mixture of the above two categories. Mesh networking provide dynamic, redundant, fault-tolerant links for building and client access.
Service Set Identifier (SSID)
Myth: Hidden (not broadcasting) the SSID makes a wireless network secure.
The SSID is the “name” of a wireless cell, this name is used to logically separate WLANs. The SSID must match exactly between the client and the access point for them to connect. The Access Point (AP) sends the SSID out in beacons.
The beacons are broadcasts that an AP sends to advertise available services, these beacons go out whether SSID is hidden or not (Clients can be configured without a SSID, where they learn the SSID from the beacons of the AP).
The Topology Basic
Extended Services Set: Two or more Basic Serve sets (Mobile clients use a single AP to connect) are connected by a common distribution system (backbone) An Extended Service Set includes a common SSID to allow roaming from AP to AP without client config.
The diagram shows the WLAN topology with 2 APs and some devices (Microsoft Icons) that I know to be Wi-Fi capable (from left to right tablet notebook, projector, PDA, smartphone, notebook).
Wireless Cell: The basic area is the RF coverage provided by an AP (Channel 1 or Channel 2 NOT both). This area is also called the “microcell“. To extend/enlarge/make bigger the basic area one simply adds APs (Recently microcell has moved to picocell reducing AP coverage by reducing power and increasing total number of AP deployed).
The basic area of an AP is called the service set, the basic area of the combined APs is called the extended services set (There is a recommended 10 – 15 % overlap between cells for data networks to allow roaming without losing RF connection. There is a 15 – 20% overlap for voice/data/video networks). Bordering cells should be set to different non-overlapping channels for best performance (more on this later).
Access Point: The name is self explanatory reverse the name Point “of” Access. As the name denotes this is the point at which client-devices connect/access the wireless network. The APs connect to then to the Ethernet backbone and facilitate the communication between wired and wireless networks
The AP is the master of a given cell and manages/controls traffic to and from the network (remote devices do not communicate with each other they communicate through the AP).
Picocell: the benefit of a picocell is better coverage, less interference, higher data rates, and fault tolerance through convergence. When an AP goes down, the neighbouring AP expands coverage by increasing power (this increases the RF range) to cover for the lost AP. (Look into WLAN Controllers cause this gets complicated to do manually quickly with say more than 5 APs)
Wireless Repeater
In environments (factory floors, doctors room, large retail, wholesale storehouses) where its just not practical to put down a wired LAN or the application of the network wouldn’t work with a wired system a wireless repeater can be put down.
A wireless repeater is a AP that is not connected to the Wired LAN (Requires 50% overlap of the AP on the Wired LAN side). This setup however has a large throughput impact where throughput is decreased by half due to the receive and retransmit time.
The SSID of the AP (the one on the left) must be configured on the wireless repeater (the one on the right). The wireless repeater uses the same channel as the AP (NB not all implementations support this).
Workgroup Bridge
Cisco Wireless Workgroup Bridge (WGB) (Reference Cisco Q&A Document) that connects to the Ethernet (RJ-45) port of any end-device (if it has a Ethernet port and is therefore network-able) that doesn’t have a WLAN Network Interface Card (NIC) (either because the end-device doesn’t have the option of a Peripheral Component Interconnect (PCI) slot, Personal Computer Memory Card International Association (PCMCIA) slot or USB slot, or software for WLAN connectivity).
A WGB provides a single MAC address connection into an AP and in turn then onto the Wired LAN backbone (The WGB cannot work in peer-to-peer mode). Another option is to connect a remote workgroups wired LAN. To implement a remote workgroup installation (i.e. multiple MAC addresses) the WGB is connected to a hub/switch switch with a Ethernet patch cable (for single MAC Address use a crossover cable) (NB not all implementations support this).
Ad-hoc mode
Ad-Hoc Mode: This is called Independant Basic Service Set (IBSS). Mobile clients connect directly without an AP.
Peer-to-Peer (P2P) a.k.a Ad-hoc mode networking is the opposite of a Server-Client model (duh). This can be in a wired or wireless environment and is where a group of end-devices come together and form an ad-hoc/P2P network with each other to share files, pictures, music, movies and applications (The ease and current application (Kazaa and Torrents) of this type of network is the main reason the RIAA hates ad-hoc/P2P networks).
In a WLAN the coverage is very limited; where all users must be in wireless reception distance of each other. There are a couple of problems with P2P “office” networks one being that security is almost non-existent, other problems being that there is no central location for any files, applications, or printing.
In most P2P environments I have found that the receptionist is given the “server-role” Pc which creates other larger problems. The person at the front desk in a company is the receptionist, in case of a theft the first computer out the door is the server. In most cases the most “spam” is received by a receptionist (classing teddy-bears, hearts and hugs, chain-mail, friend-mail, etc. as spam) being on numerous forwarding lists increases the risk of virus, trojan, worm infection. If the company allows internet access to employees its only a matter of time before the “server” begins doing its own thing.
In a WLAN it is not a good idea (iow just don’t do it) to connect a Server, or a Server-Role computer using Wireless
The roaming “feature” on wireless allows a mobile user to move from one cell to another without a drop in signal or need to manually change network settings. Roaming is enabled by complete coverage with wireless cells.
- Seamless roaming allows for users to move around from one cell to another.
- Power management lengthens the battery life of portable devices (i.e. they don’t have to search for wireless networks all the time)
- Dynamic Load Balancing distributes users among access points to increase throughput for each user.
- AP with overlapping coverage cells and redundant switches provide fault tolerant WLAN networks.
A user experiences “roaming” when one of the following conditions is met:
- The maximum data retry count is exceeded.
- The client has missed too many beacons from the access point.
- The client has reduced the data rate.
- The client intends to search for a new AP at periodic intervals.
Roaming without service interruption requires identical SSIDs, VLANs and IP subnets on all APs. The client initiates the roaming when he/she searches for another AP with the same SSID and then sends a re-authentication request (for voice and video short roaming times are important).
Roaming from one AP to another AP on the same subnet (Cell 1 to Cell 2) would be considered Layer-2 roaming (data link layer). Roaming between APs that reside on different subnets (Cell 1 to Cell3) would be considered Layer-3 roaming (network layer).
Layer-2 roaming is managed by the AP, using mulicast packets that inform switches that a devices has moved. The protocol between the APs is called Inter-Access Point Protocol (IAPP).
Layer-3 roaming is managed by either Mobile IP or Lightweight Access Point Protocol (LWAPP) with a WLAN controller.
Mobile IP: allows fixed IP addresses in an IP Subnet of a network. It relies on devices like routers (home agents and foreign agents), to runel traffic for a mobile device. This was used in Legacy WLANs.
Wireless VLAN Support
Switches use VLANs to separate traffic. WLAN APs can in turn extend the VLANs by mapping VLANs to SSIDs. The VLANs then share the same wireless cell and channel end result being virtualization of the AP.
Through the use of trunking (ISL or 802.1q) the VLANs can be mapped to APs from a/the switch allowing roaming throughout the enterprise. A Cisco Aironet AP can be configured with 8 – 16 VLANs for system design flexibility. (Some client NICs require SSID broadcast, the AP can be configured for SSID broadcast per VLAN).
Wireless Enterprise (read business) Voice Architecture
Wired LAN Voice (IP Phone) networks can be extended using the 802.11e standard that specifies QoS upstream and downstram for WLAN networks. This is very important because of the delay sensitive nature of voice.
Wireless Mes Networks
A Mesh network infrastructure is decentralized and inexpensive because each node needs to transmit only as far as the next node (WirelessAfrica). The nodes act as repeaters to transmit data from nearby nodes to peers that are too far away to reach. The result is a network that can span a large area (cost effectively if each node is owned by individuals).
Mesh Networks are reliable because each node connects to several other nodes. Wireless Mesh networks differ from conventional infrastructure wireless networks in that only a subset of nodes need to be directly connected to the wired network. Extra capacity can be added by installing more nodes. Through the use of Cisco Adaptive Wireless Path Protocol (AWP(P)) each device can find a way back to wired APs and thus by extension the network. Paths (of which there are multiple) through the network can change in response traffic load, radio conditions, or traffic prioritization. The network can cover more distance by using wireless to wireless connectivity. Unlicensed bandwidth (cheap) and wireless routing allow microcells to interconnect over wireless backhaul links.
AWP Protocol
AWP allows APs to communicate with each other to determine the best path back to the wired network. After optimal path selection is estalbished, AWP continues to run as a background service to establish alternate paths to the wired network or if topology changes or other conditions causes the link streghth to diminish. (AWP runs on each AP)
AWP is a wireless protocol by design and takes into consideration wireless radio factors like interference to make a mesh network self-configuring and self-healing. Because wireless is dynamic, addition to the network causes AWP to reconfigure paths back to the wired network automatically. AWP also uses stickiness to mitigate route flaps (disconnection/temporary disruption doesnt cause mesh change).
Notes and Notices:
This is a part of my personal BCMSN notes and research to assist myself in learning and understanding the concepts and theory for the BCMSN exam. I learn by making notes reading and writing things down and wish to file them where I can’t lose them. These notes are not to be seen, judged or mistaken for replacements to Cisco recognized and authorized training which I personally support and attend and suggest you undertake if you are going for the BCMSN Certification.