The DSL family

Which type of DSL transfers data at a high rate of speed over short distances?

1. DSL
   
2. ADSL
   
3. IDSL
   
4. VDSL
   

The answer is simple - VDSL, or Very High Speed Digital Subscriber Line.

DSL is the generic name for a family of technologies that aim to offer high speed data communication over conventional phone lines (that is the 'subscriber line' part of the acronym). Each kind of DSL offers a distinct compromise, and is suited to a particular scenario.

ADSL is the most common DSL variant. The 'A' stands for asymmetrical, which means that it works with different data rates for each direction. Usually, the download speed (from the carrier's premises to the customer location) is higher than the upload speed (that is, in the reverse direction). This asymmetry is not gratuitous, but is a consequence of the physical properties of the subscriber line, in particular the presence of crosstalk, which is the interference between adjacent pairs in the same cable. The cable starts at the C.O. as a big bundle, with hundreds of pairs; as it gets near to the customer, the bundle is split, until a single pair is left which is connect to the customer equipment. As the signal leaves the carrier's premises, the downstream signal is still strong enough to overcome the noise. But in the reverse direction -- from the customer to the carrier -- the opposite happens; the signal is attenuated over the distance, and it reaches the C.O much weaker. At this point there are hundreds of different signals in the cable, which leads to severe interference. In practice, this means that the downstream is able to work with speeds ranging from 256 kbps to a few megabits per second, while the upstream channel operates at a fraction of that speed (two to ten times slower, depending on the particular scenario).

IDSL is a variant of DSL that works using standard ISDN signaling, at 144 kbps. It's simple math: a standard ISDN line has two 64 kbps data channels (called 'B channels') and one 16 kbps channel (called 'D channel'). Sum the numbers, and you'll have 144 kbps. In practice, the main difference between ISDN and IDSL is the pricing; a flat rate for IDSL, and usage-based metering (per minute or per call) for ISDN. Due to its competitive price, IDSL was relatively popular in the beginning of the broadband access evolution but has been now widely superceded by ADSL.

VDSL was originally a generic name for several different technologies (many of them proprietary) which were developed to allow transmission rates of more than 8 Mbps over relatively short distances. In 2004, the VDSL standard was ratified as the Recommendation G.993.1, allowing speeds of up to 52 Mbps over 1500 m using conventional phone lines. More recently, ITU-T ratified the VDSL2 standard (G.993.2), with speeds of up to 100 Mbps over a few hundred meters. Many of the initial VDSL experiments were targeted at video on demand services (that's why some people used to refer to it as 'Video DSL' in its early days). Although much hyped since the late 90's, only now the technology is gaining traction, with countries such as South Korea leading the way.

IP Multicast & Ethernet MAC addressing

Which address is a valid Ethernet MAC address used for IP multicast?

1. 0100.5f80.001c
   
2. 0100.5e1b.0110
   
3. 0100.4e70.0011
   
4. 0100.4f1f.111c
   

Well, this one surprised me a little bit. You know, I've heard about it, but never thought I would have to be able to answer it on the spot. In the exam it's not possible to look up any reference material, and this question shows the level of detail that one has to master to pass.

There are a number of good tutorials on IP Multicast Addressing. The basic idea: there must be a simple and efficient way to map layer 3 multicast addresses to layer 2. The RFC 1112 (Host Extensions for IP Multicast) specifies a combination of the Ethernet native mechanism for broadcast with a clever direct mapping technique to solve this problem.

Ethernet provides a native mechanism to handle broadcast and multicast traffic; in short, any frame which has the least significant bit of the first octet of its MAC address set is a broadcast frame, and will be retransmitted by a Ethernet switch or bridge to all destinations. In the question above, all frames match the criteria for layer 2 (the first octet is 01, so the least significant bit is set), which leaves us with the direct mapping to solve the question; it uses another property of how MAC addresses are built.

Ethernet MAC address have 48 bits; the first 24 bits comprise the OUI (organizationally uniquer identifier), which is a standard prefix that is allocated by the IEEE for specific usages. Each Ethernet adapter vendor has OUIs assigned to them; the remaining 24 bits are allocated by the vendor, the only criteria being that all addresses must be unique. The IANA (Internet Assigned Numbers Authority) has a OUI reserved (01:00:5E). For IP multicast, the first bit of the second 24-bit block is set to zero. The remaining 23 bits are used to map the IP multicast address.

IP Multicast address have a standard 4-bit prefix (1110). It's equivalent to the range from 224.0.0.0 to 239.255.255.255, also known as 'Class D' addresses in the old, pre-CIDR IP allocation scheme. The remaining 28 bits are mapped to 23 bits in the MAC address. As you may have noted, that's 5 bits less than the available space; the solution is to map the lower 23 bits and forget the 5 most significant ones. It means that there is a potential clash between 32 different IP multicast addresses to the same Ethernet MAC address; in practice, it's not a problem, because there's still plenty of space to allocate IP multicast addresses.

Looking back at the question, it's clear that the answer is (2), or "0100.5e1b.0110". All the other options have different prefixes, and thus are not valid IP multicast addresses.

On Metro Ethernet Services: TLS, DVS, TSL and VLANs

Which service provider metro Ethernet service uses the customer's VLAN IDs to map to a destination site?

1. TLS
2. DVS
3. ISL
4. VLAN

Direct from Transcender, my daily Cisco question source. First step: decrypt the alphabet soup.

TLS is the acronym for Transparent LAN Services, one of the services defined in the Metro Ethernet Services framework. According to the Cisco definition, it "... provides Ethernet connectivity among geographically separated customer locations, creating a VLAN that spans those locations". It's a simple service that gives the user a direct Ethernet connection between the sites, as if all sites were physically connected in the same LAN. The user does not need to know that there is a service provider-owned network, and also, he isn't required to deploy routers or other typical WAN-oriented equipment at their premises; a direct connection to the LAN switch at every location is enough.

As a side note, TLS is also the acronym for Transport Layer Security, and is part of the SSL framework (in fact, TLS is the IETF name for SSL, but for historic reasons, everyone just says "SSL"). Be warned about the potential confusion.

DVS, or Direct VLAN Services, is a term used by Cisco to describe a slightly more complex TLS-like service. In this service, the endpoints see the service provider network as a single LAN switch, with support for VLANs. Note the difference: in a plain TLS, the curstomer gets a single LAN segment, with no support for VLAN switching. In a DVS, the customer VLANs are carried by the service provider, in such a way that separate VLANs can be transported over the Metro Ethernet network.

Note: It seems that the term DVS is being deprecated in favor of the more generic TLS name, whsoe meaning was expanded to include VLAN support. It makes sense, because it's one acronym less to remember, and also because nowadays any decent Metro Ethernet switch will support DVS anyway. One evidence is that the DVS name is not used in the Cisco documentation anymore, with the exception of the certification material.

ISL (Inter Switch Link) is a old Cisco name for a pre-802.1q VLAN trunking mechanism. It was superseded by 802.1q compliant trunks, but it's still supported by Cisco. For the most part, both protocols have comparable features. It's sometimes useful to deploy ISL inside the service provider network to configure non-802.1q trunks, which are useful to avoid some kinds of side effects in situations where VLAN Tunneling, or 1q-in-1q, is being deployed. Cisco has extensive design tips for this scenario in the 1q-in-1q-related chapters for its switches.

VLAN is the acronym for Virtual LAN, as defined by the IEEE 802.1q standard. Iit's now a standard feature for any professional switch, including most low-end devices. VLANs allow for network segmentation, and it's useful to deploy separate logical LANs over a single network infrastructure.

Now for the question at hand. The answer is (b): DVS, which uses the customer VLAN info to deliver Ethernet frames for each endpoint. (a) is a simpler service, which is not able to use the customer VLAN info. (c) and (d) are terms which define some of the technologies involved, but are not Metro Ethernet service names.

Thinking about Cisco gear

This is a brand new blog about Cisco gear. I used to work with Cisco equipment, as a system integrator, over a long period a few years ago (from 1996 to 2002). After a short period working with other stuff, I am back to the world of Cisco things. Here you will find my assorted remarks and study notes on Cisco. Don't expect anything fancy though, as this is more a scratchpad than anything else...