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Chapter 6 - Supporting Popular WAN Technologies

Cisco TCP/IP Routing Professional Reference
Chris Lewis
  Copyright © 1999 The McGraw-Hill Companies, Inc.

SMDS: Switched Multimegabit Data Service
SMDS, or Switched Multimegabit Data Service, has not yet gained significant market penetration, although it has begun to experience some growth. SMDS was viewed as a stepping stone to ATM, since some of the communications equipment and media are common to the two technologies. As SMDS is not available everywhere, and there is more interest in ATM, SMDS has had a hard time getting into the mainstream.
SMDS does, however, have some penetration; if your long-distance carrier is MCI, you may have cause to use this technology. The attraction of SMDS is that it has the potential to provide high-speed, link-level connections (initially in the 1 to 34 Mbps range) with the economy of a shared public network, and exhibits many of the qualities of a LAN.
In an SMDS network, each node has a unique 10-digit address. Each digit is in binary-coded decimal, with 4 bits used to represent values 0 through 9. Bellcore, the "keeper" of the SMDS standard, assigns a 64-bit address for SMDS, which has the following allocation:
  The most significant 4 bits are either 1100 to indicate an individual address, or 1110 to indicate a group address.
  The next 4 most significant bits are used for the country code, which is 0001 for the United States.
  The next 40 bits are the binary-coded decimal bits representing the 10-decimal digit station address.
  The final 16 bits are currently padded with ones.
To address a node on the SMDS network, all you need do is put the node's SMDS address in the destination field of the SMDS frame. In this way, SMDS behaves in a fashion similar to Ethernet or Token-Ring, which delivers frames according to MAC addresses. A key difference between SMDS and these LAN technologies, however, is the maximum frame size allowed. Ethernet allows just over 1500 bytes, and Token-Ring just over 4000 bytes, but SMDS allows up to 9188 bytes. These SMDS frames are segmented into ATM-sized 53-byte cells for transfer across the network. A large frame size gives SMDS the ability to encapsulate complete LAN frames, such as Ethernet, Token-Ring, and FDDI, for transportation over the SMDS network.
An SMDS network can accept full-bandwidth connections from DS0 (64 kbps) and DS1 (1.544 Mbps) circuits, or an access class, which is a bandwidth slice of a higher-speed link such as a DS3 (45 Mbps). These links terminate at what is known as the Subscriber Network Interface (SNI) and connect to the Customer Premises Equipment (CPE). The SNI typically is an SMDS CSU/DSU device and the CPE in this case will be a Cisco router. These elements are illustrated in Fig. 6-9.
Figure 6-9: SMDS network components
SMDS is based on the 802.6 standard for Metropolitan Area Networks (MAN), which defines a Distributed Queue Dual Bus, and is a connectionless technology. The key difference between SMDS and 802.6 is that SMDS does not utilize a dual-bus architecture; instead, connections are centered on a hub and deployed in a star configuration.
SMDS Protocols
SMDS has its own layer 1 and layer 2 protocols that specify the physical connections and voltage levels used, along with the layer 2 addresses. These protocols are implemented in the SMDS Interface Protocol (SIP). SIP has three levels, with SIP levels 2 and 3 defining the Data Link layer functions.
SMDS carries IP information quite effectively. Let's say IP as a layer 3 (in OSI terms) protocol hands a frame down to the SIP level 3 protocol. SIP 3 will place a header and trailer on this frame to form a level 3 Protocol Data Unit. (This is "layer 3" in SMDS parlance, which is a part of the layer 2 protocol in OSI terms.) This expanded frame has SMDS source and destination addresses that include the aforementioned country codes and 10-digit decimal addresses.
The expanded frame is passed down to the SIP level 2 protocol, which cuts this expanded frame into 44-byte chunks, to which get added a header and trailer, enabling reassembly of the frame once the 44-byte chunks have been received by the destination. These 44-byte chunks are termed level 2 Protocol Data Units.  The SIP level 1 protocol provides appropriate framing and timing for the transmission medium in use. The relationship between OSI and SIP layers is illustrated in Fig. 6-10.
Figure 6-10: SMDS communication layers compared to OSI
In the example given above, SMDS is acting like a LAN as far as the IP level is concerned, with the only difference being that, instead of regular ARP and the ARP table, there must be a way of resolving IP addresses to SMDS 10-digit addresses.
In SMDS, IP ARP functionality is provided by address groups, which also are referred to as Multiple Logical IP Subnetworks by RFC 1209. Address groups are a group of predefined SMDS hosts that listen to a specific multicast address as well as to their SMDS node address. In this instance, if an IP host needs to find an SMDS address for an IP address in order to send a frame, an ARP query is sent to the address group and the SMDS host with the appropriate address responds with its SMDS address. Routing broadcast updates are distributed the same way across SMDS.
Typically, SIP levels 1 and 2 are implemented in the SMDS CSU/DSU, and SIP level 3 is implemented in the Cisco router.
Configuring SMDS
Because SMDS is not as widely deployed as the other network technologies presented in this chapter, we will examine only one simple configuration. The tasks to complete to configure an interface for connection to an SMDS service are as follows:
  Define an interface to have SMDS encapsulation.
  Associate an IP address with this interface to be mapped to the SMDS address supplied by the carrier company.
  Define the SMDS broadcast address to be associated with the interface, so that ARP and routing broadcasts are passed between SMDS nodes as needed.
  Enable ARP and routing on the interface.
Figure 6-11 shows the Cisco IOS configuration commands that could be appropriate for the serial 0 port of router 1 in Fig. 6-9.
INTERFACE SERIAL 0
IP ADDRESS 164.4.4.3 255.255.255.0
ENCAPSULATION SMDS
SMDS ADDRESS C234.5678.9070.FFFF
SMDS MULTICAST IP E654.5678.333.FFF
SMDS ENABLE-ARP
Figure 6-11: Sample router configurations for SMDS
The following discusses the SMDS-specific commands in the configuration for Serial 0:
  Command encapsulation smds sets the encapsulation to SMDS for the interface. Note that although the SMDS standard allows a maximum frame size of 9188, the Cisco serial port has buffer constraints that limit it to a frame size (MTU) of 4500. If the MTU size is set to a value greater than 4500 prior to the encapsulation SMDS command, performance problems may occur.
  Command smds address c234.5678.9010.ffff defines the SMDS address for this interface supplied by the SMDS network vendor. All addresses must be entered in this notation, with dots separating each four digits. Individual node addresses start with the letter "c" and multicast addresses start with the letter "e."
  Command smds multicast ip e654.5678.3333.ffff defines the multicast address for the access group to which this interface belongs. An optional argument can be used here to associate a secondary IP address with this port and a different SMDS multicast address, so that the same port can belong to two access groups simultaneously.
  Command smds enable-arp enables the ARP function for SMDS address resolution across the SMDS network. The only restriction on this command is that the SMDS address must already be defined.

 


 
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